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Introduction 5 

Geography and Topography of the Santa Catalina Mountains 6 

Vegetation of the Santa Catalina Mountains 11 

The Desert Region 15 

The Upper Bajadas 15 

The Desert Arroyos and Canons . . . '. 19 

The Lower Desert Slopes 22 

The Upper Desert Slopes 23 

The Encinal Region 24 

The Lower Encinal 25 

The Upper Encinal 27 

The Forest Region 29 

The Pine Forest 31 

The Fir Forest 33 

Flora of the Santa Catalina Mountains 36 

Phytogeographic Relationships of the Flora 36 

The Desert Flora 36 

The Encinal Flora 38 

The Forest Flora 39 

List of Characteristic Species 41 

Climate of the Santa Catalina Mountains 46 

Rainfall 48 

Seasonal Distribution of Rainfall 48 

Altitudinal Increase of Rainfall 51 

Soil Moisture 59 

Evaporation 63 

Humidity 67 

Temperature 69 

Length of Frostless Season 70 

Normal Altitudinal Temperature Gradient 75 

The Absolute Minimum of Winter 79 

Departures from the Normal Temperature Gradient due to Cold-air Drainage 82 

Soil Temperature 86 

Correlation of Vegetation and Climate in the Santa Catalina Mountains 88 

The Normal Altitudinal Gradient of Vegetation 88 

The Vertical Distribution of Individual Species 89 

Physical Factors Involved in the Determination of the Normal Altitudinal Gradient 

of Vegetation 91 

Moisture Factors 92 

Temperature Factors 94 

The R&le of Topographic Features in Determining Departures from the Normal 

Altitudinal Gradient of Vegetation 97 

The Hole of Slope Exposure 97 

The R61e of Streams and Flood-plains 104 

The Role of Topographic Relief 107 

General Conclusions 109 




The southern half of the state of Arizona may be briefly character- 
ized as a relatively level plain studded with numerous hills and moun- 
tains. The plain rises from elevations of a few hundred feet along the 
Colorado River to as much as 4,500 and 5,000 feet near the New Mexi- 
can boundary. The lower elevations follow the Gila, Salt, San Pedro, 
and other rivers, while the higher plains surround the loftier mountains 
of the southeastern portion of the State. Between the Colorado River 
and Tucson there are no mountains of commanding elevation, and the 
area occupied by the scattered volcanic peaks and ranges is not more 
than one-tenth of the total area of the region. To the eastward of 
Tucson, however, a much greater percentage of the total area is occu- 
pied by mountain ranges, a score of which reach elevations of over 8,000 
feet. The general topographic configuration of the region has remained 
unchanged throughout a long period of geological time, and the moun- 
tains and hills have been subjected to prolonged erosion, the products 
of which have served to build up the shelving plains which form the 
intervening valleys. 

Those portions of southern Arizona which lie below 4,000 feet are 
covered with a low, open, desert vegetation, while the plains and valleys 
of higher elevation support a loose carpet of perennial grasses and 
ephemeral herbs, together with a sparse representation of succulent and 
semi-succulent types of plants. The higher mountain ranges exhibit 
a graduated sequence of vegetation from that of the desert valleys, 
through a scrub of evergreen oaks to forests of pine, spruce, and fir. 
The bodies of mesophilous vegetation which occupy these isolated 
mountain summits, and the stages which connect them with the vege- 
tation of the desert, present innumerable phenomena of the greatest 
interest to both physiological and floristic plant geography, and form 
a most fruitful field of investigation. 

The Santa Catalina Mountains are one of the most westerly of the 
high ranges of southeastern Arizona, and rise from an approximate 
basal elevation of 3,000 feet to a height of 9,150 feet. With respect to 
their vegetation these mountains are typical of a large number, not 
only in Arizona but in southern New Mexico and northern Mexico as 
well. Their location within 20 miles of Tucson and their ready acces- 
sibility from the Desert Laboratory have given opportunity for a study 
of the distribution of their vegetation and for a measurement of some 
of the physical factors upon which the existence and activities of the 
vegetation depend. It is the purpose of the present paper to give a 
brief description of the vegetistic features of the various altitudes and 



topographic situations in the Santa Catalina Mountains, to give the 
results of the climatological instrumentation which has been carried 
on, and to indicate in so far as possible the manner and degree in which 
the successive altitudinal stages of vegetation are dependent upon the 
gradients of climatic change by which they are accompanied. 

Some of the instrumental records date from the summer of 1907, 
the first year in which members of the Desert Laboratory became 
interested in the mountains, but the principal part of the data to be 
presented were secured in 1911 and subsequent years. The operation 
of instruments and the study of vegetation have been made on visits 
of 5 to 10 days, at intervals between April and October. From three 
to nine such visits have been made to the mountains in each of the 
summers since 1910. 

The practical exigencies of the work have limited the character of 
the instrumentation which could be carried out, but have not impaired 
the accuracy of the data which it was possible to secure. There is no 
respect in which the results herewith presented may be considered as 
more than a mere outline of a large and widely ramifying botanical 
problem. The central interest of the writer has been to determine 
which of the major environmental factors are responsible for the chief 
distributional features of the vegetation, and to ascertain something 
regarding the intensities of the factors responsible for the distributional 
limits of individual species, and thereby for the limitation of the types 
of vegetation themselves. Such an inquiry into the correlations exist- 
ing between physical conditions and the occurrence and activity of 
plants may do much to explain general vegetistic phenomena, but it 
does far more to open up the innumerable physiological problems which 
must be well known at the outset to underlie these correlations. 



The Santa Catalina Mountains occupy the drainage divide between 
the San Pedro River, a tributary of the Gila, and the Santa Cruz, a 
river which seldom has sufficient flow to reach an outlet in the Gila. 
The position of the mountains is between 110 30' and 111 east longi- 
tude and 32 15' and 32 35' north latitude. The general outline of the 
range is roughly triangular (see plate 40), its southern base being at 
about 3,000 feet (915 m.) elevation, its northeastern base (parallel to 
the San Pedro River) lying at approximately 3,500 feet (1,065 m.). 
To the northwest a broad grassy plain, 3,500 to 3,800 feet in elevation, 
connects the Santa Catalinas with the lower Tortilla Mountains. To 
the southeast a narrow pass, 4,300 feet (1,310 m.) in elevation, connects 
with the closely adjacent El Rincon Mountains, which reach an eleva- 
tion of 8,465 feet (2,580 m.). Southward from El Rincon range a pass 
of 4,000 feet (1,220 m.) elevation leads to the Santa Rita range (9,432 


feet, 2,875 m.). To the northeast of the San Pedro River rises the 
Galiuro range of mountains, the main ridge of which is approximately 
35 miles (57 km.) distant from the Santa Catalinas. The next moun- 
tains encountered in passing northeastward are the Pinaleno or Graham 
range, about 60 miles distant from the Santa Catalinas, and exceeding 
them in altitude by about 1,400 feet (427m.). Beyond the upper 
course of the Gila River lie the Gila Mountains, and still further to the 
northeast the White Mountains, which reach an elevation of 11,280 
feet (3,440 m.) in Escudilla Peak. The White Mountains present one 
/of the largest elevated land masses of the State, connecting to the 
northeast, through the Mogollon Mesa, with the elevated region which 
surrounds the San Francisco Peaks and supporting a heavy body of 
forest which extends from the New Mexican boundary nearly to the 
Grand Canon. East of the White Mountains the elevated country ex- 
tends for about 75 miles (121 km.) into New Mexico, breaking up into 
several diverging ranges which form a part of the Continental Divide, 
draining westward into the Gila and eastward into the Rio Grande. 

The Santa Catalina Mountains are thus seen to stand at the south- 
western terminus of a series of isolated elevations which stretch away 
from the southern edge of the Colorado Plateau. The valley of the 
Rio Grande and its tributaries, several undrained basins, and the 
valley of the Little Colorado combine to separate the entire chain of 
elevations San Francisco Mountains, Mogollon Mesa, White Moun- 
tains, and the mountains of western New Mexico from the Sangre 
de Cristo, San Juan, and Jemez mountains of northern New Mexico, 
which are virtually a part of the Rocky Mountain system. A consider- 
able degree of isolation from the north is thus given to the entire series 
of mountains in southeastern Arizona. 

To the south and southeast of the Santa Catalinas an irregular but 
close-set series of mountains gives them a connection with the Mexican 
Cordillera which is much closer than their connection with the Rocky 
Mountains. To the west the nearest forest-clad elevations are the San 
Jacinto and San Bernardino Mountains of southern California, which 
are about 300 miles (480 km.) distant. 

The relative isolation of the Santa Catalina Mountains and the 
directions in which they possess easy stages of connection with other 
elevated regions are of first importance in relation to the genesis and 
history of their flora, a subject which will be only briefly touched upon 
in this paper (see p. 36). 

The southern face of the Santa Catalinas, to which the present in- 
vestigation has been confined, is built solely of gneiss of varying degrees 
of hardness. The main ridge and the northern and eastern lateral 
ridges are worn into a relatively rounded topography, while the south- 
western corner of the range possesses rock of greater durability and is 
correspondingly more rugged in topography. 


The highest elevations lie between Mount Lemmon (9,150 feet, 
2,790 m.) and Green Mountain (7,900 feet, 2,410 m.), which are only 
7 miles (11 km.) apart. Samaniego Ridge and Oracle Ridge extend 
northward from the vicinity of Mount Lemmon, falling rapidly in 
elevation and terminating in the high plain which lies in the direction 
of the Tortilla Mountains. A very rugged ridge extends southwest- 
ward from Mount Lemmon and terminates in Pusch Ridge. To the 
south of the main ridge an extensive elongated drainage basin has been 
developed which lies parallel to the south face of the range and finds 
its outlet through Sabino Canon. Several important streams drain the 
south slopes of the main ridge and are tributary to Sabino Canon. To 
the east of Sabino two important drainages Bear Canon and Soldier 
Canon drain the eastern end of the mam ridge in the vicinity of Green 
Mountain, and west of Sabino are Pedregosa, Ventana, Pima, and other 
canons which rise hi the rugged southwestern portion of the mountain. 
All of these streams flow into the Rillito, a tributary of the Santa Cruz 
which also drains a portion of El Rincon range. The north face of the 
main ridge between Oracle and Samaniego ridges is drained by the 
Canada del Oro, which flows at first north, then west, and finally south- 
west, emptying into the Santa Cruz. On the northeast slopes of the 
range the topography is relatively simple, the high elevations falling 
away rapidly in the direction of the San Pedro River. A large number 
of minor streams drain this region and give to the San Pedro perhaps 
less than one-fourth of the total run-off of the mountains. 

The main drainageway of Sabino Canon is the only one in the Santa 
Catalinas which possesses a constant flow of water, which is due both 
to the great extent of its cachment basin and to the fact that it has 
its source on the north slopes of Mount Lemmon in the heaviest body 
of timber on the mountain. In the Canada del Oro, in Bear and Soldier 
Canons, as well as in a few of the larger canons of the north slopes, 
water may be found at all times of the year in certain localities where 
the local configuration of the valley or the occurrence of resistant dikes 
of rock forces the underflow to the surface. During the rainy seasons 
water may, of course, be found in any of the large drainageways. The 
heavy local showers of summer often convert even the smallest stream- 
ways into rushing torrents for a few hours. 

The small size of the Santa Catalinas together with their elevation 
gives a steep gradient to all of the major streams. The main stream 
of Sabino Canon falls from 7,700 feet at Webber's Cabin to 3,700 feet 
at the west end of Sabino Basin, a distance of 6 miles, or a gradient 
of fall of 667 feet per mile. From the west end of the Basin to its 
emergence onto the desert this stream falls only 1,000 feet in a distance 
of 5 miles. The Canada del Oro falls at a rate of 494 feet to the mile 
from its source, just west of Mount Lemmon, to the confluence of its 
main tributary from the west slopes of tbe Oracle Ridge, and at the 


rate of 200 feet to the mile from there to its emergence from the moun- 
tain at 3,400 feet. The result of the passage of intermittent and tor- 
rential streams through such steep drainageways has been the wearing 
down of the stream beds to solid rock throughout almost the entire 
drainage system of the mountain. There are no parks nor mountain 
meadows, such as are present in some of the largest southwestern 
mountains. The flood-plains and alluvial bottoms are all small and 
scattered. The spots in which meandering streams may be found are 
very few indeed. In Bear Canon a flood-plain nearly half a mile in 
length has been formed as a result of a large body of highly resistant 
rock, which has narrowed the canon and prevented the outwash of 
erosion material. Similarly, in Soldier Canon there is a small flood- 
plain below which the stream falls 300 feet in a very short distance 
through a narrow gorge. Although Sabino Basin is a locality in which 
several converging streams undergo a sudden reduction in their gradi- 
ent of fall, there has not been any considerable deposition at that place. 
On the contrary the region is one in which the streamways are bordered 
and bedded by large boulders in a matrix of coarse sand and are sub- 
jected to active scouring by the torrential floods of summer. 

Whatever may have been the original form of the Santa Catalinas 
they have been so far worked upon by erosion and weathering that 
they now possess almost no relatively level areas or regions of inde- 
terminate drainage. All of the higher portions of the main ridge and 
of the lateral ridges as well are extremely narrow. The only localities 
in which the topography broadens and is relatively level are at points 
where several drainages have their origin, or places just above precipi- 
tous cliffs. On the summit of Mount Lemmon there is a nearly level 
area of at least 100 acres (see plate 36 A and B), from which a flat-topped 
ridge extends eastward for half a mile, terminating in an abrupt drop, 
in the course of which two narrow ridges have their origin. This re- 
stricted area of nearly level land is a last relic of a portion of the original 
structural form of the mountain, and it will not be many centuries 
until it is reduced to the narrow form of the lower ridges. 

In the eastern and central portion of the Santa Catalinas the gneiss 
weathers readily and gives rise to a loam soil. The precipitate topog- 
raphy gives little opportunity for the accumulation of this soil, and 
it is thin in almost all localities. Throughout the lower portions of the 
range, below the pine forest, the soil has the appearance of being ex- 
tremely coarse by reason of the surface coating of angular fragments 
from 1 to 5 mm. in diameter. Surface drainage is able to move this 
material but slowly by reason of its size and angularity. Just beneath 
it may be found a fine soil, still mingled with coarse particles but held 
in place by the mulch of stones, which is analogous to "desert pave- 
ment." The outcropping rock and larger boulders serve to retard 
erosion and to preserve a soil sufficiently deep for shrubs and trees to 


find root. There are many deep soil-filled crevices through which the 
roots of trees are able to penetrate to bodies of deep-seated soil of 
favorable moisture content. 

The restricted areas of alluvial soil in the desert and lower mountain 
regions are of a fine sand or sandy loam and possess considerable humus, 
in contrast to the soils of the slopes. At the forested elevations the 
soil is similar to that of the evergreen oak region. The soil of the lower 
pine belt is scarcely superior in depth or humus content to that of the 
upper oak region. Above 7,500 feet, however, the amount of humus, 
as well as the amount of surface litter, increases with the increasing 
density of the stand of pines. On the north-facing slopes which are 
clothed with fir forest the soil is not much if at all deeper than in the 
heavy stands of pine, but is notably richer in organic matter. 

The alluvial slopes which immediately surround the mountain are 
so closely related to it in all of their physical and biological features 
that it will be necessary in the following pages to give some considera- 
tion to their vegetation. Throughout the arid southwest the long 
straight profiles presented by the outwash slopes of the hills and moun- 
tains form one of the characteristic features of the landscape. The 
distinct character of these slopes is to be attributed to the manner in 
which they have been laid down under conditions of torrential rainfall 
and of violent and intermittent stream flow, and their distinctness from 
the parabolic alluvial slopes of the humid regions has caused Tolman * 
to designate them technically by their popular Spanish name" bajada."^ 

The bajadas constitute almost the total area of all the intermontane 
valleys of southern Arizona. The only portions of the valleys topo- 
graphically separable from them are the stream beds, the flood-plains, 
and the "play as" or undrained areas into which one or more streams 
flow and deposit their load. To the student of vegetation there are 
marked differences between the upper and lower portions of all bajadas. 
The differences in the physical features presented by upper and lower 
bajadas of the same elevation have been only superficially investigated; 
the differences in their vegetation are very obvious, as will be described. 
The principal environmental features which appear to differentiate the 
high and low bajadas are the coarser character of the soil in the high 
bajadas, the possibility of higher soil moisture in them, at least at a 
depth of several feet, and the greater development of calcareous incrus- 
tations, or "caliche," in the soil of the low bajadas. The layers of 
caliche lie near the surface in some places, while in others the upper- 
most ones have been covered by deposition; they extend downwards 
for a few feet in some cases, or more frequently recur to a depth of 
100 feet or more. 

The bajada of the southern face of the Santa Catalinas has been 

* Tolman, C. F. Erosion and Deposition in the Southern Arizona Bolson Region. Jour. Geol., 
vol. 17, pp. 136-163, 1909. 
t Pronounced bahada. 


truncated at its lower edge by the Rillito, which flows at right angles 
to the slope of the bajada, and on the western side of the range the 
Canada del Oro has worn off the lower edges of the detrital slopes in 
similar matter. The well-developed bajadas which lie between Pima 
and Ventana Canons fall at the grade of 150 to 175 feet per mile. 
Between Ventana and Bear Canons the uppermost portion of the 
bajada has been worn away, so that at present a shallow valley lies 
between the base of the mountain and the lower portion of the old 
bajada, now cut into isolated and rounded hills. On the northeast 
side of the Santa Catalinas the bajada which extends down to the San 
Pedro River exhibits approximately the same grade as the bajada at 
Pima Canon. Its surface is crossed, however, by so many drainages 
from the steep northeast face of the mountain that the bajada region 
consists of a series of rounded ridges extending out from the base of 
the mountain, very unlike the relatively flat bajadas of the Santa Rita 
and El Rincon ranges. 


The journey from the base to the summit of the Santa Catalina 
Mountains brings to the eyes of the observer a constantly changing 
panorama of vegetation. New types of plants are constantly being 
encountered with increase of altitude, while types already familiar are 
being left behind. There is no portion of the mountain, at least below 
7,500 feet, in which a climb of 500 feet does not materially alter the 
physiognomy of the surrounding vegetation. The course of the vege- 
tational panorama is not merely a gradual transition from the open 
desert of succulents and microphylls to the heavy fir forest which 
occupies the summit of Mount Lemmon (plate 1). There are inter- 
posed between these vegetations two distinct belts of plant life through 
which this tremendous transition takes place. 

The arborescent cacti and the trees and shrubs of the desert give 
way gradually to evergreen oaks, leaf-succulents, sclerophyllous shrubs, 
and perennial grasses. This open but arborescent vegetation reaches 
a full development and then gives way to pine forest, with a distinctive 
accompanying carpet of herbaceous perennials. The pine forest is 
then, in turn, invaded by spruce and fir and the heavy stands of these 
trees are accompanied by still another assemblage of shrubs and her- 
baceous plants. The striking character of these gradations of vegeta- 
tion is not due solely to the contrast between the varied vegetation 
of the open desert and the monotony of the closed coniferous forests, 
but is quite as largely due to the striking types of plants which are 
to be found both in the desert and in the region of evergreen oaks. 

A first and most general observation of these vegetational stages will 
discover the distinctive regions of desert, of park-like semi-desert and 
of forest. The first is like the desert of the extensive bajada slopes 
which surround the entire mountain; the second is similar to plant 


communities which may be seen in southern Texas and in California, as 
well as in similar situations in Arizona and New Mexico; the last is 
essentially like the great body of yellow pine forest which stretches 
from southern Jalisco to British Columbia, or like the fir and Douglas 
spruce forests of the Rocky Mountains. These three major regions 



3,000 - 

Fouquieria splendens 

Quercus emoryi 

Dasyiirion wheeieri 

Quercus nypoieuca 

FIG. 1. Diagram showing vertical distribution of Desert, Encinal, Pine 
Forest, and Fir Forest in relation to slope exposure, together with dia- 
grams showing effect of slope exposure on vertical distribution of Fou- 
quieria splendens, Dasyiirion wheeieri, Quercus emoryi, and Quercus 

constitute the most natural and easily distinguished subdivisions of 
the vegetation, and depend for their distinctness on the radical dis- 
similarity of the dominant types of plants in each. They may best be 
designated by the simple terms Desert, Encinal,* and Forest. The 

* The Spanish word "encinal" signifies a grove or forest of evergreen oaks, being derived from 
encina, evergreen oak. The suitability of the word was, suggested by Prof. J. W. Harshberger. 


Encinal belt is essentially a region dominated by sclerophyllous trees 
and shrubs and by semi-succulent perennials, with an open stand of 
perennial grasses. It is what is designated by the Forest Service as 
the "woodland type" of forest. The pine and fir forests are very 
dissimilar in their floristic composition, but they are much more closely 
alike vegetistically than are any two of the three major divisions which 
have been made. A further and more careful examination of the stages 
which connect the Desert with the Forest will discover not only the 
inevitable gradations between the three major regions, but also 
several minor features which cause constantly recurring departures 
from the typical or ideal vertical distribution of the vegetation. The 
influence of slope exposure on the vertical ranges of both the individual 
species and the vegetation itself is a feature which these mountains 
share with almost all extra-tropical mountains; the distinctive vegeta- 
tion of flood-plains and streamways is also as clearly noticeable here 
as in all arid and semi-arid regions; the occurrence of the lowland 
species at higher altitudes on ridges than in the valleys is also a strong 
differentiating feature. 

In describing the salient physiognomic and floristic features of the 
vegetation, and its distributional behavior, it is expedient to recognize 
primarily the three major divisions of Desert, Encinal, and Forest, 
and then to take into account secondarily the degree to which the 
components of these regions intermingle and the extent to which the 
topographic irregularities of the mountain cause an alternation and 
interdigitation of the three regions. 

The basal slopes of the mountain between 3,000 and 4,000 feet (915 
and 1,220 m.) present few vegetational distinctions from the upper 
bajadas, and almost no distinctions of flora. Between 4,000 and 5,000 
feet (1,220 and 1,525 m.) there is a rapid elimination of all but a very 
few of the characteristic desert species, and on north slopes at the 
latter elevation nearly all of the dominant Encinal forms have made 
their entry. The upper limit of the Desert may be placed at 4,000 feet 
for north slopes and 4,500 feet (1,472 m.) for south slopes. The upper 
edge of the Desert exhibits an attenuated occurrence of all of the larger 
desert plants and the presence of many perennial grasses and semi- 
woody plants which occur both in the Encinal Region and on the bajadas 
of equal or slightly greater elevation in the neighboring portions of 
Arizona. The extreme upper limit of desert forms is 7,000 feet (2,133 
m.) , an elevation which is reached by a single succulent species. Follow- 
ing the dissimilarity of the lower and upper portions of the Desert Region 
they have been described separately. 

The Encinal Region extends from the occurrence of the first ex- 
tremely open groves of evergreen oaks on north slopes at 4,000 feet up 
to the first elevation at which the larger pines begin to dominate the 
physiognomy of the vegetation, at about 6,300 feet (1,920 m.) on south 


slopes. A few of the dominant species of the Encinal are found on 
the higher bajadas, above 3,500 feet (1,067 m.) elevation, but in the 
mountains none of its species is to be found so low as this except on 
north slopes or near arroyos of large drainage area. At 4,000 feet, on 
north slopes, several of the larger Encinal plants are encountered, and 
at 4,300 feet (1,310 m.), on north slopes, several additional dominant 
species are found. Within the Encinal Region it is possible to recognize 
a lower and an upper portion, distinguished chiefly by the openness of 
the former and the closed character of the latter. The closed Upper 
Encinal merges gradually into the Forest, losing some of its character- 
istic species at 6,000 to 6,300 feet (1,830 to 1,920 m.), while others range 
to 7,800 feet (2,380 m.) and a few to 8,300 feet (2,530 m.) on south 

The lowest occurrence of Forest is on north slopes at about 5,800 
feet (1,768 m.) and on south slopes at about 6,300 feet (1,920 m.). 
The Forest is at first rather open and is superposed, as it were, upon 
the closed Encinal, but it becomes heavier and the Encinal elements 
within it become more sparse at elevations of from 6,300 to 6,800 feet 
(1,920 to 2,073 m.), according to the slope exposure. The upper limit 
of Forest is not reached in the Santa Catalina Mountains at their 
highest elevation of 9,150 feet (2,790 m.), nor in the adjacent Pinaleno 
Mountains (Mount Graham) at 10,516 feet (3,205 m.). The forest of 
yellow pine occupies all south slopes up to the summit of Mount 
Lemmon. A forest dominated by fir, spruce, and Mexican white pine 
occupies the north slopes above 7,500 feet (2,287 m.), the earliest 
occurrence of these species being about 1,000 feet (305 m.) lower. 

The description of vegetation which is given in the following pages 
applies only to the south face of the Santa Catalinas. The north face 
presents more abrupt slopes than the south, with most of its ridges 
running north from the main ridge. This circumstance obscures the 
influence of slope exposure, since it presents opposed slopes, facing 
east and west, which are identical in their vegetation.* Furthermore 
the north face of the range is mineralogically diversified, presenting 
exposures of shale, sandstone, limestone, diorite, and gneiss, whereas 
the south face presents an exposure of gneiss only, with a resultant 
mineral identity of soils from base to summit. It has thus been possible 
to carry out a study of climatic influences over a vertical gradient of 
6,000 feet (1,830 m.) with uniform soil, and the east and west ridges 
of the south face have furnished opposed north and south slopes at all 

* Differences between the vegetation of east and west slopes have been pointed out by Blumer 
for El Rincon Mountain, but the differences noted were of another character from those commonly 
existent between north and south slopes. See: Blumer, J. C. A Comparison between two Moun- 
tain Sides. The Plant World, 13: 134-140. 1910. 



Under the designation of "Desert" are comprised all those portions 
of the Santa Catalina Mountains in which the vegetation is open, low, 
and diversified in the assemblage of growth-forms, with a predominance 
of microphyllous trees and shrubs and an abundance of cacti. Such 
a vegetation is to be found covering the upper bajadas and extending 
up the slopes of the mountain to elevations of 4,000 to 4,500 feet 
(1,220 to 1,372 m.), according to slope exposure. The vegetation of the 
Upper Bajadas will be described for the sake of the contrast which it 
affords with the vegetation of the upper portions of the mountain, as 
well as to give a picture of the plant life by which the mountain is 
surrounded and from which it has derived many of its characteristic 
species. The desert slopes of the mountain itself exhibit at first a close 
resemblance to the bajada, and then lose most of the larger bajada 
plants before the entry of the dominant plants of the Encinal region. 
This circumstance admits of a subdivision of the Desert region of the 
mountain into Lower Desert Slopes and Upper Desert Slopes. The 
latter region is much poorer than the former in cacti and much richer 
in grasses, both from the standpoints of the number of species and the 
number of individuals. The Upper Desert is similar in vegetation to 
many of the Upper Bajadas, such as those to the northwest of the Santa 
Catalinas and to the east and west of the Santa Rita Mountains, and 
might well be designated as "semi-desert" or "desert-grassland transi- 
tion." It is, however, essentially similar to the desert plains in its 
vegetational make-up, and in no part of Arizona does it serve as a 
transition to true grassland. The largest canons of the Santa Catalinas 
possess some plant communities that are radically unlike the vegeta- 
tion of the desert itself, but not unlike the communities which surround 
the springs and wells of the desert plains. These are the groups of 
aquatic and palustrine plants which accompany the streams and pools 
of the canons. The smaller canons and arroyos t present distinctive 
features of vegetation, departing more and more from the large canons 
and approaching more nearly the character of the desert areas away 
from water. All of these areas have been treated as a part of the Desert 


The Lower Bajadas of the Tucson region are covered by a vegeta- 
tion in which Covillea tridentata (jediondia, creosote bush) is always the 
predominant plant and is often almost the sole plant of more than 
2 feet in height over areas many square miles in extent. The plants 
which most commonly enter this community are Prosopis velutina 
(mesquite), Opuntia fulgida, Opuntia spinosior (both arborescent cylin- 

* The word "region" is not here used in any of the technical senses in which it has been em- 
ployed in phytogeography. 

t The Spanish word arroyo is in common use in the southwestern United States to designate 
streamways which are usually without water. 


dropuntias), Opuntia toumeyi, Opuntia blakeana (procumbent plato- 
puntias), and Acacia paucispina. 

The shorter and steeper Upper Bajadas which fringe the southern 
and southwestern edge of the Santa Catalinas are clothed with a much 
more diversified vegetation, hi all respects similar to that of other 
Upper Bajadas which lie below 3,500 feet (1,067 m.) in other localities 
in southwestern Arizona. The freedom of the soil from caliche is here, 
as elsewhere, responsible for the existence of a diversified vegetation 
rather than a pure stand of Covillea. 

The Upper Bajadas, as exemplified along the south face of the Santa 
Catalinas at about 3,000 feet elevation (915 m.), bear what may be 
regarded in many respects as the most highly developed type of desert 
vegetation to be found in southern Arizona or northern Sonora. In the 
Upper Bajadas may be found a greater number of species of perennial 
plants than in any other distinctly desert situations. In them also 
the number of individual perennial plants per unit area is greater than 
in any areas outside the flood plains of such rivers as the Santa Cruz 
and Gila. The only areas that compare with the High Bajadas in 
these respects are the volcanic hills in which basaltic rock has weathered 
to a fine clay which is very retentive of soil moisture, as is well exempli- 
fied in Tumamoc Hill, the site of the Desert Laboratory. The andesitic 
and rhyolitic hills in the vicinity of Tumamoc are much poorer than 
it is in the number of individual plants per unit area, although perhaps 
nearly as rich in their flora. 

On the Upper Bajadas there often occur, in almost equal admixture, 
from 15 to 25 perennial species of plants of such size as to dominate the 
physiognomy of the vegetation. These same species may be found on 
the more nearly level Lower Bajadas, but any one of them may often 
be absent for many miles, may be sporadically represented by a few 
individuals, or may occur in dense but local colonies (particularly in 
the case of the cacti). Occasionally as many as 5 to 10 of the species 
may be within sight at the same time. 

The flora which characterizes the Upper Bajadas of the Santa 
Catalinas ranges without substantial loss down to sea-level on the gulf 
of California,* and the vegetation formed by their commingling may 
be found as a belt covering the high bajadas which encircle all of the 
mountain ranges and clothing all of the low basaltic hills. A climb of 
2 hours from the base of the Santa Catalinas will discover greater 
changes of vegetation and flora than can be encountered in the 150 
miles (242 km.) between Tucson and Adair Bay. 

The Upper Bajadas present the desert characteristics of openness 
of stand, lowness of stature, and commingling of diverse vegetation 
types. The first of these features is common to the vegetation of all 

*SeeHornaday,W.T. Camp Fires on Desert and Lava. New York, Scribner, 1909. MacDougal, 
D. T. Across Papagueria. The Plant World, 11: 93-99, 123-131, 1908. 


deserts, the last is at least characteristic of the less pronounced deserts 
of the southwestern United States and of Mexico. The openness of the 
stand is such that it is possible in all places to ride a horse through the 
vegetation and to take whatever course the rider may wish, with only 
occasional digressions of a few yards from the general direction of 
travel. The stature of the vegetation is such that it would be possible 
for the rider to keep almost constantly in view another mounted man 
half a mile distant. The columnar giant cacti reach a maximum height 
of 40 feet (12 m.) and the trees a height of 20 to 25 feet (6 to 8 m.). 
The great bulk of the shrubs and succulents, however, are not more 
than 6 feet (2 m.) in height, and many of them are less than 4 feet 
(1.2m.). Among the commonest vegetation types are stem-succulents, 
microphyllous and sclerophyllous trees and shrubs, macrophyllous decid- 
uous shrubs, perennial grasses, and root-perennial and ephemeral her- 
baceous plants. 

Largest and most conspicuous of the succulents is Carnegiea gigantea 
(saughuaro, giant cactus), which is here in its optimum habitat and 
very abundant (plate 3, B). Among the microphyllous trees the most 
abundant are Prosopis velutina, Acacia greggii, Acacia paucispina, and 
the green-barked Parkinsonia microphylla (palo verde). The much- 
branched arborescent types of cacti are represented by Opuntia versi- 
color, which attains a maximum height of 12 feet (4 m.), and by Opuntia 
fulgida and Opuntia mamillata (cholla), remarkable for the brilliance 
of their glistening straw-colored spines. Opuntia blakeana, Opuntia 
engelmanni, Opuntia toumeyi, and Opuntia discata are abundant repre- 
sentatives of the platopuntia group. The evergreen Covillea is greatly 
outnumbered by Fouquieria splendens (ocotillo). The globular Echino- 
cactus wislizeni (bisnaga) attains a height of 4 feet (1.3 m.) with an 
even greater girth. Similar in form but never exceeding a foot in 
height are Echinocereus fendleri and Mamillaria grahami. The sclero- 
phyllous Simmondsia calif ornica (jojobe) and the relatively large-leaved 
deciduous Jatropha cardiophylla are frequent, while a large number of 
less striking shrubs are common, including Franseria deltoidea, Isocoma 
hartwegi, Encelia farinosa, Zizyphus lycioides var. canescens, Lycium 
torreyi, Momisia pallida, Krameria glandulosa, Trixis angustifolia var. 
latiuscula, Crassina pumila, and Psilostrophe cooperi. 

The seasonal rains of whiter and those of summer cause activity of 
foliation and growth on the part of all of the smaller shrubs. The 
winter rams cause foliation in Parkinsonia and Fouquieria, but not in 
Prosopis and the species of Acacia. Neither do they initiate growth 
in Parkinsonia, Fouquieria, nor any of the cacti. The two widely 
separated seasons of rain bring forth two wholly distinct sets of herba- 
ceous ephemeral plants, at the same time that each season causes activ- 
ity upon the part of some of the root-perennials. The ephemeral plants 
may form a dense carpet over both the Upper Bajadas and the Lower 



Bajadas in seasons of well-distributed and copious rainfall. With less 
favorable conditions they may form a very light cover or may be almost 
absent. The total flora of root-perennials and ephemerals is large, and 
the relative abundance of the various species fluctuates tremendously 
from spot to spot, and in the same spot it is by no means the same from 
year to year. This flora is nearly identical with that of the basaltic hills 
in the vicinity of Tucson, and has been fully listed by Thornber,* with 
a subdivision of vegetation types. 

In the following list have been brought together the names of the 
most characteristic plants of the Upper Bajadas, grouped vegetistically 
and briefly described. Asterisks indicate the relative abundance of 
the species three indicating that a plant is extremely common, two 
that it is very common, and one that it is fairly common. Figures 
follow the descriptions, indicating the average height of each species. 
A comparison of all the maximum heights given will convey an impres- 
sion of the low stature of the commonest components of the vegetation. 

Vegetistic Grouping of the Characteristic Species of the Upper Bajadas. 

Perennial Non-succulent Trees and Shrubs: 
*** Acacia greggii, microphyllous, winter- 
deciduous. 2 to 3 m. 
** Covillea tridentata, microphyllous, 
evergreen. 1 to 2.5 m. 

* Crossosoma bigelorii, sclerophyllous, 

evergreen. 1 to 1.5 m. 

* Ephedra trifurca, aphyllous, green- 

stemmed. 0.5 to 1 m. 
*** Fouquieria splendens, macrophyllous, 

drought-deciduous. 2 to 4 m. 
** Jatropha cardiophylla, macrophyllous, 
winter-deciduous. 0.5 to 1 m. 

* Kaberlinia spinosa, aphyllous, green- 

stemmed. 0.5 to 1 m. 
** Krameria glandulosa, microphyllous, 

evergreen. 1 to 2 m. 
** Lycium berlandieri, microphyllous, 

evergreen. 1 to 2 m. 

* Lycium fremontii, microphyllous, 

evergreen. 1 to 2 m. 
** Momisia pallida, sclerophyllous, ever- 
green. 1.5 to 2.5m. 

* Olneya tesola, microphyllous, ever- 

green (foliage occasionally winter- 
killed). 3 to 6m. 

*** Parkinsonia microphylla, microphyl- 
lous, drought-deciduous, green- 
stemmed. 2 to 5 m. 

*** Prosopis velutina, microphyllous, 

winter-deciduous. 3 to 6 m. 
** Zizyphus lycioides var. canescens, 
microphyllous, evergreen 1 to 2 m. 

Perennial Succulent Plants: 

*** Carnegiea gigantea, columnar, 

branched. 5 to 14 m. 
** Echinocactus wislizeni, cylindrical. 

0.5 to 1.5 m. 
** Echinocereus fendleri, cylindrical, cse- 

spitose. 0.1 to 0.4 m. 
** Mamillaria grahami, cylindrical, soli- 
tary or csespitose. 0.1 to 0.2m. 
** Opuntia blakeana, flat-jointed, pro- 

*** Opuntia discata, flat-jointed, pro- 
cumbent or semi-erect. 
** Opuntia fulgida, cylindrical-jointed, 

arborescent, 1 to 2 m. 
*** Opuntia mamillata, cylindrical- 
jointed, arborescent. 1 to 2 m. 
** Opuntia toumeyi, flat-jointed, pro- 
*** Opuntia versicolor, cylindrical-jointed, 

arborescent. 1 to 4 m. 
Perennial Shrublets (all less than 0.7m. high): 

* Coldenia canescens, sclerophyllous. 
** Crassina pumila, dissected leaves. 

*** Encelia farinosa, macrophyllous, 

slightly drought-deciduous. 
*** Franseria deltoidea, sclerophyllous. 
*** Isocoma hartwegi, dissected leaves. 
*** KaUiandra eriophytta, dissected leaves. 
** PsUostrophe cooperi, sclerophyllous. 

* Trixis angustifolia var. latiuscula, 

sclerophyllous, slightly drought- 

* Thornber, J. J. Vegetation Groups of the Desert Laboratory Domain. Carnegie Inst. 
Wash. Pub. 113, Chapter IV, 1909. 



Ephemeral Summer-active Herbaceous Plants Continued. 

Root Perennials (all facultative evergreens): 
*** Abutilon incanum, sclerophyllous. 
** Brodicea, capitate var. pauciflora, 

bulbous, linear leaves. 
** Cassia coyesti,sclerophyllous,branched 

* Dalea parryi, microphyllous. 

*** Muhlenbergia porteri, semi-scandent. 

** Pentstemon wrightii, macrophyllous. 

** Perezia wrightii, macrophyllous. 

** Verbena ciliata, macrophyllous, hairy. 
Ephemeral Summer-active Herbaceous Plants: 

** Bahia absinthifolia. 
*** Baileya multiradiata. 

** Boerhaavia plerocarpa. 

* Boerhaavia watsoni. 
*** Bouteloua aristidoides. 
*** Cladothrix lanuginosa. 

** Euphorbia florida. 

* Euphorbia melanadenia. 

Ephemeral Summer-active Herbaceaus Plants: 

*** Pectis papposa. 
*** Wedelia incarnata. 
Ephemeral Winter-active Herbaceous Plants. 
** Actinolepis lanosa. 
** Anisolotus trispermus. 
*** Baeria chrysostoma. 
** Chorizanthe breincornu. 
** Cryptanthe intermedia. 

* Eremocarya micrantha. 
*** Gilia floccosa. 

** Lepidium lasiocarpum. 
** Lesquerella gordoni. 

* Mentzelia albicaulis. 

* Orthocarpus purpurascens. 
** Pectocarya linearis. 

*** Plantago fasligiata. 
*** Plantago ignota. 


In crossing the Upper Bajadas it is often possible to detect, by means 
of the vegetation, the approach to a very shallow drainageway through 
which water runs for only a few hours after the severest summer rains. 
The larger arroyos are still more conspicuous by reason of the still 
heavier stand of vegetation along their margins, and in the largest 
canons is found the culmination of the influence of surface streams 
and underflows for the support of vegetation. The effect of the most 
transitory of the small streams is merely the raising of the moisture of 
adjacent soil to such a point that it will present favorable conditions 
for plant activity for a longer time after the close of the rainy periods 
than will the soil of the bajada in general. There is only a negligible 
and short-lived underflow in these smallest arroyos, and their only 
differences from the bajada are that in the rainy seasons they present 
slightly more favorable conditions with respect to soil moisture and 
that the effect of the rainy season is slightly prolonged in them, while 
the periods of drought are correspondingly shortened. In the larger 
arroyos there may not be a constant underflow, but there is at least a 
relatively high percentage of soil moisture for periods of sufficient length 
to greatly reduce the influence of the arid periods upon their plants. 
In the largest arroyos and in the mountain canons themselves there is 
either a constant underflow, maintaining high moistures in the soil of 
the banks and bed of the arroyo, or else there is constant water, either 
running or standing in pools. 

The smallest arroyos, which are very frequent on the Upper Bajada 
in the close proximity of the mountain, present no peculiar species, but 
merely a closer stand of the same plants that are to be observed 
throughout the bajada, notably Prosopis, Acacia greggii, and Momisia 
pallida. Along somewhat larger arroyos are to be found still heavier 
stands of the above species, together with Parkinsonia torreyana, Celtis 


reticulata, Baccharis sarothr aides (batamote), Franseria ambrosioides, 
Lycium fremontii, Verbesina encelioides, and Bebbia juncea. 

In the canons and arroyos large enough to have a heavy flow of 
storm water but not large enough to have even pools of water which 
are constant throughout the year, there may be found several additional 
species of plants which also occur on the sandy flood-plains of the 
largest canons. Prominent among these are Chilopsis linearis, Hymeno- 
clea monogyra, and Baccharis glutinosa, all of which are large shrubs 
or in the case of Chilopsis may attain the size of small trees. Also 
characteristic of these sands are Franseria ambrosioides, Rumex hymeno- 
sepalus, Euphorbia pediculifera, Clematis ligusticifolia, and Calyptridium 

In the largest canons of the south side of the Santa Catalinas it is 
possible to witness the occurrence of communities of mesophilous, 
palustrine, and aquatic plants which are limited in area but are made 
up of species which stand strongly in contrast with the predominant 
forms of the bajadas. The existence of streams and pools adjacent to 
rocky slopes makes it possible in several places for Callitriche and 
Isnardia to grow within 20 feet of Carnegiea and Fouquieria. 

At the mouth of Soldier Canon the rocky slopes of the streamway 
are clothed with typical bajada plants together with a few forms which 
are particularly abundant on cliffs and in rocky situations, both in the 
larger mountains of the region and in the volcanic hills. Among the 
latter are Opuntia bigelovii, Hyptis emoryi, Lippia wrightii, Anisacan- 
thus thurberi, Enceliafarinosa, Eriogonum wrightii, Chrysoma laricifolia, 
and Crossosoma bigelovii. Among the boulders bordering the stream- 
way are Janusia gracilis, Plumbago scandens, Maurandia antirrhini- 
flora, Mimitanthe pilosa, and Stemodia plumieri, as well as occasional 
individuals of several species which are common away from streams 
at elevations of 4,000 to 5,000 feet, as, for example, Dasylirion wheeleri, 
Nolina microcarpa, Erythrina flabelliformis, Ingenhousia triloba, and 
Mimosa biuncifera. When the sands of the arroyo have not been 
recently scoured by floods they support scattered individuals of Ama- 
ranthus palmeri (celite), Cassia leptocarpa, Nicotiana trigonophylla, 
Bebbia juncea, Hymenoclea monogyra, Franseria xanthocarpa, Asclepias 
linifolia, Baccharis sarothroides, Mentzelia gracilenta, and Carduus sp. 

In Ventana, Bear, and Sabino Canons it is possible at all times of 
the year to find small colonies of palustrine and aquatic plants, and 
the vicinity of such localities is the optimum habitat for Prosopis and 
Populus. An underflow passes out at the mouth of Sabino Canon which 
is heavier and more constant than that of any other canon in the range; 
this gives Sabino Canon its abundant mesophilous vegetation and also 
causes the arroyo through which its flood waters reach the Rillito to 
be occupied by a much richer stand of vegetation than is to be found 
along any of the arroyos of the adjacent region. The sandy and 


boulder-strewn bed is from 100 to 300 feet in width and the portions 
covered by the storm floods vary from season to season. Trees not 
only occupy the banks of the arroyo but occur scattered throughout 
its bed, where they may persist for many years, only to be eventually 
uprooted by some exceptionally severe freshet. Among these trees 
are Populus sp., Fraxinus toumeyi, Juglans major, Platanus wrightii, 
Sapindus drummondii, Prosopis velutina, and Sambucus mexicana, some 
of which reach a height of 50 to 60 feet (15 to 18 m.). Such shrubs as 
Chilopsis linearis, Baccharis sarothroides, Baccharis glutinosa, and 
Hymenoclea monogyra are also common in and along this arroyo. 

The floor of Sabino Canon from its mouth to the Sabino Basin is 
occupied by an irregular and broken forest of Prosopis, Populus, Frax- 
inus, Platanus, and Salix (Salix wrightii and Salix sp.). Among these 
common trees are scattered a few individuals of three of the oaks 
characteristic of the Encinal region: Quercus oblongifolia, Quercus 
arizonica, and Quercus emoryi. These oaks occur near the mouth of 
the canon at an elevation of 2,700 feet (823 m.), although their lowest 
occurrence on slopes, away from the proximity of an underflow, is at 
4,200 to 4,500 feet (1,280 to 1,372 m.). Cupressus arizonica occurs in 
the upper half of the canon and in Sabino Basin at elevations above 
3,200 feet (975 m.). It is confined to the proximity of streams up to 
an elevation of 6,000 feet, above which it is occasionally found on 

The shrubby vegetation of the floor of Sabino Canon includes all 
of the species which have been mentioned as occurring in the smaller 
canons and arroyos, together with a number of shrubs and perennials 
which are more common along the arroyos and streams of the Encinal 
region. Among the latter are those species mentioned as occurring 
in Soldier Canon and also Vitis arizonica, Bouvardia triphylla, Amorpha 
californica, and Brickellia californica. 

Other characteristic plants are the shrubs Dodoncea viscosa var. 
angustifolia, Eysenhardtia orthocarpa, Indigofera sphcerocarpa, and the 
woody climber Nissolia schottii. 

Among the herbaceous plants common in and along the pools and 
water-courses of Sabino Canon may be mentioned: 

Cattitriche sp. 
Carex sp. 

Cerastium texanum. 
Cyperus infiexus. 
Hdenium thurberi. 
Hydrocotyle ranunculoides. 
Isnardia palustris. 
Juncus arizonicus. 
Juncus bufoniw. 
Juncus interior. 
Juncus spharocarpus. 
Linaria canadcnsis. 
Mecardonia peduncularis. 

Mimulus langsdorfii. 
Mimitanthe pilosa. 
Montia perfoliata. 
Myosurus cupulatus. 
Phalaris intermedia. 
Platystemon californicus. 
Polygonum sp. 
Senedo lemmmi. 
Specularia biflora. 
Stachys coccinea. 
Stemodia durantifolia. 
Tagetes lemmoni. 
Tillcea erecta. 



On leaving the uppermost edge of the bajada and commencing the 
ascent of the mountain over the rather abrupt slopes which lie between 
the larger canons, a region is entered upon in which the physical con- 
ditions differ from those of the bajada chiefly in the pronounced slope 
exposure to the south, southwest, or southeast, and in the occurrence 
of large masses of rock in situ, with the coarse soil limited to small 
benches, pockets, and fissures. The vegetation of these lowest slopes 
is very similar to that of the Upper Bajadas, and is composed of a 
nearly identical flora. Prosopis, Parkinsonia, and Acacia are repre- 
sented by smaller and less frequent individuals, and both the cylindro- 
puntias and platopuntias occur somewhat less frequently. Carnegiea 
gigantea is even more abundant on the slopes than on the bajadas, 
being represented by smaller individuals, among which relatively few 
have reached the size at which branching begins. For Carnegiea and 
the above-mentioned trees the relatively rapid erosion of the soft gneiss 
and the shifting of the shallow soil are apparently too great to permit 
the attainment of great size or age. Fouquieria, Encelia, and Chrysoma 
laricifolia are even more abundant on the slopes than on the Upper 
Bajada, and Opuniia bigelovii, the most densely spiny of all the cylin- 
dropuntias, is found exclusively on southerly slopes and cliffs, in very 
rocky substratum, at elevations below 3,500 feet (1,067 m.). Olneya 
tesola and Covillea have not been detected on the mountain slopes (see 
plates 4 and 5). 

The summer and winter ephemerals of the bajada are nearly all 
to be found on the Desert Slopes of the mountain, but rarely in such 
abundance as they attain on level ground. Among the most common 
of the ephemerals and root-perennials to be observed in the summer 
are Cladothrox lanuginosa, Pectis papposa, Euphorbia florida, Bcer- 
haaina pterocarpa, Bouteloua aristidoides, Andropogon saccharoides, 
Wedelia incarnata, Machceranthera tanacetifolia, Triodia mutica, Evol- 
vulus arizonicus, Allionia gracillima, and Cassia covesii. The bases of 
boulders and partially shaded ledges of rock are the habitats of Selagi- 
nella rupincola, Cheilanlhes lindheimeri, and Notholcena hookeri. The 
ferns are not common and are conspicuous only during rainy periods, 
but the Selaginella is abundant here and becomes even more so at 
slightly higher elevations, where it frequently clothes the rocky walls 
of steep canons to such an extent that their usual grayness is converted 
to a vivid green a few hours after a heavy rain (see plate 7). 

The ascent from 3,500 to 4,000 feet (1,067 to 1,220 m.) witnesses 
the first essential changes in the vegetation. At the latter elevation 
nearly all of the typical desert forms may be found, but Opuntia has 
become infrequent and Carnegiea gigantea, Echinocactus wislizeni, and 
Fouquieria splendens are conspicuously confined to southerly slopes 
(see plates 6 and 8). Parkinsonia torreyana, which is confined to arro- 


yos on the bajada, is found here growing with Parkinsonia microphylla, 
which it eventually exceeds in vertical distribution by nearly 500 feet 
(153 m.). Prosopis is even more abundant at 4,000 feet (1,220 m.) 
than it is on the lowest slopes, and attains a trunk circumference of 
6 feet (2 m.) at 4,200 feet elevation (1,280 m.), within 600 vertical 
feet (183 m.) of its upper limit. Such common shrubs of the bajada 
as Lycium, Zizyphus, Krameria, Jatropha, and Momisia are now very 
sporadic in their occurrence, and the compact, hemispherical and 
vividly green Chrysoma laricifolia has become very frequent and con- 
spicuous, together with the white-tomentose Artemisia ludoviciana and 
the less conspicuous Eriogonum wrightii. 

On northerly slopes, just below 4,000 feet (1,220 m.), are encountered 
the first individuals of the rosaceous tree Vauquelinia californica and 
of Agave palmeri and Dasylirion wheeleri (sotol). Along the arroyos 
the most conspicuous forms are Erythrinaflabelliformis, the large leaves 
and brilliant scarlet flowers of which recall its tropical congeners, 
Manihot carthaginensis, with leaves of striking form, and Ingenhousia 
triloba (wild cotton), with tripartite leaves and large white flowers 
which strikingly resemble those of the cotton plant. 


The slopes lying between 4,000 and 4,500 feet (1,220 and 1,372 m.) 
constitute the upper edge of the desert. On these slopes all the char- 
acteristic species of the bajada are confined to southerly slopes, and 
all but half a dozen of them find their uppermost limits. On the Upper 
Desert slopes Vauquelinia becomes common, although confined to 
ledges of rock, and Juniperus pachyphlcea, Quercus oblongifolia, and 
Quercus arizonica occur for the first tune away from canons. On the 
northerly slopes, where these trees form the lowest attenuated edge of 
the Encinal region, Dasylirion occurs in abundance together with the 
lowest individuals of Nolina microcarpa (bear grass), Arctostaphylos 
pungens (manzanita), Agave schottii, and Yucca macrocarpa. 

The physiognomy of the Upper Desert slopes is made distinctive 
from that of the Lower Desert slopes not only by the entrance of these 
plants of striking form, and the exit of the desert species, but also by 
the abundance of perennial grasses, root-perennials, and small shrubs, 
which combine with the ephemeral plants, or their dead remains, to give 
a much more complete ground cover than is to be found in any part of 
the bajadas. The compact and extended patches of Agave schottii are an 
important element in this low cover, and so are the scattered plants of 
Boutelouarothrockii&nd the bunches of Boutelouacurtipendula, Bouteloua 
oligostachya, Muhlenbergia dumosa, Andropogon scoparium, Eragrostis 
lugens, and Heteropogon contortus (see plates 8 and 9). 

Commonest among the low shrubs and other perennials of the Upper 
Desert are: Chrysoma laricifolia, Acacia suffrutescens, Eriogonum 


wrightii, Dalea wislizeni, Calliandra eriophylla, Hymenopappus mexi- 
canus, Artemisia ludoviciana, Dalea albiflora, Asdepias linifolia, Fran- 
seria tenuifolia, Baccharis thesioides, Ayenia microphylla, and Anisolotus 
argensis. The commonest summer ephemerals are Eriogonum abertia- 
num and Eriocarpum gracile. 

The flood-plains and the banks and beds of the arroyos in the Upper 
Desert are, in general, more like the arroyos of the desert in their 
vegetation than like those of the Encinal region. The largest tribu- 
taries of Sabino Canon are somewhat less rich in aquatic and palustrine 
plants than the lower portion of the canon, and merely because of 
their steeper gradient and less regular flow. The forest which occupies 
the flood-plains of Sabino Basin is chiefly made up of Quercus emoryi, 
Quercus arizonica, Platanus wrightii, and Cupressus arizonica. The 
smaller flood-plains and arroyos of the Upper Desert have few of these 
trees but occasional individuals of Populus and open thickets of Bac- 
charis emoryi and Baccharis sarothroides, together with Franseria 
ambrosioides, Ingenhousia triloba, Erythrina flabelliformis, Croton texen- 
sis, Calliandra eriophylla, Brickellia californica, Gymnosperma corym- 
bosa, Amorpha californica, Bouvardia triphylla, and Stachys coccinea 
(see plate IOA). 


Some of the distinctive species of the Lower Encinal are found at 4,000 
feet and other forms, characteristic of the Upper Encinal, extend upward 
into the Forest Region as far as 8,000 to 8,600 feet. The Lower Encinal 
may be said to have its commencement, however, in the open orchard- 
like stands of Quercus oblongifolia and Quercus arizonica, which occupy 
northerly slopes at about 4,300 feet. At an approximate elevation of 
5,000 feet the open Encinal may be found on all slopes except the steepest 
southerly ones, while on steep northern slopes it already forms nearly 
closed stands. The dense stands of the Upper Encinal region begin to 
appear on southerly slopes at about 5,800 feet, and persist to the eleva- 
tion of 6,200 to 6,400 feet, where large pines begin to dominate the 
physiognomy and the true forest may be said to begin. 

The activities of growth and flowering which are so conspicuous on 
the Desert in the season of winter rains are very much reduced in the 
Lower Encinal and are practically absent in the Upper Encinal. 
Leaves are retained by the evergreen oaks and the sclerophyllous shrubs 
throughout the winter and are shed in April or May, simultaneously 
with the first growth of shoots and the renewal of foliage. Extremely 
few of the ephemerals which often carpet the Desert in January are 
to be found in the Encinal region. There is some activity on the part 
of root perennials in the Lower Encinal during the months of March 
and April, and flowers may be found on species of Sphceralcea, Calo- 
chortus, Verbena, Pentstemon, Eriogonum, and Lesquerella. Such activ- 
ity is commonly stopped by the advent of the arid fore-summer, and 


relatively little activity is observable in May and June, at least away 
from arroyos and springs. In the Upper Encinal the early months of 
spring partake of the rest which is then predominant in the Forest 
region. The deciduous trees begin foliation in late April or early May 
and some of the root perennials are not far behind them in their earliest 
activity. The duration of the arid fore-summer being slightly less in 
the Upper Encinal than in the Lower Encinal, and its intensity being 
also less, there is not so decided a break, among the herbaceous peren- 
nials between the first activity of spring and that of the humid mid- 
summer, as there is in the Lower Encinal and the Desert. 


The species which chiefly characterize the Lower Encinal at its 
desert edge have already been mentioned : Quercus oblongifolia, Quer- 
cus arizonica, Juniperus, Vauquelinia, Dasylirion, Nolina, Yucca macro- 
carpa, Arctostaphylos pungens, Agave palmeri, and Agave schottii. All 
of these are much more abundant at 5,000 feet than at 4,500 except 
Quercus oblongifolia, which is a tree of very narrow vertical range, rarely 
occurring above 5,200 feet and reaching its limit at 5,600 feet on steep 
south slopes. At 5,000 feet the Encinal has been augmented by the 
appearance of the common trees Quercus emoryi and Pinus cembroides 
(pinon) and by the shrubs Garrya wrightii, Mimosa biuncifera, Rhus 
trilobata, and Rhamnus crocea var. pilosa (see plates HA, 15, and 16). 

The only characteristic Desert species which persist throughout the 
Lower Encinal are: Carnegiea gigantea, a single young individual of 
which has been seen at 5,100 feet; Opuntia versicolor, which reaches 
5,500 feet; Fouquieria and Echinocactus wislizeni, which reach 5,600 
to 5,800 feet; and Mamillaria grahami, which ascends to 7,000 feet. 
So far as known, no other plants occurring on the Bajadas or in any 
of the other non-palustrine desert habitats range to elevations above 
6,000 feet.* There are at least a few species found in canons and near 
constant water which range from the elevation of the Desert to more 
than 6,000 feet. Several of the typical desert genera are represented 
at higher elevations by species which seldom range as low as the Upper 
Desert region. Two species of Opuntia (platopuntias) are found 
throughout the Encinal, growing in thin soil or on rocks, and reaching 
their highest occurrence solely on ridges or upper slopes. One of these 
species has been found on a sharp rocky ridge at 7,200 feet, which is 
the highest known occurrence of a platopuntia in the Santa Catalinas. 
Mamillaria arizonica ranges from the Upper Desert to nearly 7,000 
feet; Echinocereus polyacanthos ranges from about 5,000 feet to 7,800 
feet, which is the highest elevation at which any cactus has been found 
in these mountains. Agave palmeri and Yucca schottii are also fre- 

* This statement is made only with respect to the Santa Cataliuas. The influence of the 
character of the underlying rock and of the elevation of the surrounding desert each serves to 
determine indirectly the vertical limits of desert species. 


quently found up to elevations of 7,000 to 7,200 feet, and the latter 
reaches its uppermost limit at 7,800 feet (see p. 30). 

The ground cover of low perennial plants, grasses, succulents, and 
herbaceous species which has been mentioned as characterizing the Upper 
Desert is likewise to be found throughout the Lower Encinal, but does 
not form as close a carpet in the latter region as it does in the former (see 
plates 12, 15, and 16). Throughout the year this irregular carpet does 
much to lend character to the landscape, varying but little in its density 
with the alternating seasons of vegetative activity and of drought rest. 
The scattered polsters of Chrysoma laricifolia are green at all seasons, and 
there is no change in the gray-green foliage of Eriogonum wrightii nor 
in the white tomentose leaves of Artemisia ludovidana. The perennial 
grasses, many of the other perennial herbaceous plants, and all of the 
ephemerals are either in a resting state or dead throughout the arid 
fore-summer and the arid after-summer, but the only change which 
their rest or death registers in the landscape is a change of its color tone 
from a greenish gray to an almost uniform gray and grayish brown. 

All of the low shrubs and root perennials which were mentioned as 
characteristic of the Upper Desert are to be found occasionally or 
commonly in the Lower Encinal, excepting Franseria tenuifolia and 
Ayenia microphylla. The winter and spring ephemerals are extremely 
few at 4,500 to 5,000 feet, but there is much activity of growth and 
much blooming among the root perennials and low shrubs during the 
months of February and March, and sometimes during the early part 
of April. The humid mid-summer is a season of even greater activity 
on the part of the smaller elements of the vegetation. Relatively few of 
the conspicuous herbaceous plants which are active at 5,000 feet in the 
mid-summer have extended upward from thebajada, and the number of 
summer ephemeral species is very small as compared with the Desert. 

Among the small shrubs, root perennials and other herbaceous plants 
which are common during the humid mid-summer at 4,500 to 5,500 
feet, in the Lower Encinal, may be mentioned: 

Baccharis pteronoides. 
Baccharis thesioides. 
Bouteloua hirsuta. 
Bouteloua rothrockii. 
Castilleja integra. 
Cordylanthus wrightii. 
Croialaria lupulina. 
Dalea albiflora. 
Dalea wislizeni. 
Eriocarpum gracile. 

Eriogonum pharnaceoides. 
Eupliorbia heterophylla. 
Gilia multiflora. 
Gnaphalium wrightii. 
Hymenothrix wrightii. 
Linum neomexicanum. 
Muhlenbergia gracillima. 
Pappophorum wrightii. 
Pentslemon palmeri. 
Phaseolus wrightii. 

On the flood-plains and along the streamways of the Lower Encinal 
may be found a greater number of individuals of the evergreen oaks 
than on the surrounding slopes (see plate 10s), and also Juglans major, 
Platanus wrightii, and Populus sp., not to mention the restricted occur- 
rence of Cupressus arizonica. Shrubs occasionally found along the 


arroyos are Rhus trilobata, Baccharis emoryi, Erythrina flabelliformis, 
Bouvardia triphylla, Amorpha californica, Fendlera rupicola, Morus 
celtidifolia, and the climber Vitis arizonica. 

On the flood-plains of the larger canons in the Lower Encinal may 
be found the lowest examples of several species which become common 
in the forested region of the mountain, and these are indeed the lowest 
members of the forest flora, if aquatics are excepted. At 4,900 feet 
Ceanothus fendleri and Prunus virens are both to be found, growing 
not only on a flood-plain but in the shade of evergreen oaks. Berberis 
wilcoxii is found at 5,200 feet growing in shade near a constant spring, 
and Rhamnus ursina is infrequent at 5,000 feet near streamways. 

During the mid-summer there is an abundant stand of herbaceous 
perennials and ephemerals on the flood-plains of the Lower Encinal, 
giving them a much closer carpet of vegetation than is to be found on 
the adjacent slopes. Abundant and characteristic among them are: 

Artemisia sp. 
Asdepias tuberosa. 
Brickellia californica. 
Castilleja Integra. 
Chamcecrista lepladenia. 
Comandra pallida. 
Cordylanthus wrightii. 
Crolalaria lupulina. 
Diodia teres. 
Eriocarpum gracile. 

Euphorbia crenulata. 

Gymnolomia multiflora. 

Hymenolhrix wrightii. 

Malvastrum sp. 

Monarda pectinata. 

Solanum douglasii. 

Solidago sparsiflara var. subcinerea. 

Sporobolus confusus. 

Stachys coccinea. 

Stenophyllus capillaris. 


During the ascent from 5,000 to 6,000 feet the most notable change 
in the vegetation is the gradual increase in the density of the stand of 
evergreen trees and shrubs (see plates 18, 19, and 20), a change which 
forms the chief distinction of the Upper Encinal from the Lower 
Encinal. Quercus emoryi and Quercus arizonica are still the dominant 
trees, while Pinus cembroides and Juniperus pachyphloea are somewhat 
less common. Arctostaphylos pungens and Garrya wrightii are the most 
common of the larger shrubs and Mimosa biund/era of the smaller ones. 
Dasylirion wheeleri, Nolina microcarpa, and Agave palmeri remain 
abundant, at least on southern slopes, up to 6,000 feet and Agave 
schottii remains common up to its upper limit at that elevation. With 
the increasing abundance of the oaks, however, these semi-desert 
species as well as the cacti become infrequent and are confined to the 
summits of ridges and the crevices of rocks. 

On steep north slopes, between 5,300 and 6,000 feet, many almost 
pure stands of Pinus cembroides are to be found and also the lowest 
individuals of Quercus reticulata, here a low-branched tree of 20 feet 
in height. Pinus chihuahuana first appears at about 5,900 feet on 
south slopes, being the only one of the trees which is not found at 
much lower elevations on north slopes than on south ones indeed it 
is not common on north slopes at any elevation. 


The heaviest stands of the Upper Encinal constitute a relatively 
dense thicket in which the trees are from 18 to 30 feet hi height and 
so closely placed that it is very difficult for a mounted man to make 
his way among them. This is partly due to the fact that the oaks, the 
juniper, and the pinion all branch freely from a point near the ground, 
and partly to the size and hemispherical habit of Arctostaphylos, in 
which many of the stiff branches are placed in a nearly horizontal 
position near the ground. These dense stands of the Upper Encinal, 
between 5,600 and 6,200 feet, are made up of the same species that 
form the very open Lower Encinal in so far as concerns the trees, shrubs 
and larger perennials. There are, however, many root-perennial her- 
baceous plants in the Upper Encinal which are not to be found below 
5,500 feet, nearly all of which extend upward into the lower portions 
of the Forest region. 

Quercus emoryi is still a common tree at 5,600 feet, Quercus arizonica 
is replaced by the closely similar Quercus reticulata, Quercus hypoleuca 
makes its first appearance, and Juniperus pachyphlcea and Pinus cem- 
broides reach their maximum abundance between 5,500 and 6,500 feet. 
Dasylirion, Nolina, and Yucca are still conspicuous elements of the 
vegetation even in the most dense stands of oaks, but Agave schottii 
is no longer found and Agave palmeri, like the cacti, is found only on 
ridges and rocks. A common tree of the lower forest region, Arbutus 
arizonica, is first found in the Upper Encinal, where its isolated indi- 
viduals are conspicuously different from the oaks. The only trees of the 
mountain, excepting the desert species, the ranges of which lie wholly 
below the Upper Encinal, are Vauquelinia calif ornica and Quercus oblong- 
ifolia, while Quercus arizonica reaches its upper limit in this region. 

Phoradendron californicum, the mistletoe, which so commonly infests 
Prosopis and the other trees of the desert, is found throughout the 
Desert region of the mountains, while in the Encinal Phoradendron 
flavescens var. villosum is found on several hosts and Phoradendron 
juniperinum is extremely common on Juniperus, but does not extend 
with it to its highest occurrences. 

The vegetation of the Upper Encinal is extremely poor in shrubs 
of the type so common in the Upper Desert and still frequent in 
the Lower Encinal. In the open spots there may be found a few 
individuals of Artemisia ludoviciana, Parosela wislizeni, Anisolotus 
argensis, and other dwarf shrubs of the Lower Encinal, while in the 
shade of the heaviest stands of oaks are to be seen Pteris aquilina var. 
pubescens, Muhknbergia affinis, Polygala alba, Comandra pallida, 
Hymenopappus mexicanus, Cordylanthus wrightii, Chenopodium fre- 
montii, and other species of root-perennials. The vegetation of rocks 
and exposed ridges is still suggestive of the desert, both in its physiog- 
nomy and in its phyletic relationships. In the crevices of rocks, where 
the amount of soil is extremely scant and the supply of moisture must 


be very uncertain, Laphamia lemmoni is found, a small composite 
known only from the Encinal region and only from this habitat. In 
crevices more favorably situated with respect to moisture may be found 
Heuchera sanguined, and on north slopes at 6,000 feet, in moist crevices, 
may be found the lowest colonies of Saxifraga eriophora, a plant which 
occurs infrequently up to the summit of the mountain. 

Beds of Selaginella rupincola are still to be found at 6,000 feet and 
the several species of drought-resistant ferns, which are confined to the 
shade of rocks at lower elevations, are common on the floor of the heavy 
stands of Pinus cembroides, or grow among the boulders in more open 
situations. Among these the most common are : Cheilanthes fendleri, 
Notholcena sinuata, Notholcena ferruginea, and Gymnopteris hispida. None 
of these species extend upward into the Forest region (see plate 14). 

The drier flood-plains and arroyos of the Upper Encinal are charac- 
terized by the same oaks and evergreen conifers that occur on the 
adjacent slopes, while the moister streamways bear a number of decidu- 
ous trees and shrubs, notably Juglans rupestris and Platanus wrightii, 
extending upward from streamways at lower elevations, and Prunus 
virens, Rhamnus ursina, Rhus trilobata, Robinia neomexicana, and Rhus 
elegantula. Less frequent are Ceanothus fendleri, Berberis wilcoxii, and 
Bouvardia triphylla, and Vitis arizonica is still common. Pinus chihua- 
huana is not infrequent along the drier arroyos at the lower edge of its 
range, and Cupressus arizonica is found along the streams and on the 
lower slopes of Sabino and Bear Canons and some of their tributaries. 

The commonest herbaceous perennials of the flood-plains of the 
Upper Encinal are : 

Apocynum sp. 
Artemisia dracunculoides. 
Asclepias tuberosa. 
Carduus rothrockii. 
Euphorbia crenulata. 
Geranium ccespilosum. 
Gomphocarpus hypoleucus. 
Gymnolomia multiflora. 
Monarda pectinata nutt. 
Muhlenbergia sp. 

Oenothera sp. 

Pentstemon lorreyi. 

Picradenia biennis. 

Pteris aquilina var. pubescens. 

Rubus oligospermus. 

Senecio neomexicanus. 

Solidago sparsiflora var. subcinerea. 

Sporobolus confusus. 

Thaliclrum fendleri var. wrightii. 

Zauschneria calif ornica. 


One of the most striking changes encountered in the vegetational 
gradient of the Santa Catalinas is that from the closed and relatively 
low Encinal to the open forest of Pinus arizonica, with trees 50 to 60 
feet in height. This pine, the Arizona yellow pine, is closely related 
to Pinus ponderosa, the western yellow pine, and is the common tree 
of the forested altitudes of the mountain, extending upward on south- 
erly slopes to the summit of Mount Lemmon. The lowest stands of 
pine which possess sufficient density to be regarded as forest occur on 
northerly slopes at 5,800 to 6,000 feet, or on southerly slopes at 6,000 
to 6,400 feet, the limits depending in each particular locality upon the 


steepness of the slope and its soil characteristics, particularly with 
respect to the soil moisture supply (see plate 21). 

Much more gradual and inconspicuous is the transition from the 
Pine Forest to that in which Abies concolor (white fir) is the dominant 
tree. This type of Forest occupies the northern slopes of the highest 
summits and ridges of the range from 7,500 feet upward, but there are 
no elevations in the Santa Catalinas sufficiently great to bring the Fir 
Forest onto the south slopes. 

Throughout the Pine Forest there are trees, shrubs, and herbaceous 
plants which may be found in the Encinal, at least in its upper portion, 
but only in the lowest edge of the Pine Forest may plants be found 
which suggest the genera or vegetation types characteristic of the desert. 
A single cactus (Echinocereus polyacanthos) , a Yucca, and an Agave are 
the sole representatives of the succulent and semi-succulent forms of 
the lower elevations, and they are rare above 7,000 feet and absent above 
7,800 feet. 

The Pine Forest is not, however, without vegetational features which 
suggest the effects of a climate not far removed in character from that 
of the desert. The openness of the lowest stands of Pinus arizonica, 
the high mortality among the seedlings of the pine, the character of 
the foliage of the shrubs and herbaceous perennials, and the deep-seated 
root systems of the latter plants, all point to the existence of a pre- 
carious soil-moisture supply and to atmospheric conditions conducive 
to active transpiration. In the Fir Forest none of these features is 
observable, and the vegetation as a whole presents a much more 
mesophilous aspect. 

In the Forest region the winter is a season of almost absolute rest, 
save for the photosynthetic activity which is doubtless carried on by 
the conifers, and possibly by the evergreen oaks and shrubs. The 
deciduous trees and shrubs are leafless from early or mid October until 
April or May, and only a few herbaceous perennials are active during 
this period, such as the evergreen species of Pyrola and the early vernal 
plants, such as Frasera. The amount of activity on the part of the 
perennial herbaceous plants during the arid fore-summer is largely 
dependent on the amount of winter precipitation and the date of its 
termination. In the lower portion of the Pine Forest it often happens 
that almost all activity is in abeyance until the first rains of the humid 
mid-summer, while in the upper Pine Forest and in the Fir Forest it 
is always possible to find a majority of the common herbaceous plants 
in activity in May and June. There is a notable scarcity of annual 
plants above 6,000 feet, and the only ones that have been detected in 
the Forest region are: 

Androsace arizonica. 
Bidens sp. 
Ceraslium sericeum. 
Dalea polygonaides. 

Drymaria sperguloides. 
Drymeria tenella. 
Muhlenbergia sp. 



In the lowest stands of Pine Forest many of the dominant Encinal 
forms are still to be found, but in no case do the evergreen oaks fail 
to become more and more scattered in occurrence as the forest of pines 
becomes more dense. Quercus emoryi and Pinus cembroides are scarcely 
concerned in the overlapping of the Chaparral and Forest, as the former 
reaches its upper limit at 6,300 feet, while the latter becomes confined 
to the rocky non-forested or lightly forested ridges at about the same 
elevation, although it persists as a rare shrub to an elevation of 7,800 
feet. Arctostaphylos and Garrya are likewise of infrequent occurrence 
in stands of forest. The oaks which are characteristic of the closed 
forest are Quercus reticulata and Quercus hypoleuca. The former is com- 
monly a low-branching shrub which often forms thickets on the steep 
slopes of the highest peaks, where it extends upward to about 8,600 feet. 
The latter oak is a shrub near its lower and upper limits at 6,000 and 
8,500 feet respectively, but attains a height of 40 feet and a girth of 
3 to 4 feet between 6,500 and 7,500 feet. Juniperus pachyphloea is of 
occasional occurrence in the Forest up to 7,900 feet, and Arbutus 
arizonica (Arizona madrona), at first infrequent, becomes common at 
7,000 to 7,500 feet and reaches its upper limit at 7,800 to 8,000 feet. 

The composition of the Forest itself is extremely simple from its 
lower limit around 6,000 feet to 7,500 feet, and above that elevation 
is equally simple on southerly slopes up to the summit of Mount 
Lemmon. Pinus chihuahuana reaches its limit at about 6,700 feet and 
forms a very inconsiderable portion of the forest throughout the upper- 
most 500 feet of its vertical range. Pseudotsuga mucronata begins to 
occur on steep northerly slopes at 6,100 feet and Pinus slrobiformis 
(Mexican white pine) at 6,800 to 7,000 feet, but neither begins to 
affect the composition of the Forest in general until higher elevations 
are reached. At 6,000 feet the streamways and flood-plains are char- 
acterized by several deciduous trees in addition to the pines themselves. 
Platanus wrightii is near its upper limit at this elevation, Juglans 
rupestris, Prunus virens, and Acer interior are of frequent occurrence, 
while at 6,500 to 6,800 feet are found the lowest individuals of Quercus 
submollis and Alnus acuminata. 

Throughout the Pine Forest are to be found a large number of her- 
baceous perennials, a few of which occur in the Upper Encinal, the great 
majority of which, however, accompany the closed stands of pine, with 
additions and eliminations with increasing altitude. In addition to these 
plants is another large group which is confined in occurrence to the near 
proximity of streams and streamways; some of the members of the group 
being thus restricted in occurrence at lower altitudes, while they are of 
more general occurrence on heavily wooded slopes at higher elevations. 

In the clear park-like stretches of Pine Forest where no evergreen 
oaks happen to occur, the most conspicuous plants on the forest floor 



are the low thorny shrub Ceanothus fendleri, or varieties of it, and the 
bunch-grass Muhlenbergia virescens. The commonest of the herbaceous 
perennials are low, small plants such as Hedeoma hyssopifolia, Hous- 
tonia wrightii, Poa fendleriana, Calliandra reticulata, and Calliandra 
humilis, or else they are somewhat taller but relatively inconspicuous, 
as Pseudocymopterus montanus var. tenuifolius, Erigeron neomexicanus, 
Lithospermum multiflorum, Lotus puberulus, and others. In the dense 
shade of the Upper Encinal Pteris aquilina var. pubescens is common, 
and it again becomes common in the pines above 7,500 feet, but is infre- 
quent in the lower portion of the forest region. 

The Pine Forest gives the impression of possessing a much richer 
flora of herbaceous plants than is found in any other habitat of the 
mountain. This impression is due to the fact that a large number of 
species enter into the vegetation as very common components of it. 
As there are almost no rare or infrequent species to be found in the 
Pine Forest away from streams and springs, the total flora involved 
is not so great as might be supposed on first examination. Following 
is a list of the characteristic species found between 7,000 and 8,000 
feet, the relative abundance of which is indicated by asterisks: 

Characteristic Herbaceous Plants of the Pine Forest. 

*** Latliyrus graminifolius. 






Achillea lanulosa. 
Agastache pallidiflora. 
Anisololus puberulus. 
Antennaria marginata. 
Anthericum lorrcyi. 
Apocynum scopulorum. 
Bidens sp. 

Brickellia grandiflora. 
Calliandra reticulata. 
Carpochate bigelovii. 
Castilleja gloriosa. 
Cologania longifolia. 
Commelina dianthifolia. 
Desmodium arizonicum. 
Desmodium grahami. 
Dugaldia hoopesii. 
Erigeron macranthus. 
Erigeron neomexicanus. 
Eupatorium arizonicum. 
Eupatorium pauperculum. 
Eupatorium rothrockii. 
Geranium c<espitosum. 
Gilia thurberi. 
Gnaphalium decurrens. 
Gnaphalium wrightii. 
Gomphocarpus hypoleucus. 
Hedeoma hyssopifolia. 
Hieracium discolor. 
Houstonia wrightii. 
Hymenopappus mexicanus. 
Ipomoea muricata. 
Kceleria cristata. 


Lilhospermum multiflorum. 

Lupinus sp. 

Microstylis montana. 

Monarda pectinala. 

Monarda scabra. 

Muhlenbergia virescens. 

Muhlenbergia sp. 

Oenothera hookeri. 

Onosmodium thurberi. 

Panicum bulbosum. 

Pentstemon torreyi. 

Perityle coronopifolia. 

Phaseolus retusus. 

Pinaropappus foliosus. 

Poa fendleriana. 

Potentilla subviscosa. 

Pseudocymopterus montanus 

Pseudocymopterus montanus 

Pteris aquilina var. pubescens. 

Salvia arizonica. 

Senecio neomexicanus. 

Solidago bigelovii. 

Solidago marshallii. 

Stevia sp. 
* Tradescantia pinetorum. 
** Trifolium pinetorum. 
** Vicia americana. 
** Woodsia mexicana. 







The pure or nearly pure stands of Pinus arizonica which occur be- 
tween 8,000 and 9,000 feet are increasingly poor in the evergreen oak 
shrubs, which have disappeared at the latter altitude. The clumps of 
young Quercus submollis give the forest its only deciduous element at 
this altitude, and the low patches of Ceanoihus, so common at 8,000 
feet, give way at 9,000 feet to Symphoricarpos oreophilus and to the 
much less frequent Holodiscus dumosus. Very many of the commonest 
herbaceous perennials of the Pine Forests which lie between 7,000 and 
8,000 feet do not reach 9,000 feet, or are replaced hi the physiognomy 
of the forest by closely related species. On the summit and southern 
slopes of Mount Lemmon the commonest herbaceous plants are: 
Koeleria cristata, Dugaldia hoopesii, Erigeron neomexicanum, Pteris 
aquilina var. pubescens, Gnaphalium decurrens, Hieracium lemmoni, 
Senecio sp., Antennaria marginata, Silene greggii, and Helianthella 
arizonica. Throughout the higher Pine Forest Arceuthobium divari- 
catum and Arceuthobium robustum are common on the trunks and limbs 
of Pinus arizonica. 

At about 6,800 feet Alnus acuminata, Acer interior, and Quercus 
submollis become frequent along streams (see plates 30 and 31). The 
first two are confined to this habitat throughout their vertical range, 
while the oak, which is the only deciduous member of the genus in 
these mountains, is found even hi some of the driest situations above 
7,600 feet. Quercus submollis occurs characteristically either as single 
trees of considerable size, up to 40 feet in height and 4 feet in girth, 
or else as crowded circumscribed groups of young trees, which doubt- 
less owe their juxtaposition to the accidents of seed dispersal. Salix 
taxifolia is also a common streamside shrub above 6,800 feet, and 
in certain portions of the mountain Rosa fendleri is abundant in the 
proximity of streams. 

Herbaceous plants are to be found in increasing numbers at or near 
the banks of streams between 6,000 and 7,400 feet. Prominent among 
them are: Juncus arizonicus, Aquilegia chrysantha, Thalictrum fendleri 
var. wrightii, Scrophularia sp., Trifolium pinetorum, Fragaria ovalis, 
Potentilla thurberi, Hypericum formosum, Lobelia gruina, Agrimonia 
brittoniana var. occidentalis, Gaura suffulta, and Tagetes lemmoni. 


Between 7,000 and 7,400 feet is a rapid change in the character of 
the forest stands on northerly slopes, due to the increasing occurrence 
of Pseudotsuga mucronata and Pinus strobiformis, the lower limits of 
which have already been mentioned, and to the appearance of Abies 
concolor. These three species occur in mixed stands together with 
Pinus arizonica on northerly slopes up to about 7,500 feet, above which 
elevation the latter becomes a very infrequent tree on slopes facing 
directly north, although it still occurs in admixture with Pseudotsuga 


and Abies at 9,000 feet on eastern and western exposures. Above 7,500 
feet Pinus strobiformis ceases to be confined to the proximity of streams, 
and occurs in admixture with Pseudotsuga and Abies, but is not so 
common as they in the heaviest stands of this type of forest. On the 
north slopes of Mount Lemmon is a small colony of Abies arizonica. 
which is not known from any other locality on the mountain. 

Slopes of due south or southwestern exposure are held by Pinus 
arizonica up to the summit of Mount Lemmon at 9,150 feet, with a 
slight occurrence of Pseudotsuga and Pinus strobiformis above 8,000 
feet. The Pseudotsuga and Abies forest is found in fine development 
at 7,500 feet on steep north exposures, and reaches its maximum devel- 
opment in stature and size of the trees on the north slopes of Mount 
Lemmon at 8,500 to 9,100 feet (see plates 1 and 35). The altitude of 
the Santa Catalina Mountains is nowhere sufficient to admit of the 
occurrence of extended bodies of such forest, nor of their existence on 
southerly slopes. 

In the Fir Forest the last relicts of the Encinal have disappeared : 
Quercus hypoleuca, Quercus reticulata, and Juniperus pachyphlcea are 
nowhere to be found in association with Pseudotsuga and Abies, 
although they may grow very near them on opposed slopes. Arbutus 
arizonica, which is more common in the Pine Forest than in the Encinal, 
is likewise absent from the Fir Forest. The deciduous Quercus submollis 
and the widely distributed Populus tremuloides are the commonest 
of the subordinate trees, the latter often becoming dominant over 
areas of an acre or more in extent, where it ultimately gives way to 

The floor of the Fir Forest is much more heavily and continuously 
shaded than that of the densest stands of pine, a circumstance which 
is of great importance in determining the nature of the forest reproduc- 
tion and also in conditioning the character of the shrubby and herba- 
ceous vegetation. The dense shade, the heavy litter, and the high humus 
content of the soil tend to preserve its moisture throughout the arid 
fore-summer (see p. 61), so that the seedling trees and other plants of 
these situations are very far removed from the desiccating influences 
which are operative in the open Pine Forest. The Fir Forest near the 
summits of ridges is somewhat more open than that which is found on 
middle and lower slopes, and this difference is accompanied by a dis- 
similarity in the herbaceous flora of upper and lower slopes. On the 
latter may frequently be found communities of plants which differ little 
in their specific make-up from the communities which occupy flood- 
plains, although they are much less dense. 

The heaviest stands of Abies and Pseudotsuga, like most heavy conif- 
erous forests, are relatively poor in both shrubs and herbaceous plants. 
A few of the shrubs common to the water-courses are to be found also 
in the Fir Forest, such as Jamesia americana, Symphoricarpos oreo- 


philus, Ribes pinetorum, and Rubus neomexicanus. The trifoliate 
maple, Acer glabrum, also occurs locally on the north slopes of Mount 
Lemmon. The poverty in the stand of herbaceous species on the floor 
of the Fir Forest is contrasted with the large number of species to be 
found, which is probably not so great, however, as the number charac- 
teristic of the open Pine Forest. Most common are : Bromus richard- 
sonii, Cystopterisfragilis, Geranium ccespitosum, Frasera speciosa, Thalic- 
trum fendleri var. wrightii, Galium asperrimum, Smilacina sessilifolia, 
Osmorhiza nuda, Disporum trachycarpum, Viola canadensis var. rydbergii, 
Oxalis metcalfii, Fragaria ovalis, Trifolium rusbyi, and Draba helleriana. 
On Abies the parasitic Phoradendron bolleanum is not infrequent. 

The banks of constant and intermittent streams and the narrow 
flood-plains of the Fir Forest region form a series of habitats with 
closely similar physical conditions and with nearly identical vegetation. 
In them are to be found a greater abundance and variety of trees and 
shrubs than occur in topographically analogous habitats at lower ele- 
vations. Abies, Pscudotsuga, Pinus strobiformis, and even Pinus 
arizonica occur in this habitat. Its commonest woody plants, however, 
are those which do not occur in other situations, as Alnus acuminata, 
Acer interior, Acer brachypterum, Salix scouleriana, Salix exigua, Salix 
taxifolia, Sorbus dumosa, Cornus stolonifera var. riparia, Jamesia ameri- 
cana, Sambucus vestita, Symphoricarpos oreophilus, Rubus arizonicus, 
Ribes pinetorum, and Salix sp. 

In this same series of habitats, which are the most elevated of the 
moist habitats of the mountain, is the most dense stand of herbaceous 
vegetation that occurs on the Santa Catalinas. This vegetation is rich 
in species and varies in its make-up from place to place according to 
the amount of soil moisture present and according to the openness or 
shade. In the following list are given the characteristic plants of these 
situations. The two species of Mimulus are the only plants invariably 
confined to the immediate proximity of water. Such plants as Dugaldia 
andAgrimonia, on the other hand, are found only in the unshaded flood- 
plains. A comparison of this list with that just given for the floor of the 
Fir Forest will show that the latter habitat has few distinctive species. 

Characteristic Herbaceous Plants of Flood-Plains, Stream Banks, and Lower Slopes in the 

Fir Forest. 

Aconitum columbianum. 

Actcea viridiflora. 

Agrimonia brittoniana var. occiden- 


Agrostis scabra var. subrepens, 
Aralia humttis. 
Aspidium filix-mas. 
Bromus richardsonii. 
Carex sp. 
Carex sp. 

Cerastium sericeum. 
Delphinium scopulorum. 

Disporum trachycarpum. 
Draba helleriana. 
Dugaldia hoopesii. 
Epilobium novomexicanum. 
Equisetum robustum. 
Frasera speciosa. 
Galium asperrimum. 
Gentiana microcalyx. 
Geranium ccespitosum. 
Glyceria nervata. 
Gyrostachys sp. 
Heracleum lanatum. 






Hurmdus lupidus var. neomexicanus. 

Hypericum formosum. 

Juncus brunnescens. 

Juncus interior. 

Limnorchis sparsiflora. 

Listera sp. 

Microstylis porphyrea. 

Mimulus cardinalis. 

Mimulus guttatus. 

Osmorhiza nuda. 

Oxalis metcalfii. 

Oxalis wrightii. 

* Polygonum douglasii. 
Pyrola chlorantha. 
Pyrola secunda. 
Rubus arizonicus. 
Rudbeckia laciniaia. 
Scrophularia sp. 
Smilacina amplexicaulis. 
Smilacina sesstiifolia. 
Solanum fendleri. 
Thaliclrum fendleri var. wrightii. 
Viola canadensis var. rydbergii. 

* Viola nephrophylla. 


The wide range of physical conditions embraced within the area of 
the Santa Catalina Mountains gives them a relatively large flora, which 
has been estimated by Professor J. J. Thornber to be about 1,500 
species. Although the exploitation of this flora is not completed it is 
nevertheless sufficiently well advanced to show that elements are pres- 
ent which are common to each of many diverse regions lying north, 
south, east, and west. 

The desert at the foot of the mountains stands in unbroken connec- 
tion with the deserts of Sonora and Sinaloa. The Encinal and Forest 
regions, on the other hand, are isolated from other areas possessing the 
same physical conditions. Areas of Encinal are numerous and near, 
both on the low desert mountains and on the elevated plains of southern 
Arizona ; while bodies of forest are to be found only at greater distances 
and more remotely separated from each other. The floristic history 
of the Encinal and Forest regions of the Santa Catalinas is quite as 
intimately bound up with the controlling influences of climatic con- 
ditions as is the present limitation of the vegetation. In fact the floras 
of the two isolated regions are a resultant between the physical con- 
ditions which they have presented in the remote and recent past and 
the operation of natural agencies of dispersal. 


It would not be within the scope of this paper to enter upon a detailed 
discussion of the floristic relationships of the isolated mountain areas 
of Encinal and Forest in southern Arizona, even if all the evidence 
bearing on such a discussion were now in hand. It will be instructive, 
however, to point out very briefly some of the principal floristic rela- 
tionships of the Santa Catalinas in order to demonstrate the extensive 
and diversified area over which members of its flora may be found. 


The flora which occupies the bajadas of the Santa Cruz valley and 
the lower slopes of the Santa Catalina Mountains derives many species 
from each of two Mexican desert regions, the one lying at low elevations 


between the Sierra Madre Occidental and the Gulf of California, in the 
States of Sonora and Sinaloa, the other lying at higher elevations in 
the States of Chihuahua and Zacatecas. There are strong diversities 
of flora between these two Mexican deserts, although they do not fail 
to have many species in common. The Sierra Madre forms an effective 
barrier between them in Mexico, but north of the International Bound- 
ary the continental divide is formed by scattered mountain ranges and 
broad valleys rather than by a continuous elevated range, and these 
valleys, lying between 4,000 and 5,000 feet, have permitted the inter- 
mingling of species from the two desert floras, at the same time that 
they have constituted a barrier to many species presumably unable to 
withstand the winter temperature conditions of the elevated valleys. 
The deserts which border the lower course of the Colorado River in 
Arizona and California, the Mojave Desert, and other desert regions 
in southern California and Nevada lying below 4,000 feet, possess a 
very small number of distinctive species as contrasted with the two 
Mexican desert regions, and have contributed almost no species to the 
flora of the Santa Cruz valley, although many species of wide Mexican 
occurrence are represented in both localities. The deserts of the Great 
Basin have likewise contributed no distinctive elements to the flora 
of the Santa Cruz Valley and the Desert region of the Santa Catalinas. 

Among the many species characteristic of the Arizona-Sonora Desert 
which do not cross the continental divide are: Carnegiea gigantea, 
Parkinsonia microphylla, Encelia farinosa, Olneya tesota, Hyptis emoryi, 
Franseria deltoidea, Simmondsia californica, Jatropha cardiophylla, and 
Crossosoma bigelovii. Among the desert species which are common to 
the Arizona-Sonora region and to the Texas-Chihuahua desert are: 
Fouquieria splendens, Kceberlinia spinosa, Chilopsis saligna, Momisia 
pallida, Coldenia canescens, Opuntia leptocaulis, Ephedra trifurca, 
Hilaria mutica, and Baileya multiradiata. 

It would be possible to place perhaps 90 per cent of the desert flora 
of southern Arizona in one or the other of the categories just mentioned. 
There are a few local and endemic species, but very few species exhibit 
ranges extending chiefly to the west, north, or east. Among the two 
Mexican elements many species range far south of Mexico, as witness 
the following, which are found in the deserts of Chile: Calandrinia 
menziesii, Bowlesia lobata, Daucus pusillus, Parietaria debilis, and 
Hydrocotyle ranunculoides. A large number of the genera found in the 
Desert flora also possess representatives in the deserts of Argentine 
and Chile, as: Covilka, Franseria, Encelia, Actinella, Krameria, Gutier- 
rezia, Viguiera, Chorizanthe, Coldenia, Perezia, Menodora, Nama, 
Amsinckia and many others. Other genera found in the Santa Cruz 
Valley have many representatives in tropical South America or in the 
West Indies, as Hyptis, Dodoncea, Erythrina, and Gymnolomia, or have 
a world-wide representation, as Tragia and Stemodia. 


Enough has been said to show that both the specific and generic 
relationships of the Desert flora are with the desert regions of Mexico, 
the deserts of Argentine and Chile, and even with the moist tropical 
regions of South America. The plants which dominate the Desert 
landscape in southern Arizona are members of genera, or even of 
species, which characterize a much greater area to the south than to 
the north, and they are in the main members of genera which reach 
their maximum development in number of species and in abundance 
of individuals in similar desert regions. The plants of the Desert which 
are of tropical relationship are usually the sole and northernmost repre- 
sentatives of families or genera which are much more richly represented, 
both in types and in individuals, in the tropical zone. These plants 
are often so infrequent and inconspicuous as scarcely to interest the 
student of vegetation, except for the fact that their seasonal behavior 
and habitat relations are such as to give them the most moist conditions 
which the Desert affords. Among them may be mentioned : Passiflora, 
Stemodia, Maurandia, and Rivina. 

The few members of genera of northern dominance, such as Populus 
and Salix, or Anemone and Delphinium, are either to be sought in the 
vicinity of streams and ponds, as is the case with the former two, or 
are to be found in activity only in the late winter and early spring, as 
is true of the latter two. The still fewer species of transcontinental 
range are almost solely palustrine plants, as Cephalanthus occidentalis, 
Scirpus americanus, Cyperus diandrus, and others, and are to be found 
only in palustrine situations in Arizona. 


The type of vegetation which is designated as Encinal in this paper 
is found throughout southern Arizona and New Mexico at elevations 
of 5,000 to 7,000 feet. It is pre-eminently a community of evergreen 
oaks and nut pines, with many sclerophyllous shrubs. With many 
floristic modifications this type of Encinal extends into western Texas, 
Colorado, and inner California, usually as a belt connecting the treeless 
plains or desert with the forested mountain tops. Encinal similar to 
that of southern Arizona is found throughout the mountainous portions 
of Sonora, Chihuahua, Sinaloa, and Zacatecas, and with many modi- 
fications it extends still further south. 

The dominant species of the Encinal of the Santa Catalinas range 
far to the south along both sides of the Sierra Madre, whereas but few 
of them range further north than the southern edge of the Mogollon 
Plateau in central Arizona, and some of them not even so far as that. 
The 14 commonest woody or semi-succulent perennials in the Encinal 
of the Santa Catalinas are all plants of extended Sonoran and Chi- 
huahuan distribution; all of them occur in southern New Mexico and 
eight of them in western Texas. Only one of the plants reaches Call- 


fornia and only one of them has been reported from Colorado. These 
plants are: 

Quercus oblongifolia. 
Quercus arizonica. 
Quercus emoryi, Tex. 
Vauquelinia californica. 
Juniperus pachyphlcea, Tex. 

Arctoslaphylos pungens, Cal. 
Garrya wrightii, Tex. 
Dasylirion wheeleri, Tex. 
Agave palmeri. 
Nolina microcarpa. 

Pinus cembraides, Tex. 
Mimosa biuncifera, Tex. 
Chrysoma laricifolia, Tex. 
Eriogonum imghtii, Col., Tex. 

The Encinal likewise comprises a number of plants which reach their 
maximum occurrence on the Great Plains or else possess areas of dis- 
tribution which are chiefly to the northeast of Arizona. Among these 
are Bouteloua obligostachya, Bouteloua hirsuta, Bouteloua curtipendula, 
Polygala alba, Artemisia ludoviciana, Artemisia dracunculoides, and 
Stephanomeria rundnata. 

The elements which are common to the flora of California are few, 
as is true of the Desert, and are almost solely comprised in the follow- 
ing: Zauschneria californica, Amorpha californica, Bouvardia triphylla, 
and Brickellia californica, not to add Arctostaphylos pungens, which has 
its maximum extension southward into Mexico. The Encinal contains 
a number of forms which have been but recently segregated from well- 
known species, among them Rhus racemulosa, Rhamnus ursina, and 
Prunus virens. So little is known of the ranges of these species that 
it is impossible to state in how far they may represent contributions 
from distant floras or to what extent they represent forms that have 
been differentiated in the Arizona-Sonora region. 

The only northern element in the Encinal flora seems to be that 
which has been mentioned as occurring also in the Great Plains, while 
the mountainous regions of Colorado and Utah have contributed even 
fewer species than has the Calif ornian region. 


The Forest region of the Santa Catalinas possesses strong floristic 
affinities both with the Mexican cordillera and with the Rocky Moun- 
tains of Colorado and their southern extension in New Mexico. The 
majority of the plants which take a conspicuous place in the vegetation 
of the Forest are members of northern genera. Many of these members 
are identical with Rocky Mountain species, while many others have 
their chief range in the mountains of northern Mexico. There are also 
representatives of a few genera which are distinctively Mexican, a few 
species of northwestern relationship, and a few apparently of restricted 
range in the desert mountains of Arizona and New Mexico. 

As examples of the large Rocky Mountain contingent in the Forest 
flora may be mentioned: 

Abies concolor. 
Pseudotsuga mucronata. 
Disporum trachycarpum. 
Salix scauleriana. 
Populus tremuloides. 

Acer globrum. 
Jamesia americana. 
Symphoricarpos oreophilus. 
Frasera speciosa. 
Dugaldia hoopesii. 

Erigeran macranthus. 
Heuchera rubescens. 
BrickeUia grandiflora. 
Gilia thurberi. 
Achillea lanulosa. 


Some of the members of this group are of wide distribution in the 
north, as Populus iremuloides, Achillea lanulosa, and Disporum trachy- 
carpum. In the northern mountain contingent are also a few species 
which range eastward to the Atlantic coast, a few which are found at 
least as far south as Maryland (Herackum lanatum, Rudbeckia lacini- 
tata, Apocynum androscemifolium, Vicia americana, and Asplenium tri- 
chomaries), not to mention Achillea lanulosa, which scarcely deserves 
separation from the cosmopolitan Achillea millefolium. 

The relationship with northern California and the northwestern 
states is weakly expressed in the occurrence of Salix lasiolepis and 
Prunus emarginata. Genera characteristic of the sub-arctic regions are 
sparingly represented at higher elevations by species of Primula, Saxi- 
fraga, and Androsace. 

Some of the most conspicuous components of the vegetation belong 
to northern genera, but to species which are characteristic of the Mexi- 
can cordillera, as Pinus arizonica, Pinus strobiformis, Alnus acuminata, 
Salix bonplandiana, Quercus hypoleuca, and Quercus reticulata. Such 
genera of herbaceous plants as Solidago, Eupatorium, Erigeron, Pentste- 
mon, Mimulus, Potentilla, Gilia, and Gentiana all of which are richly 
developed in the Rocky Mountains are chiefly represented in the 
Santa Catalinas by species not found in Colorado nor Wyoming. The 
extent to which these species are characteristic of the Arizona-New 
Mexico region or are components of the flora of the higher Mexican 
mountains is only partially known. 

The relationship of the Forest flora to that of the extended mountain 
regions to the south is still further strengthened by the occurrence of 
members of genera which are not found in the Rocky Mountains of 
Colorado and northern New Mexico, as Arbutus, Calliandra, Micro- 
stylis, Drymaria, Cologania, Stevia, and Tagetes. 

To summarize for the mountain as a whole, it may be said that the 
floristic relationships of the Desert and Encinal regions are almost 
wholly with the Mexican deserts and foothills to the south, while those 
of the Forest region are divided between the Mexican Cordillera and 
the Rocky Mountains. The Mexican group is the more conspicuous 
in the make-up of the vegetation, while the Rocky Mountain contin- 
gent is apparently preponderant in number of species. 

It will be impossible to summarize the floristic relationships of the Santa 
Catalinas in a thorough manner until very much more is known of their 
own flora and also of the floras of the many adjacent mountain ranges and 
desert valleys, both in the United States and in Mexico. For the explana- 
tion of these relationships a closer acquaintance is needed with the 
actual mechanisms of transport which are effective in the dispersal of the 
Beeds of desert and mountain plants. A fuller knowledge is also required 
of the fluctuations of climate within recent geological time, and of the 
consequent downward and up ward movements of the Encinal and Forest 
belts of all the southwestern mountains. Such movements would alter- 



nately establish and break the connections between the vegetations of 
the various mountain ranges and elevated plains, thereby permitting the 
dispersal and subsequent isolation of species which might find no means 
of movement across the desert valleys under existing conditions. 


The lack of a single taxonomic work covering the entire flora of the 
Santa Catalina Mountains makes it desirable to bring together here a 
list of the plant names which are used throughout this paper, together 
with some of the commoner synonyms. The list comprises only those 
plants which are common and characteristic components of the vege- 
tation of some particular .region or habitat of the mountain. The writer 
wishes to express here his very great indebtedness to Professor J. J. 
Thornber, of the University of Arizona, for determining numerous sets 
of plants from the Santa Catalinas and for verifying the following list. 


Aspidium filix-mas (L.) Sw. 

= Dryopteris filix-mas (L.) Schott. 

Asplenium trichomanes L. 

Cheilanthes fendleri Hook. 

Cheilanthes lindheimeri Hook. 

Cheilanthes wrightii Hook. 

Cystopteris fragilis (L.) Bernh. 
= Filix fraoilis (L.) Underw. 

Gymnopteris hispida (Mett.) Underw. 
= Gymnogramme hispida Mett. 

Notholcena ferruginea (Desv.) Hook. 

Notholcena hookeri D. C. Eaton. 

Notholcena sinuata (Sw.) Kaulf. 

Pettaea wrightiana Hook. 

Pteris aguilina var. pubescens Underw. 

Woodsia mexicana Fee. 

Eguisetum robustum A. Br. 
SKT.AC n.NKi.i.ACE.E : 

Selaginella rupincola Underw. 

Selaginella sp. 

Abies arizonica Merriam. 

Abies concolor Lindl. & Gord. 

Cupressus arizonica Greene. 

Juniperus pachyphloea Torr. 

Pinus arizonica Engelm. 

Finns cembroides Zucc. 

Pinus chihuahuana Engelm. 

Pinus strobiformis Engelm. 

Pseudotsuga mucronata (Raf.) Sudw. 

= Pseudotsuga taxifolia (Lam.) Britton. 

Ephedra trifurca Torr. 

Agrostis scabra var. subrepens Hitchck. 

Andropogon saccharoides Sw. 

= Amphilophis saccharoides (Sw.) Nash. 

Andropogon scoparium Michx. 

Aristida americana var. bromides (H. B. K.) 
Scribn. & Merr. 

Aristida dicergens Vasey. 

Aristida scheidiana Trin. & Rupr. 

Bouteloua aristidoides (Kunth) Griseb. 

Bouteloua curtipendula (Michx.) Torr. 

GRAM INE^E Continued ' 

Bouteloua hirsuta Lag. 

Bouteloua oKgostachya (Nutt.) Torr. 

Bouteloua polystachya (Benth.) Torr. 

Bouteloua rothrockii Vasey. 

Bromus richardsonii Link. 

Diplachne dubia (Nees) Benth. 

Eragrostis lugens Nees. 

Eragrostis neomexicana Vasey. 

Eragrostis pilosa (L.) Beauv. 

Heteropogon contortvs (L.) Beauv. 

Hilaria cenchroides H. K. B. 

Hilaria mutica (Buckl.) Benth. 

K(jcleria cristata (L.) Pers. 

Leptochloa mucronata (Miehx.) Kunth. 

Muhlenbergia affinis Trin. 

Muhlenbergia dumosa Scribn. 

Muhlenbergia disiichophylla (Presl) Munro. 

Muhlenbergia gracillima Torr. 

Muhlenbergia porteri Scribn. 

Muhlenbergia vaseyana Scribn. 

Muhlenbergia virescens (H. B. K.) Trin. 

Muhlenbergia sp. 

Panicularia nenata (Willd.) Kze. 

Panicum bulbosum H. B. K. 

Panicum bulbosum var. minor Vasey. 

Panicum hallii Vasey. 

Panicum hirticaulum Presl. 

Papphorum wrightii Wats. 

Poa fendleriana (Steud.) Vasey. 

Sitanion elymoides Raf. 

Sporobolus confusus (Fourn.) Vasey. 

Stipa neomexicana (Thurb.) Scribn. 

Carex sp. 

Carex sp. 

Cyperus fendlerianus Boeckl. 

Cyperus inflexus Muhl. 

Cyperus speciosus Vahl. 

Eleocharis montana (H. B. K.) R. & S. 

Fimbristylis sp. 

Hemicarpha micrantha (Vahl) Britt. 

Stenophyttus capillaris (L.) Britt. 
('< >M M KI,I x A <]:.]: : 

Commelina dianthifolia DC. 

Tradescantia scopulorum Rose. 

Tradescantia pinetorum Greene. 



List of Characteristic 


Juncus arizonicus Wieg. 
Juncus brunnescens Rydb. 
Juncus bufonius L. 
Juncus interior Wieg. 


Anthericum torreyi Baker. 
Brodiiea capitata var. pauciflora Wats. 
Calochortus nuttallii T. & G. 
Dasyllrion wheeleri Wats. 
Disporum trachycarpum (Wats.) B. & H. 
Nolina microcarpa Wats. 
Smilacina amplexicaulis Nutt. 

= Faffnero amplexicaulis (Nutt.) Morong. 
Smilacina sessilifolia Nutt. 
Yucca macrocarpa (Torr.) Coville. 
Yucca schottii Engelm. 

Agave palmeri Engelm. 
Agate parryi Engelm. 
Agate schottii Engelm. 

Sisyrinchium arizonicum Rothr. 

= Oreolirion arizonicum (Rothr.) Bicknell. 
Gyrostachys sp. 

Limnorchis sparsiflora (Wats.) Rydb. 
Listera sp. 
Microstylis corymbosa Wats. 

= Achroanthes corymbosa (Wats.) Greene. 
Microstylia montana Rothr. 

= Achroanthes montana (Rothr.) Greene. 
Microstylis porphyrea Ridley. 

= Achroanthes porphyrea (Ridley) Greene. 

Populus angustifolia James. 
Populus tremuloides Michx. 
Populus sp. 

near to Populus wislizeni (Wats.) Sarg. 
Salix bonplandiana H. B. K. 
Salix exigua Nutt. 
Salix scouleriana Barr. 
Salix taxifolia H. B. K. 
Salix wrightii Anders. 
Salix sp. 


Jutland major (Torr.) Hell. 

Alnus acuminata H. B. K. 

= Alnus oblongifolia Torr. 

Quercus arizonica Engelm. 

Quercus emoryi Torr. 

Quercus hypoleuca Engelm. 

Quercus oblongifolia Torr. 

Quercus reticulata Humb. & Bonpl. 

Quercus submollis Rydb. 

Species Continued. 


Comandra pallida A. DC. 

Arceuthobium dimricatum Engelm. 

= Razoumofskya dimricata (Engelm.) Kze. 

Arceuthobium robustum Engelm. 

= Razoumofskya robusta (Engelm.) Kze. 

Phoradendron bolleanum Eichl. 

Phoradendron californicum Nutt. 

Phoradendron flavescens var. villosum 

Phoradendron juniperinum Engelm. 

Chorizanthe brevicornu Torr. 

Eriogonum abertianum Torr. 

Eriogonum pharnaceoides Torr. 

Eriogonum wrightii Torr. 

Polygonum douglasii Greene. 

Rumex hymenosepalus Torr. 

Chenopodium fremontii Wats. 


Amaranthus palmeri Wats. 

Cladothrix lanuginosa Nutt. 

Frcelichia floridana (Nutt.) Moq. 

Gomphrena ccespilosa Torr. 

Gomphrena nitida Rothr. 

Allionia gracillima Standley. 

Boerhaavia pterocarpa Wats. 

Boerhaavia watsoni Standley. 

Wedelia incarnata (L.) Kze. 

Calandrinia memiesii (Hook.) T. & G. 

Calyptridium monandrum Nutt. 

Montia perfoliata (Donn.) Howell. 

Talinum patens var. sarmentosum (Engelm.) 


Arenaria confusa Rydb. 

Cerastium sericeum Wats. 

Cerastium texanum Britt. 

Drymaria sperguloides Gray. 

Drymaria teneUa Gray. 

Silene laciniata var. greggii (Gray) Wats. 

Aconitum columbianum Nutt. 

Acted mridiflora Greene. 

Aquilegia chrysantha Gray. 

Clematis ligusticifolia Nutt. 

Myosurus cupulatus Wats. 

Thalictrum fendleri var. wrightii Gray. 


Berberis wilcoxii Kearney. 

Eschscholtzia mexicana Greene. 

Platystemon californicus Benth. 

Momisia pallida (Torr.) Planch. 

= Celtis pallida Torr. 
Celtis reticulata Torr. 

Cdtia occidentalis var. reticulata (Torr.) 


Humulus lupulu* var. neomezicanus Nels. & 

Morus celtidifolia H. B. K. 

Draba helleriana Greene. 
Draba spectabiliti Greene. 
Lepidium lasiocarpum Nutt. 
Lesquerella gordoni (Gray) Wats. 
Thelypodium linearifolium Gray. 

Sedum stettiforme Wats. 
Tilloea erecta Hook. & Arn. 



List of Characteristic Species Continued. 


Fendlera rupicola Engelm. & Gray. 
Heuchera rubescens Torr. 
Heuchera sanguinea Engelm. 
Jamesia americana T. & G. 

= Edwinia americana (T. & G.) Hell. 
Ribes pinetorum Greene. 
Saxifraga eriophora Wats. 

=Micranthcs eriophora (Wats.) Small. 

Platanus wrightii Wats. 

Crossosoma bigelovii Wats. 

Agrimonia brittoniana var. occidentalis Bick- 


Cowania stansburiana Torr. 
Fragaria ovalis (Lehm.) Rydb. 
Hulodiscus dumosus (Nutt.) Hell. 
Potenlilla thurberi Gray. 
Potentilla subviscosa Greene. 
Primus virens (Woot. & Stand.) 

= Padua virens Woot. & Stand. 
Rosa fendleri Crepin. 
Rubus arizonicus Greene. 
Rubus neomexicanus Gray. 
Rubus oligosperma Thornb. 
Sorbus dumosa Greene. 
Vauguelinia californica (Torr.) Sarg. 

Acacia greggii Gray. 

Acacia paucispina Wooton. 

Acacia suffrutescens Rose. 

Amorpha californica Nutt. 

Anisolotus argensis Coville. 

Anisolotus puberulue (Benth.) Woot. & Stand. 

= Hosackia puberula Benth. 
Anisolotus (rispermus(Greene) Woot.A Stand. 

= Lotus trispermus Greene. 
Cassia covesii Gray. 
Cassia leptadenia Greenm. 

= Chamcecrista leptadenia (Greenm.) 


Cassia leptocarpa Benth. 
Calliandra eriophylla Benth. 
Calliandra reticulata Gray. 
Calliandra humilis Benth. 
Cologania longifolia Gray. 
Crotolaria lupulina Raf. 
Dalea albiflora Gray. 

=Paroaela albiflora (Gray) Vail. 
Dalea parryi T. & G. 

= Parosela parryi (T. & G.) Hell. 
Dalea polygonoides Gray. 

= Parosela polygonoides (Gray) Hell. 
Dalea wislizeni Gray. 

= Parosela wislizeni (Gray) Vail. 
Desmodium ariaonicum Wats. 

= Meibomia arizonica (Wats.) Vail. 
Desmodium bigelovii Gray. 

= Meibomia bigelovii (Gray) Kze. 
Desmodium grahami Gray. 

= Meibomia grahami (Gray) Kze. 
Desmodium psilocarpum Gray. 

= Meibomia psilocarpa (Gray) Kze. 
Erythrina flabelliformis Kearney. 
Eysenhardtia orthocarpa (Gray) Wats. 
Indigofera iphoerocarpa Gray. 

LEGUMINOS/E Continued: 

Lathyrus graminifolius (Wats.) White. 

Lupinus sp. 

near to Lupinus palmeri Wats. 

Mimosa biuncifera Benth. 

Nissolia schottii (Torr.) Gray. 

Parkinsonia microphylla Torr. 

Parkinsonia torreyana Wats. 

Cercidium torreyanum (Wats.) Sarg. 

Phaseolus retusus Benth. 

Phaseolus terightii Gray. 

Prosopis velutina Wooton. 

Robinia neomexicana Gray. 

Trifolium pinetorum Greene. 

Vicia americana Muhl. 

Vicia melilotoides Woot. & Stand. 

Geranium casspitosum James. 

Oxalis albicans H. B. K. 

^lonoxalis albicans (H. B. K.) Small. 

Oxalis metcalfii (Small). 

= Ionoxalis metcalfii Small. 

Linum lewisii Pursh. 

Linum neomexicanum Greene. 

Covillea tridentata (DC.) Vail. 

= Larrea tridentata (DC.) Coville. 

Ptelea cognata Greene. 

Janusia gracilis Gray. 


Krameria glandulosa Rose & Painter. 

Polygala alba Nutt. 

Croton texensis (Klotsch) Muell. Arg. 

Euphorbia crenulata Engelm. 

Euphorbia florida Engelm. 

Euphorbia heterophylla L. 

Euphorbia melanadenia Torr. 

Euphorbia pediculifera Engelm. 

Jatropha angustidens Muell. Arg. 

Jatropha cardiophylla (Torr.) Muel. Arg. 

Manihot carthaginensis Muel. Arg. 

Callitriche sp. 

Simmondsia californica Nutt. 

Rhus aromatica var. mottis (Gray) Ashe. 

Rhus elegantula Greene. 

Rhus rydbergii Small. 

= Toxicodendron rydbergii (Small) Greene. 

Rhus trilobata Nutt. 

A ( ' K li A CE.T3 I 

Acer brachypterum Woot. & Stand. 

Acer glabrum Torr. 

Acer interior Britt. 

Dodoncea viscosa var. angustifolia. (L. f.) 

Sapindus drummondii Hook. & Arn. 

Ceanothus fendleri Gray. 

Ceanothus fendleri var. venosus Trel. 

Rhamnus crocea var. pilosa Trel. 

Rhamnus ursina Greene. 



List of Characteristic 

RHAMNACE.E Continued : 
Zizyphus lycioides var. canescens Gray. 

= Condalia lycioides (Gray) Webcrbaur. 
VITACE>E: . . 

Parthenocissus dumetorum var. lacmuua 

= Parthenocissus quinquefolia var. tacimata 


Vitis arizonica Engelm. 

Abutilon incanum (Link) Sweet. 
Ingenhousia triloba DC. 

= Thurberia thespesioides Gray. 
Malmstrum sp. 
Sphaeralcea pedata Torr. 

Aj/enia microphylla Gray. 

Hypericum formosum H. B. K. 

Fouguieria splendens Engelm. 


Viola canadensis var. rydbergii (Greene) 


Viola nephrophylla Greene. 

Mentzelia albicaulis Dougl. 

Carnegiea gigantea (Engelm.) Britt. & Rose. 

= Cereus giganteus Engelm. 
Echinocactus wislizeni Engelm. 
Echinocereus fendleri (Engelm.) Rumpl. 
Echinocereus polyacanthus Engelm. 
Mamillaria arizonica Engelm. 
Mamillaria grahami Engelm. 
Opuntia bigelovii Engelm. & Bigel. 
Opuntia blakeana Rose. 
Opuntia engelmanni Salm Dyck. 
Opuntia fulgida Engelm. 
Opuntia Icevis Coult. 
Opuntia leptocaulis DC. 
Opuntia mamillata Schott. 
Opuntia santa-rita (Griff. & Hare) Rose. 
Opuntia spinosior (Engelm. & Bigel.) 


Opuntia toumeyi Rose. 
Opuntia nersicolor Engelm. 
Opuntia sp. 
Opuntia sp. 

Epilobium novomexicanum Hausk. 
Gaura suffulta Engelm. 
Isnardia palustris L. 

=Ludwigia palustris (L.) Ell. 
(Enothera hookeri T. & G. 

-Onagra hookeri (T. & G.) Small. 
(Enothera mexicana Spach. 
Zauschneria californica Presl. 

Aralia humilii Cav. 

I M 1 : 1 I ! . I M H 1 I 

Daucus pusillus Michx. 
Heracleum lanatum Michx. 
Hydrocolyle ranunculoides L. f . 
Osmorhiza nuda Torr. 

= Washingtonia obtusa C. & R. 
Pteudocymopterus nwntanus var. purpureus 
C. 4R. 

Species Continued. 

Pseudocymopterus montanus var. tenuifolius 

(Gray) C. & R. 

Cornus stolonifera var. riparia (Rydb.) 

Garrya wrighlii Torr. 

Arbutus arizonica (Gray) Sarg. 

Arctostaphylos pringlei ParrV. 

Arctostaphylos pungens H. B. K. 

Hypopitys sanguinea Hell. 

Pterospora andromedea Nutt. 

Pyrola chlorantha Sw. 

Pyrola secunda L. 

Androsace diffusa Small. 

Androsace arizonica Gray. 

Primula rusbyi Greene. 

Fraxinui altenuata Jones. 

Fraxinus toumeyi Britt. 

Menodora scabra Gray. 

Apocynum androscemifolium L. 

Apocynum scopulorum Greene. 

Haplophyton cimicidium A. DC. 

Asclepias linaria Cav. 

Asclepias tuberosa L. 

Gomphocarpus hypoleucus Gray. 


Evolvulus arizonicus Gray. 

Ipomaa capillacea Don. 

Ipomcea coccinea var. hederifolia Gray. 

Ipomcea muricata. Cav. 


Gilia floccosa Gray. 
Gilia multifiora Nutt. 
Gilia thurberi Gray. 
Linanthus aureus (Nutt.) Greene. 
Ellisia torreyi Gray. 
Emmenanthe penduloeflora Benth. 
Nama hispida Gray. 
Phacelia distans Benth. 


Amsinckia tessettata Gray. 
Coldenia canescens DC. 
Cryptanthe intermedia (Gray) Greene. 
Cryptanthe pterocarpa (Torr.) Greene. 
Eremocarya micrantha (Torr.) Greene. 
Lithospermum multiflorum Torr. 
Onosmodium thurberi Gray. 
Pectocarya linearis DC. 

\"K HI) E S ACE.K : 

Lippia vtrightii Gray. 
Verbena ciliata Benth. 
Verbena wrightii Gray. 

Agastache pallidiflora (Hell.) Rydb. 
Hedeoma hyssopifolia Gray. 
Hyplis emoryi Torr. 
Monarda pectinata Nutt. 
Monarda scabra Beck. 
Katcia arizonica Gray. 
Slachys coccinea Jacq. 
Trischostema arizonicum Gray. 



List of Characteristic 

Lycium berlandieri Dunal. 

Lycium fremontii Gray. 

Lycium partiflorum Gray. 

Nicotiana trigonophylla Dunal. 

Solanum fendleri Gray. 

Castilleja gloriosa Britt. 

Castilleja integra Gray. 

Cordylanthus wrightii Gray. 

Linaria canadensis L. 

Maurandia antirrhinifolia (Poir.) Willd. 

Mecardonia peduncularis (Benth.) Greene. 

Mimitanthe pilosa (Benth.) Greene. 
= Mimulus pilosus Wats. 

Mimulus cardinalis Dougl. 

Mimulus guttatus (L.) DC. 

Mimulus langsdorfii Sims. 

Orthocarpus purpurascens Benth. 

Pentstemon barbatus (Cav.) Nutt. 

Pentstemon spectabilis Thurber. 

Pentstemon torreyi Benth. 

Pentstemon wrightii Hook. 

Scrophularia sp. 

Slemodia durantifolia (L.) Sw. 

Chilopsis linearia (Cav.) Sweet. 
= Chilopsis saligna Don. 

Stenolobium incisum Standley. 

Anisacanthus thurberi (Torr.) Gray. 

Carlowrightia arizonica Gray. 

Plantago fastigiata Morris. 

Plantago ignota Morris. 

Bouvardia triphylla Salisb. 

Diodia teres Walt. 

Galium asperrimum Gray. 

Galium rothrockii Gray. 

Galium wrightii Gray. 

Houstonia ivrightii Gray. 

Sambucus mexicana Presl. 

Sambucus vestita Woot. & Stand. 

Symphoricarpos oreophilus Gray. 

Valeriana arizonica Gray. 

Lobelia gruina Cav. 

Specularia biflora (R. & P.) Fisch. & Mey. 

Achittea lanulosa Nutt. 

Actinolepis lanosa Gray. 

Antennaria marginata Greene. 

Artemisia dracunculoides Pursh. 

Artemisia ludoviciana Nutt. 

Artemisia sp. 

Artemisia sp. 

Baccharis emoryi Gray. 

Baccharis glutinosa Pers. 

Baccharis pteronoides DC. 

Baccharis sarothroides Gray. 

Baccharis thesioides H. B. K. 

Bceria chrysostoma Fisch. & Mey. 

Bahia absinthifolia Benth. 

Baileya multiradiata Harv. & Gray. 

Bebbiajuncea (Benth.) Greene. 

Species Continued. 
COMPOSITE Continued: 
Bidens sp. 
Brickellia californica (T. & G.) Gray. 

= Coleosanthus californicus (T. & G.) Kze. 
Brickellia grandiflora Nutt. 

= Coleosanthus grandiflorus (Hook.) Kze. 
Carduus rothrockii (Gray) Greene. 
Carduua sp. 

Carpochcete bigelovii Gray. 
Chrysoma laricifolia (Gray) Greene. 

= Aplopappus laricifolius Gray. 
Crassina pumila (Gray) Kze. 

= Zinnia pumila Gray. 
Dugaldia hoopesii (Gray) Rydb. 
Encelia farinosa Gray. 
Erigeron macranlhus Nutt. 
EriQeron neomexicanus Gray. 
Erigeron wootoni Rydb. 
Eriocarpum gracile (Nutt.) Greene. 

= Aplopappus gracilis (Nutt.) Grey. 
Eupatorium arizonicum (Gray) . 

= Eupatorium occidentale var. arizonicum 


Eupatorium pauperculum Gray. 
Eupatorium rothrockii Gray. 
Franseria cordifolia Gray. 
Franseria deltoidea Torr. 

= Gcertneria deltoidea (Torr.) Kze. 
Franseria tenuifolia Gray. 

= Gcertneria tenuifolia (Gray) Kze. 
Franseria ambrosioides Cav. 
Gnaphalium decurrens Ives. 
Gnaphalium wrightii Gray. 
Guardiola platyphylla Gray. 
Gymnolomia multiflora Rothr. 
Gymnosperma corymbosa DC. 
Helenium thurberi Gray. 
Helianthella arizonica (Gray). 

= Helianthella guincrueneniia var. arizonica 


Hieracium discolor. 
Hieracium lemmoni Gray. 
Hymenoclea monogyra T. & G. 
Hymenopappuy mexicanus Gray. 
Hymenothrix wrightii Gray. 
Isocoma hartwegi (Gray) Greene. 

= Bigelovia hartwegi Gray. 
Laphamia lemmoni Gray. 
Laphamia sp. 

near to Laphamia halimifolia Gray. 
Machceranthera tanacetifolia (H. B. K.) Nees. 
Pectis papposa Gray. 
Perityle coronopifolia Gray. 
Picradenia biennis (Gray) Greene. 

=Actinella biennis Gray. 
Pinaropappus foliosus Hell. 
Psilo&trophe cooperi (Gray) Greene. 

= Ridellia cooperi Gray. 
Rudbeckia laciniata L. 
Senecio neomexicanui Gray. 
Solidago bigelovii Gray. 
Solidago marshallii Rothr. 
Solidago sparsiflora var. subcinerea Gray. 
Stephanomeria runcinata Nutt. 
Stevia sp. 

Tagetes lemmoni Gray. 
Trixis angustifolia var. latiuscula Gray. 
Verbesina encelioides (Cav.) B. A H. 

= Ximenesia encelioides Cav. 




The latitude of the Santa Catalina Mountains and their position in 
the midst of a continental desert give to their lower slopes the climate 
which is well known to characterize southern Arizona : a low unequally 
distributed rainfall, a short winter with frequent severe frost, and a 
long summer with high maximum temperatures and low atmospheric 
humidity. The longitudinal position of the Santa Catalinas, between 
the Pacific Coast and the southern Great Plains, gives to their climate 
also some of the characteristics of both these regions, notably in respect 
of the incidence of the rainfall seasons. Both the winter rains of the 
Pacific Coast and the summer rains which are prevalent on the Great 
Plains extend in attenuated form to Tucson and to the Santa Catalinas, 
giving them a short rainy season in July and August, often extending 
over into September, and a longer less pronounced rainy season from 
December to February or March.* Although the amount of rain in these 
seasons increases with altitude, the duration of the seasons themselves 
is essentially the same from Tucson to the summit of Mount Lemmon, 
and in fact throughout southeastern Arizona. 




FIG. 2. Schematic representation of rainfall seasons and length of frostless season at Tucson 
and in the Santa Catalina Mountains, showing averaged limiting dates of rainfall seasons 
for 8 years and averaged limits of the frostless season for 1909, 1910, and 1911 (A A), and 
for 1912, 1913, and 1914 (B B). 

The long frostless season characteristic of Tucson and the foothills 
of the Santa Catalinas naturally decreases in length with altitude until 
at 8,000 feet it is only one half as long. The curves of decreasing length 
of frostless season and a diagrammatic representation of the incidence 
of the rainy seasons are shown in figure 2. 

The gentle rains and occasional snowfall of the winter season serve 
to replenish the moisture of the soil at all altitudes, but on the desert 

* See Shreve, Forrest. Rainfall as a Determinant of Soil Moisture. 
17: 9-26, 1914. 

The Plant World, 


their effect is soon overcome by the desiccating conditions of March 
and April. The hot and rainless weeks which precede the mid-summer 
have been designated the "arid fore-summer." On the desert this is 
a season in which the temperature conditions are conducive to activity 
on the part of plants, while the soil moisture conditions are increasingly 
deterrent to it. As a result of these conflicting conditions activity may 
be observed in the trees which grow near a constant water supply, as 
Populus sp. (cottonwood) and Salix sp. (willows), trees which possess 
deep-seated root systems, as Prosopis velutina (mesquite), and plants 
which contain stores of water, as all species of cacti. The activity of 
Populus and Prosopis consists in both flowering and leafing-out, as well 
as in shoot growth; in the cacti it consists in flowering and in some 
species also in growth. Among all desert plants other than those 
indicated the arid fore-summer is a period of drought-rest. 

With respect to the water relations of plants the arid fore-summer 
is the most trying season of the year, combining low soil moistures with 
atmospheric conditions that compel active transpiration . In all respects 
in which moisture conditions may be critical for the survival of individuals 
or the limitation of the distribution of species it is in the arid fore-summer 
that the critical intensity of these conditions must be sought. 

The retardation of spring which accompanies increasing altitude 
results in a shortening of the arid fore-summer from a length of 15 
weeks on the desert to 11 weeks at 6,000 feet and 6 weeks at 8,000 feet 
(see fig. 2). Not only does this trying season decrease in length with 
altitude, but its physical conditions become ameliorated, as will be shown. 

The "humid mid-summer" commences on July 8 and lasts until 
September 12, these being the average dates, for 8 years, of the first 
and last rams of 0.50 inch or more. In this season the moisture con- 
ditions of desert and mountain top are more nearly alike than at any 
other time. It is the season of greatest vegetative activity on the 
desert and in the forest also. On the desert it is the only season in 
which germinations take place among the perennials, and it is the chief 
season of growth among all perennial plants, including those that have 
been in leaf during the arid fore-summer. In the Encinal region the 
evergreen oaks renew then- foliage at the advent of spring, but the great 
mass of vegetative activity awaits the humid mid-summer. In the 
Forest the pines also commence growth with the cessation of frost, 
but make their chief growth during July and August. The humid 
mid-summer is also the chief period of activity for the herbaceous 
perennials and small shrubs of the forested elevations. Heavy snow- 
fall during mid-winter or the occurrence of exceptionally late winter 
rains may bring about growth among the herbaceous perennials of the 
forest during the arid fore-summer. In fact a few species, notably 
Frasera speciosa and Dugaldia hoopesii, commence growth before the 
last frosts of spring. 


At the higher altitudes the shortness of the growing season and the 
coldness of its nights are inimical to the activity of the herbaceous 
perennials. These circumstances make very difficult the introduction 
into the Forest region of plants which would seem calculated to nourish 
in a region of similar moisture conditions. 

After the close of the humid mid-summer the desert is subjected 
to a variable period of 6 to 10 weeks of arid conditions, a season known 
as the "arid after-summer." Although the temperature, humidity, 
soil moisture, and evaporation may reach as extreme values in the 
arid after-summer as in the arid fore-summer, nevertheless the total 
duration of such extremes is not as great in the former season. A 
general cessation of vegetative activity occurs in September and Octo- 
ber at the higher elevations and in October and November at the lower 
ones. On the desert it sometimes happens that occasional rains during 
the arid after-summer prolong the activity of the shrubs and even of 
the summer ephemerals to such a late date that they may be seen in 
flower side by side with root-perennials which are characteristic of the 
winter season. 


The figures for the monthly average rainfall at Tucson, as determined 
from the 38-year record (1876 to 1913), show that the year falls natur- 
rally into two humid and two arid seasons (see fig. 4). Without regard 
to the average dates upon which the heavy rains of the humid seasons 
commence or terminate, the humid winter may be seen to fall within 
December, January, February, and March, and the humid mid-summer 
within July, August, and September. Making this artificial division 
by months between the rainfall seasons, the percentages of the total 
annual precipitation which fall in the four seasons are as follows: 
humid winter 31.1 per cent, arid fore-summer 5.9 per cent, humid mid- 
summer 50.6 per cent, arid after-summer 12.4 per cent. The two rainy 
seasons yield 81.7 per cent of the total annual rainfall, and the light 
rains of the two arid seasons (which form the remaining 18.3 per cent) 
are of very slight influence upon vegetation. The rains of November 
may bring forth some of the winter herbaceous perennials, without any 
effect on the large perennials other than the inducing of leaves on Fou- 
quieria and Parkinsonia. The rains of the arid fore-summer are usually 
too light and too widely separated to bring into activity either the 
summer ephemerals or the perennial plants. 


On the Pacific Coast the monthly distribution of rainfall brings over 
75 per cent of the annual total within the whiter months. On passing 
eastward through Arizona this predominance of winter rain is gradually 
lost until it becomes less than 20 per cent of the annual total at the 
Rio Grande River in New Mexico. Conversely, the precipitation of 



the summer months is almost negligible on the Pacific Coast and grad- 
ually increases on passing eastward until it reaches 50 per cent at Tucson. 
Between Tucson and the Rio Grande it remains at about 50 per cent, but 
from the basin of the Rio Grande eastward the rainfall seasons of the 
Tucson region cease to be a natural division of the year (see table 1). 

TABLE 1. Percentages of summer rainfall and of winter rainfall to the annual rainfall for 
a series of stations stretching from the Pacific coast to the Rio Grande River, through 
southern Arizona. 







76 7 

Riverside, Cal ... 




Indio Cal 

73 3 


85 3 


20 3 

79 3 

Gila Bend, Ariz 

47 5 

35 5 


45 9 


79 9 

43 7 

37 6 

81 3 

Tucson, Ariz 




24 8 

57 5 

82 3 

34 9 

48 5 

83 4 

Lordsburg, N. Mex 




22 9 

54 8 

77 7 

Agricultural College, N. Mei. . . 




In figure 3 are given curves showing the percentages of the annual 
rainfall which are formed by summer rains and by winter rains for a 
chain of 13 stations stretching from Los Angeles to Mesilla Park, New 
Mexico, on the Rio Grande River. 

TABLE 2. The total annual rainfall, the summer rainfall, and the percentage of the latter to 
the former for very wet and very dry years at Tucson. 


Eight wet years 
(14 inches or over). 


Eleven dry years 
(9 inches or less) . 




















1 1886 


i 1891 







Average percentage 




Average percentage 



The fall of approximately half the annual precipitation in the humid 
mid-summer is by no means a constant occurrence at Tucson. In 
1881 the summer rainfall was 80.3 per cent of the annual, and in 1884 



it fell to 11.7 per cent. Neither does the percentage of summer rain 
fluctuate in relation to the occurrence of very wet or very dry years. 
In 8 of the wettest years since 1876 (14 inches or above) the summer 
rain was 53.1 per cent of the total, and in 11 of the driest years (9 inches 
or less) the summer yielded 47.6 per cent of the total (see table 2). 

The impossibility of securing figures for the winter precipitation in 
the Santa Catalina Mountains makes it necessary to estimate the 
annual totals of rainfall at different altitudes from the known figures 


FIG. 3. Graphs showing percentage of winter rainfall to annual total 
(light line), and of summer rainfall to annual total (heavy line), for 
a chain of 13 stations from the Pacific to the Rio Grande. 

for the summer rain. The average rainfall at the stations at 7,600 feet 
and 8,000 feet for the years 1907 to 1914 is 17.45 inches (443 mm.), 
from which it may be assumed that the annual average is approximately 
35 inches (889 mm.). The summer rain at Tucson during 1907 to 1914 
was 54.7 per cent of the annual total. If the seasonal distribution of 
rain is the same on the mountain that it is at Tucson, the above esti- 
mate of the annual total for the mountain is correct within 1 or 2 inches. 
The influence of altitude on the seasonal distribution of rainfall in 
Arizona is a matter which can not be determined without further data 



than are now in hand. During the years 1907 to 1912 the percentage 
at Benson, Arizona (3,523 feet), was 60 per cent, that at Globe, Arizona 
(3,525 feet), was 42.1 per cent, the average of the two 51.0 per cent. 
The average of the percentages for Fort Huachuca (5,100 feet), Fort 
Apache (5,200 feet), and Bisbee (5,500 feet) is 55.5 per cent. Although 
these figures indicate that up to 
5,000 feet there is about the same 
percentage that holds at 2,400 feet 
(at Tucson), nevertheless at Flag- 
staff (6,907 feet) the summer rain 
was only 40.7 per cent of the total 
in the years mentioned. At Chlar- 
son's Mill (7,200 feet) an incom- 
plete record indicates that in 1907, 
1909, and 1910 the summer rain 
was far below the percentages for 
Tucson for those years. At Greer 
(9,200 feet), on the Mogollon Pla- 
teau, the summer rain was a much 
greater percentage of the annual total in 1905 than it was at Tucson, 
while in 1906 and 1908 the percentages were nearly identical. It can 
only be said, therefore, that a much larger body of data is necessary to 
determine the possible change of seasonal distribution of rain due to 
altitude. The evidence at hand indicates that there is little probability 
of a marked influence. (See table 3.) 

TABLE 3. The average annual rainfall for 1907 to 1912, the average summer rainfall for the 
same years, and the percentage of the latter to the former for stations at different altitudes 
in central and southern Arizona. 


Fio. 4. Diagram showing monthly distribution 
of rainfall at Tucson. Averages of record for 
38 years, 1876 to 1913 inclusive. 








11 54 

6 66 

57 7 



10 51 

6 31 

60 \. 



17 01 

7 16 

42 1 f* 

Fort Huachuca 


15 53 

9 24 

59 5 1 

Fort Apache 


16 95 

7 53 

44 4 Us 5 



20 68 

12 95 

62 6 J 



22 60 

9 20 

40 7 


The measurements of summer rainfall on the Santa Catalina Moun- 
tains were begun in 1907 by the installation of a metal gauge at 7,600 
feet, where the record was secured until 1911, after which it was re- 
moved to a nearby ridge at 8,000 feet. During 1908 and 1909 readings 
were secured at the base of the mountain and at 6,000 feet, in 1910 at 



6,000 feet only. In 1911 a series of stations was selected at vertical 
intervals of 1,000 feet, from the base of the mountain, 3,000 feet, to 
the station at 8,000 feet, and in 1912 a station was established on 
Mount Lemmon, at 9,000 feet. These stations have been continued 
in the succeeding summers. 

The readings of the gauges have been made at irregular intervals, as 
opportunity afforded; the water has been protected from evaporation 
by the use of kerosene, and has been measured volumetrically. The 
installation of the gauges has been made each spring in time to secure 
the first of the summer rain, and the final readings have been made in 
September, closing in 1911 on the 22d to the 25th, in 1912 on the 28th 
to 30th, in 1913 on the 25th to the 27th, and in 1914 on October 10th 
to llth. The location of the gauges at the various altitudes has been 
such as to give them comparable topographic surroundings. Each 
station is at the summit of a ridge with a commanding opening to the 
south and without nearby trees. A record of rain has also been secured 
at the Xero-Montane Garden at 5,300 feet, near the head of Soldier 
Canon and just below the 6,000-foot station. A recapitulation of all 
the readings of mountain rainfall is given in table 4. 

TABLE 4. Summer Rainfall in the Santa Catalina Mountains. 

All readings cover the total precipitation of July, August, and September. Starred figures include 
some October rainfall. Figures followed by plus are incomplete, owing to the overflowing of 










Averages of 
perfect records. 

























The only record of daily rainfall for the Santa Catalinas is one 
secured in Marshall Gulch, at 7,600 feet, from June to August 1911, 
by Professor J. G. Brown, of the University of Arizona. A comparison 
of the daily rainfall at Marshall Gulch and at 8,000 feet with that at 
the Desert Laboratory (2,663 feet) for the period of these observations 
is given in table 5. The number of rainy days on the desert was greater 
than the number on the mountain top 31 and 19 respectively owing 
to the 16 days with only a trace of rain at the Laboratory. The total 
rainfall of the three months was 5.42 inches at the Laboratory (for 



exactly the same days covered by the Marshall Gulch record), and 
14.86 inches at the mountain station. The general correspondence 
between the dates of heavier rains at these stations, 5,000 vertical 
feet apart, indicates the close relationship of the atmospheric factors 
which determine the rainfall of all altitudes. 

TABLE 5. Comparative daily incidence of rainfall at the Desert Laboratory (2,663 feet) and 
at the Montane Garden in Marshall Gulch (7,600 feet), for June, July, and August 1911. 

Day of 




Day of 




D L 

M G 

D L. 

M G 

D L 

M G 

D L 

M G. 

D. L. 


D. L. 












19th . . 










6th . 

22d. . . 









24th . . 






10th. . 
llth. . 
13th. . 
14th. . 





















Total rainfall: Desert Laboratory, 5.42 in.; Montane Garden, 14.86 in. Total number of 
rainy days: Desert Laboratory, 15 (or 31, including days with T) ; Montane Garden, 19. 

Another comparison which it is possible to institute between the 
summit of the Santa Catalinas and the desert is the summer rainfall 
totals from 1907 to 1914 inclusive (see fig. 9). The directions of the 
curves which show the march of the summer precipitation from year 
to year indicate an almost complete lack of relationship between the 
mountain and the plain. It is obvious that the curve of altitudinal 
increase of rainfall determined in such a year as 1910 would be very 
unlike the curve determined in 1911. 

It has been suggested by Smith * that there may be a relative 
increase of rainfall at the higher altitudes as the summer advances, 
which is to say that the gradient of increase of rainfall with altitude is 
steeper for the late summer than it is for the early summer. In order 
to test this possibility the series of ten readings taken in the humid mid- 
summer of 1911 and the one set taken in the early arid after-summer 
have been grouped into totals for five periods of approximately one 
month each (table 6). An inspection of the table shows that the maxi- 
mum rainfall occurred between July 18 and August 24 at 3,000, 4,000, 

* Smith, G. E. P. Groundwater Supply and Irrigation in the Rillito Valley. Ariz. Agric. 
Exper. Sta. Bull. 64, 1910. 



5,000, and 8,000 feet, and between June 20 and July 18 at 6,000 and 
^OOo' feet. In similar manner the less frequent readings of 1912 and 
1913 have been divided into the early summer and late summer falls, 
by the latest July reading, and the averaged curves for early summer 
and late summer rain are of nearly the same shape, but the late summer 
curve is not so steep. This short record does not seem, therefore, to 
corroborate the suggestion of Smith. 

TABLE Q.Intraseasonal distribution of summer rainfall at the Desert Laboratory and at 6 
elevations in the Santa Catalina Mountains for 1911. 

Rainfall of the maximum period in heavy type. 

Apr. 25-27 

June 20-21 

July 18-19 

Aug. 22-24 

Sept. 22-25 








June 20-21. 

July 18-19. 

Aug. 22-24. 

Sept. 22-25. 

Oct. 12-14. 

Des. Lab . . . 







3,000 feet... 







4,000 feet... 







6,000 feet... 







6,000 feet... 







7,000 feet... 







7,600 feet... 







The increase of rainfall which accompanies increase of altitude is a 
phenomenon of general occurrence throughout the southwestern 
United States. The curves by which such increase may be expressed 
differ from each other most strikingly, according to the horizontal dis- 
tance of the successive stations from each other, according to the 
coastal or continental position of the series of stations, or according 
to the size of the mountain range on which the successive elevations 
are secured. Although it is possible to deduce mathematical formulae 
for the vertical increase of rainfall, it is necessary to introduce into all 
such formulae a constant for the particular region or mountain involved, 
and the figures thus secured are merely in the nature of hypothetical 
means near which the normal conditions may fall. It would be of very 
great interest in the extension of plant geography to possess data on 
the actual amounts of rainfall at successive elevations in a large number 
of mountains and shelving plains throughout the southwest. The mean 
rainfall conditions which are expressed in a gradient based on a long 
climatological record are of great importance in connection with vege- 
tation, but only when consideration is also given to the extremes of 
rainfall, and particularly to the lower extremes, if a semi-arid country 
is under consideration. The securing of typical normal gradients of 
altitudinal increase of rainfall is not of so much importance in plant 
geography, therefore, as a knowledge of the actual oscillations of the 
rainfall conditions from year to year throughout the series of stations 
or localities involved. 



Smith* has deduced two curves of altitudinal increase of rain, one 
applicable to Pima and Final Counties, Arizona (the counties in which 
the Santa Catalinas lie), the second to Graham and Cochise Counties. 
These curves are based on records of various lengths, chiefly from sta- 
tions located in the valleys of these mountainous counties. Smith's 
curves are reproduced in figure 5, in which they have been brought 
half way down toward the base line in order to make them comparable 
with the curve expressing the average summer rainfall of the Santa 
Catalina Mountains for 1911, 1912, and 1913. The portion of Smith's 
Graham-Cochise curve extending above 5,500 feet is based on a single 
short record at 6,000 feet. , 

The curve of altitudinal rise of ram for 1911, 1912, and 1913 in the 
Santa Catalinas is merely a simple average of the actual readings for 
the three summers, without any attempt to correct in accordance with 

8,000 9.000 

&000 ftOOO 

Flo. 5. Graph showing altitudinal increase of summer rainfall on the Santa Catalina Mountains 
in 1911, 1912, and 1913 (solid line) ; together with Smith's curves for Pima and Final Counties, 
Arizona (dotted line), and for Cochise and Graham Counties (broken line). 

FlO. 6. Graphs showing vertical increase of summer rainfall in the Santa Catalina Mountains in 
1911 (solid line), 1912 (broken line), and 1913 (dotted line). 

the departure of the neighboring lowland rainfall from the normal 
during these years, without the application of any rainfall formula, 
and without the smoothing of the lines. Reference to table 4 will show 
that the record for 7,000 feet is based on two years only, and the record 
for 9,000 feet on one correct summer's reading and the reading of one 
summer in which the gauge overflowed. 

A comparison of Smith's curves with the curve for the Santa Cata- 
linas shows the latter to have a sharper rise from 3,000 to 4,000 feet, 
and to have a relatively level stretch from 4,000 to 6,000 feet, where 
the former curves have their sharpest ascent. 

* Smith, G. E. P., loc, tit. 



Figure 6 gives the actual curves for the three summers for the Santa 
Catalinas. It will be noted that in each curve there is a sharp rise 
from 3,000 to 4,000 feet, a rise which continued at the same gradient 
to 5,000 feet in 1911 and 1913. From these submaxima, reached at 
5,000 feet in 1911 and 1913 and at 4,000 feet in 1912, there is a fall to 
a subminimum at 6,000 feet in the two former years and at 5,000 feet 
in the latter year. There is then a pronounced rise in the curve to 
7,000 and 8,000 feet. The rainfall at 9,000 feet in 1912 was probably 
an inch or more greater than indicated by the curve, in any case was 
greater than that at 8,000 feet; whereas in 1913 the precipitation at 
9,000 feet was less than that at 8,000 feet, in fact less than that at 
5,000 feet. 

The horizontal distances between the rainfall stations were unequal 
(see plate A), the angle of rise from 3,000 to 4,000 feet being very 

Fio. 7. Graph showing vertical increase of summer rainfall in the Santa Catalina Mountains 

in 1911 (solid line) , together with averaged vertical increase in a series of 13 Weather Bureau 

stations in Arizona (broken line). 
Fio. 8. Graph showing vertical increase of summer rainfall in the Santa Catalina Mountains 

in 1912 (solid line), together with averaged vertical increase in a series of 21 Weather Bureau 

stations in Arizona (broken line). 

sharp, that from 4,000 to 5 000 slightly less sharp, and that from 5,000 
to 6,000 still less sharp and exactly equal to the angle of rise from 6,000 
to 7,000 feet. The stations at 8,000 and 9,000 feet are located at the 
west end of the main ridge and are consequently not in line with the 
lower stations. The sharp rise in elevation between the 3,000 and 4,000 
foot stations is doubtless partially accountable for the rapid increase 
of rainfall between them. The steep rise of the rainfall graphs between 
6,000 and 7,000 feet may indicate an influence due to the position of 
the 7,000-foot station on the north rim of Bear Canon, with a very 
abrupt wall immediately below it. There is no topographic cause, 
however, to which it is possible to attribute the dip in the rainfall 
curves for 6,000 feet in 1911 and 5,000 feet in 1912. 

In order to institute a comparison between the mountain gradients 
of rainfall and those of the valley stations of the Weather Bureau the 
data have been collated which are expressed in the curves of figures 7 


Plate A 



and 8. These figures compare the summer rainfall curves of the Santa 
Catalinas and those of selected stations for the same summers. In 
figure 7 the rainfall of July, August, and September 1911 has been used, 
for 13 stations located in southeastern Arizona, east of Phcenix and 
south of Fort Apache. The rain has been averaged for each group of 
stations lying within the same thousand-foot interval of altitude. 
Figure 8 shows the curve for the Santa Catalinas for 1912 and the curve 
for 21 stations in the same area. A single record above 5,000 feet has 
been available for this curve, that at Chlarson's Mill, in the Pinaleno 

The significance of the comparison of these rainfall records for a 
single season is entirely different from that of averages for long series 
of years. Such a comparison as this makes possible the contrasting of 
records which are strictly contemporaneous and serves to show the 
way in which the same complex of meteorological conditions affected 
the precipitation at various altitudes, and how these conditions affected 
the rainfall of a single mountain range in comparison with that of an 
extended adjacent area lying at different levels. The extremely small 
number of rainfall records secured at localities above 5,500 feet in 
southern Arizona does much to vitiate such a comparison. In 1911 the 
gradient of rise was greater between 3,000 feet and 5,000 feet in the 
Santa Catalinas than it was in the Weather Bureau stations. In 1912 
the rise was sharper in the mountains from 3,000 to 4,000 feet than 
it was in the valleys, but the fall from 4,000 to 5,000 feet was paralleled 
by a rise in the curve of the valley stations. The fall at 7,200 feet at 
Chlarson's Mill was far below that at 8,000 feet in the Santa Catalinas 
for the same period. 

The shape of the averaged curve of rainfall in the Santa Catalinas 
for the three summers is correlated with the nature and movement 
of the convectional storms to which the summer precipitation is due. 
It would appear that certain rains are derived from low-lying clouds 
which form over the desert and are then driven against the mountain 
wall by the prevailing southwest winds of summer. These rains in- 
crease in intensity as they pass up the mountain slopes and yield their 
maximum downpour at about 4,000 or 5,000 feet, according to the 
conditions. The rainfall at the Xero-Montane Garden was greater 
than that at the 6,000-foot station (700 feet above it and only half a 
mile distant) for four of the six summers in which records have been 
kept in the two localities (see table 4). The Garden is located at the 
head of Soldier Canon, and just above it there is a sharp increase in 
the gradient of the mountain slopes. It is probable that the head of 
the canon is the terminating point in the course of many of the desert 
rain storms. The rapid increase of rainfall between 6,000 and 7,000 
feet may be due to a similar topographic cause, as mentioned in a 
preceding paragraph, or it may give indication that the rains of the 



higher elevations are derived from a higher cloud level, probably from 
convectional clouds which form at times when the atmospheric con- 
ditions cause condensation at a greater distance from the earth. When 
a long series of records shall have been secured from the 9,000-foot 
station it will probably show that its average rainfall is greater than 
that at 8,000 feet, but the 9,000-foot record for 1913 indicates that 
there will be occasional years, at least, in which the maximum for the 
mountain is recorded at 8,000 feet. This probably means that at 
10,000 feet on adjacent mountains there is a constantly lower rainfall 
than at 8,000 or 9,000 feet. 

The check in the vertical increase of rainfall which has been described 
as occurring between 4,000 and 6,000 feet appears to be absent from 
all curves derived from widely separated valley stations. The writer 
has seen no such plateau in any 
curves derived from southwest- 
ern data, but there is always the 
possibility that a plateau has 
been smoothed out of the curves 
or that the data have been sub- 
jected to the influence of a 
straight-line equation. The 
character of the increase of pre- 
cipitation with altitude in a sin- 
gle small range of mountains is 
no more a special case than is the 
increase in a widely separated 
series of stations in any locations 
whatsoever. In so far as con- 
cerns the study of meteorological 
dynamics, such a mountain 
range as the Santa Catalinas offers exceptional opportunities for investi- 
gation, and much more might be learned in a single summer of intensive 
meteorological study on its slopes than could be ascertained by an exami- 
nation of records of rainfall covering a period of a thousand years. 

As regards vegetation, the most important feature of the study of 
rainfall conditions is the determination of the extremes of variation in 
the amount and seasonal distribution of rain, and the ascertaining of 
the effect of these extremes upon the conditions of soil moisture. Years 
of heavy precipitation are important for the maintenance of the forest 
which clothes the higher mountain slopes and for the general restora- 
tion of the supplies of soil moisture and ground water. The years of 
low precipitation, and especially the series of consecutive years with 
deficient rainfall, are of first importance to the vegetation which occu- 
pies the Encinal region of the mountains. During such years, and 
particularly during the arid fore-summer of such years, the lowest 

Fia. 9. Graph showing lack of relation between 
Bummer rainfall at Marshall Gulch (7,600 feet) 
and at Desert Laboratory (2,663 feet) from 1907 
to 1914. 


individuals of all Encinal and Forest species are subjected to conditions 
of water supply which are perhaps below any conditions that have 
previously occurred during their lives, or are surely in the case of 
perennials the most trying conditions when considered in the light 
of the plants having grown to greater size and heavier water-demand 
than during the dry periods of their earlier existence. 


It is obvious, from a consideration of the monthly distribution of 
rainfall in the Santa Catalinas, that at all elevations there are annually 
two periods of high soil moisture, coinciding with the humid mid- 
summer and the humid winter, and two periods of decreasing soil mois- 
ture content, coinciding with the arid fore-summer and arid after- 
summer. The influence of the earliest rains of summer and winter is 
quickly exerted in an elevation of the soil moisture, but at the close 
of these seasons it is with relative slowness that the soil falls to low 
percentages of moisture, particularly at the highest altitudes. The 
minimum moisture content of the year is usually to be detected just 
before the first heavy rain of the humid mid-summer, but the content 
in September or October may sometimes be quite as low. 

At low elevations in the Santa Catalinas the annual march of soil 
moisture may be expected to be analogous to that which has been 
described by the writer for Tumamoc Hill, the site of the Desert 
Laboratory.* Marked differences will result from a comparison of the 
two localities, however, owing to the difference in the character of 
the soil. The very fine clay of Tumamoc Hill is conservative in its 
changes of moisture content, both with respect to increases and de- 
creases of moisture, while the coarse loam found at the lower elevation 
in the Santa Catalinas possesses a greater permeability and a lesser 
holding power. The soils of elevations of 7,000 feet and more are 
richer in organic matter than those of the Desert and Encinal regions 
of the mountain, and are doubtless more like the clay of Tumamoc 
Hill in the smoothness of their curves of change in moisture content. 

The few readings of soil moisture content that have been made were 
directed toward a determination of the soil conditions in the most arid 
portion of the year. It is obvious that it is these annual minima which 
are of the greatest importance to plants, particularly to such plants 
as are near, the lowest limit of their vertical occurrence. Much less 
interest attaches to the high moisture contents which might be found 
in the midst of the rainy seasons. It is true that these high moistures 
are the ones which call forth general vegetative activity and condition 
the appearance of ephemeral plants at the lower altitudes. It is like- 
wise possible that high and protracted soil moistures may be of some 
importance as a limiting factor for desert species at the upper edges 

* Shreve, Forrest. Rainfall as a Determinant of Soil Moisture. The Plant World, 17: 9-26, 1914. 


of their ranges. It was impossible, nevertheless, to secure a set of 
soil samples in the humid mid-summer which would be representative 
of the maximum moisture conditions and at the same time comparable 
for the various altitudes. A set of samples taken at the same interval 
after a rain of the same amount, at each of the several elevations, 
would comply with the requirements. 

All samples of soil for moisture content were taken from a depth 
of 15 cm. The conditions at this depth are of importance for ephemeral 
herbaceous plants and for some shrubs, but the trees and larger shrubs 
are, of course, dependent for their supplies on much more deep-seated 
bodies of soil. The rocky character of the substratum means that the 
largest perennial plants are dependent to a great extent upon the 
moisture contained in the soil which occupies the crevices of the rock 
in situ. It is particularly noticeable that the lowest trees of the Encinal 
region grow in the uppermost part of talus slopes or along the bottoms 
of cliffs. In such situations it is doubtless possible for the roots of 
these trees to reach soil-filled crevices which are fed by gravity with 
the water of large veins of soil above. 

The samples of soil were secured by digging with a hand trowel and 
transferring quickly to bottles, which were tightly stopped, and then 
coated over the stopper with vaseline. The soils were dried in the 
original bottles by heating to 100 C. until they showed constant 
weight. The percentages of moisture have been calculated on the dry 
weight as unity. The physical texture of all samples taken was very 
similar, but there was a greater amount of humus in those from the 
higher elevations. 

Three series of soil samples were taken at various times to determine 
the conditions prevailing in the arid fore-summer. These samples 
were taken at 1,000-foot intervals, from the vicinity of the rainfall 
stations, and were secured in pairs, one sample being from a south 
slope and one from a north slope. The localities chosen for sampling 
were typical of the slopes at the several elevations, and in every case 
the pair of samples was secured in the midst of the dissimilar vegeta- 
tions which occupy the opposed slopes. 

On April 27 to 29, 1911, a series was secured from 3,000 feet to 7,000 
feet (see table 7). For the three months preceding the taking of these 
samples there had been only light and infrequent rains over the sur- 
rounding region, the rainfall of the mountains themselves for this period 
being unknown. At Tucson there was a rainfall of 0.28 inch on April 
2, and there was no appearance of rain on the mountains after that 
date. On June 9 to 11 another series of samples was secured at the 
same stations, together with a pair from the station at 8,000 feet. 
There had been no rain between the securing of the two sets of samples. 

A comparison of the percentages of moisture in April and in June 
shows them to be of about the same order of magnitude. The relative 



dryness of the three months preceding the taking of the first set of 
samples, together with the 25 days of rainless weather just preceding 
the taking of the samples, had reduced the moisture of the superficial 
soil to an amount which was near the minimum for the year, as repre- 
sented by the percentages for June. The percentages for June were 
all slightly lower than those for April. The reading of 5.2 per cent for 
the south slope at 5,000 feet in April is undoubtedly too high. The 
fall in moisture on the north slope at 7,000 feet from 9.2 per cent in 
April to 3.2 per cent in June is doubtless significant of the long reten- 

TABLE 7. Soil moisture in the arid fore-summer at a depth of 15 cm. on north and south 
exposures at seven altitudes on the Santa Catalina Mountains. 


Apr. 27 to 29, 

June 9 to 11, 

May 15 to 20, 

Average of 
the three de- 

3,000 south 





4 000 south 

2 7 




4 000 north 





5,000 south 





5 000 north 

3 8 




6 000 south 





6,000 north 





7 000 south 

3 1 




7 000 north 





8,000 south 




8 000 north 




9 000 south 



9.000 north . . . 



The percentages are based on dry weight. 

tion of moisture derived from whiter rains, characteristic of the forested 
elevations. In June the north slope at 8,000 feet had fallen to a slightly 
lower percentage of moisture than the north slope at 7,000 feet in April. 

Between May 15 and 20, 1914, another series of moisture samples 
was secured at the same localities and extended to the 9,000-foot 
station. The preceding winter had been slightly below the average in 
precipitation, but the rainfall for March had been above the average. 
At the time of the taking of the samples there had been no rains for 
six weeks. This series of percentages is similar to those secured in 
1911, and the three may be taken together as indicating the average 
soil moisture conditions of the arid fore-summer. 

A significant feature of all three of the series of moisture determina- 
tions is the fact that there is no appreciable increase of soil moisture 
up to an elevation of 7,000 feet, beyond which elevation there is a 
sharp rise in the percentages, particularly those for the north slopes. 
In other words, so far as the superficial soil moisture conditions are 
concerned, the arid fore-summer carries the desert up to the lower 
limit of the Pine Forest. 

One of the underlying causes of the importance of slope exposure 
for vegetation is revealed in a comparison of the percentages of soil 



moisture for north and south slopes. For the stations at 4,000, 5,000, 

and 6,000 feet the percentages for the two slopes scarcely differ by 

more than the error which may be attributed to the inequalities of the 

moisture in adjacent bodies of soil. Nevertheless, in all but two cases 

there is a slightly greater moisture 

content on the north slope than on 

the sou th . At 7,000 feet the difference 

between the two exposures becomes 

greater, and is still greater at 8,000 

and at 9,000 feet. An inspection of 

the averages (see fig. 10) shows that 

the south slope at 7,000 feet has a 

lower soil moisture than the north 

slope at 6,000 feet. The south slope 

at 8,000 feet, however, has a higher 

moisture than the north slope at 

7,000 feet. 

The fact that there is a very slight 
difference between the soil moisture 
of north and south slopes at lower 
elevations and a greater difference 
with increasing altitude would sug- 
gest that there might be a more pronounced set of vegetational phe- 
nomena resulting from slope exposure at higher elevations than at lower 
ones. This is, indeed, the case, and will be discussed under a later 
heading (see p. 98). 

TABLE 8. Soil moisture in the arid fore-summer and arid after-summer at a depth of 15 cm. 
on north and south exposures, in shade and open, at various altitudes on the Santa Catalina 

FIG. 10. Graph showing vertical increase 
of soil moisture at 15 cm. in the Santa 
Catalina Mountains on north slopes 
(heavy line) and south slopes (light line). 
Average of three determinations in arid 
fore- summer. 






May 27, 1910. . 

6 300 


2 I 

Do. .. 




2 4 


5 300 



3 8 

May 29, 1910 






of slope 


May 30, 1910 

5 030 


of ridge 


5 7 

Sept. 24, 1910.. 

4 600 


2 2 


4 600 




Do.. . 

5 300 







9 ft 

In the summer of 1910 several samples of soil were taken for the 
determination of the moisture conditions on opposed slopes and in 
shaded and open soil, as well as on the top and at the base of a slope. 
These data are shown in table 8. The September readings, when taken 


in comparison with the April, May, and June readings, show moistures 
of about the same amount, indicating that the after-summer is often 
a season of as great soil aridity as the fore-summer. 

The data for shaded and unshaded soil, both in May and September, 
corroborate similar determinations made on Tumamoc Hill and go to 
show that in the arid seasons the influence of shade in sustaining the 
moisture of soil is so slight as to be negligible. The influence of shade 
in retarding the desiccation of the soil just after a rain is not without 
its importance, but in the Desert and Encinal regions the soil in the 
shade of trees will soon reach as low a percentage as that in the full sun. 


It has been frequently pointed out, in recent botanical literature, 
that the measurement of the evaporative power of the air affords a 
concise expression of the combined effects of temperature, humidity, 
and air movement in so far as these factors affect the loss of water by 
plants. The obvious importance of these factors and consequently 
of evaporation in the environmental complex of the Santa Catalinas 
led to the early installation of a series of atmometers (or evaporimeters) 
at several elevations in these mountains. In the summer of 1906 Dr. 
B. E. Livingston secured data from three porous cup atmometers at 
elevations of 6,000, 7,500, and 8,000 feet.* In 1908 and 1910 the 
writer installed series of atmometers at five elevations, from which 
readings were secured which are not sufficiently complete and reliable 
to be worthy of publication. In 1911 a new series of atmometers was 
installed at the six rainfall stations, from 3,000 to 8,000 feet inclusive, 
at 1,000-foot intervals. These instruments were exposed in pairs, on 
north and south exposures, and were operated in the most careful 
manner, in accordance with the experience of the two preceding years. 
The atmometers were read at fortnightly intervals, or nearly so, and 
at each reading fresh cups were installed. The actual readings were 
reduced to standard by the use of an average between the original and 
the final coefficients of correction. Only good distilled water was used, 
and it was conveyed in tin canteens (rather than galvanized iron ones) 
from which the resin remaining from the soldering had been removed 
with carbon bisulphide. The bottles used for the atmometers had a 
capacity of 1 gallon at the lower stations and of 2 quarts at the higher 
stations, such ample amounts of water providing against the possible 
danger of the atmometers going dry. The stoppers in the mouths of 
the bottles were made very tight, to prevent the cups from blowing 
loose, but were provided with grooves to admit air. These grooves 
were stopped with loose cotton, to prevent the entrance of ants, and 
the stoppers were covered by aprons to exclude ram. The atmometers 
were all exposed in situations such that they received full insolation 

* Livingston, B. E. Evaporation and Plant Habitats. The Plant World, 11: 1-9, 1908. 



throughout the day, except in the case of the instrument on the north 
slope at 8,000 feet, which it was impossible to place in such a way as 
to avoid a slight amount of light shade in the mid-morning and in the 

Readings were secured at the stations from 3,000 to 7,000 feet from 
April 25 to 27 until September 5 to 6, and at 8,000 feet from June 7 
until September 5. The actual amounts of the readings are given in 
table 9, in terms of the average loss per day in cubic centimeters from 
a standard cup. 

TABLE 9. The average daily evaporation (in cubic centimeters), for the periods indicated, on 
north and south exposures, at 6 elevations in the Santa Catalina Mountains. 



4,000 feet. 

5,000 feet. 

6,000 feet. 

7,000 feet. 

8,000 feet. 












Apr. 25-27 to May 18-19 . 
May 18-19 to June 6- 7 . 
June 6- 7 to June 20-22 . 
June 20-22 to July 6- 7 . 
July 6- 7 to July 18-21 . 
July 18-21 to Aug. 8-9. 
Aug. 8- 9 to Aug. 22-23 . 
Aug. 22-23 to Sept. 5- 6. 
Sept. 5- 6 to Sept. 22-25 . 












In order to ascertain the altitudinal gradient of evaporation rate 
the readings from the north and south slopes at each altitude have 
been averaged. The averaged total evaporation of the summer for 
each station has been subdivided, so as to show the amount for the 
arid fore-summer as shown by the first three series of readings, and 
for the humid mid-summer as shown by the last six series. The curves 
in figure 12 show the altitudinal fall in evaporation rate during the 
two seasons, in terms of the average daily loss from the atmometer. 
These curves bring out in striking manner the low evaporation rates 
of the humid mid-summer as contrasted with the arid fore-summer, 
the latter being nearly twice as great as the former. There is a strong 
parallelism between the two curves, but the one for the humid season 
is slightly flatter than the one for the arid season. This means that, 
so far as concerns the evaporation conditions alone, there is a less 
differentiation between Desert and Forest in the summer rainy season 
than there is in the arid portion of the summer. The pronounced drop 
in evaporation from 7,000 to 8,000 feet is particularly significant, as 
the former elevation marks the lower edge of the Forest, while the 
latter is in the midst of the best stands of pine. It is possible that the 
forest itself interferes with air movements near the ground in such a 
way as to be responsible for the sharp fall in evaporation. 

In order to exhibit the seasonal march of evaporation rate at the 
several altitudes the curves of figure 11 have been drawn. These 



curves show the averages between the south and north slopes for each 
station at each reading, being expressed in cubic centimeters of evapo- 
ration per day. Here again is brought out the pronounced fall in 









i i i r 


Fio. 11. Graphs showing seasonal march of evaporation rate at 6 altitudes in the Santa Cata- 
linas in 1911. Amounts are average daily losses from the atmometer, and each reading is 
the average of one on a north slope and one on a south slope. 

rate which follows the advent of the summer rains and the cloudy 

and relatively humid weather by which they are accompanied. After 

the period of heavy rams by which the 

humid mid-summer was opened in the last 

days of June and early days of July there 

was a slight rise in evaporation, followed 

by a slight fall in late August and early 

September. The curves for 3,000 and 

4,000 feet accompany each other closely 

after the first two readings, and the curves 

for 6,000 and 7,000 feet accompany each 

other closely throughout the summer. 

The curve for 8,000 feet stands always 

well apart from that for 7,000 feet. The 

grouping of these curves is, therefore, 

analogous to the natural subdivisions of 

the vegetation. The readings taken in the 

3.000 -.COO 

FIQ. 12. Graphs showing altitudinal 
decrease in rate of evaporation in 
the Santa Catalinas in arid fore- 
Hummer (heavy line) and humid 
mid-summer (light line) of 1911. 

Desert region at 3,000 and 4,000 feet, those taken in the Encinal and the 
lower edge of the Forest at 5,000, 6,000, and 7,000 feet, and the one series 



taken in the heart of the Forest stand apart in three loosely defined 
groups in close parallelism to the zonation of the vegetation itself. 
5 During the arid fore-summer the evaporation at 5,000 feet is similar 
to that at 4,000 feet, while during the humid mid-summer it is more 
nearly like that of the 6,000-foot station. In other words, the advent 
of the rains causes the evaporation conditions of the Upper Encinal 
and lower Forest region to extend downward into the Lower Encinal. 
The significance of slope exposure in determining evaporation rate 
is indicated in figures 13 and 14. In these graphs the vertical gradients 
of evaporation at the six elevations are shown separately for the instru- 
ments on the south slopes and the north slopes at each station. The 
gradients for the arid fore-summer and for the humid mid-summer 
are shown, as well as the curves for the entire summer. In the arid 
season there is even a slightly greater evaporation on north slopes at 
4,000 and 5,000 feet than there is on south slopes, but this condition 

Fio. 13. Graphs showing altitudinal decrease in rate of evaporation in the Santa Catalinas on 

Bouth-facing slopes (heavy line) and on north-facing slopes (light line) in arid fore-summer 

of 1911. 
FIG. 14. Graphs showing altitudinal decrease in rate of evaporation in the Santa Catalinas on 

south-facing slopes (heavy line) and on north-facing slopes (light line) in humid mid-summer 

of 1911. 

is reversed at the higher elevations. In the humid season there is also 
a slightly greater rate of evaporation on the north slope at 4,000 feet, 
while all of the higher stations show an almost uniformly greater rate 
on the south slopes. It is impossible to explain the cases in which the 
evaporation was greater on north slopes than on south ones. It is 
possible, of course, that they require no explanation but are typical of 
the extremely arid conditions of the lowest elevations at the driest 
time of the year. They are at least accordant with the fact that the 
soil moisture is sometimes greater on the south slopes. 

The summer averages show a difference of from 5 to 10 c.c. per day 
between the evaporation on opposed slopes, in readings of 35 to 45 c.c. 
per day or less. As the actual amounts of evaporation fall with increas- 
ing altitude, the difference between the opposed slopes becomes pro- 
portionately greater. 

As in the case of all climatological data, it would be impossible to 
state the normal conditions of evaporation at the various altitudes 


and on opposed slopes without instrumental data for several series of 
stations and covering several years. It is possible, nevertheless, to 
state from the data presented that (a) the rate of evaporation through 
the arid and humid summer seasons is about 3^ times as great on the 
desert as it is at 8,000 feet; (fc) the rates of evaporation are approxi- 
mately half as great in the humid mid-summer as they are in the arid 
fore-summer; (c) at the middle and higher altitudes the evaporation 
on north slopes is less than on south slopes; (d) the difference between 
the amounts of evaporation on north and south slopes becomes greater 
with increase of altitude, in proportion to the amounts of each. 


The prevalence of low atmospheric humidities is one of the most 
pronounced features of desert climate and is an extremely potent factor 
in causing the high rates of evaporation that have been shown to occur 
at the lowest stations in the Santa Catalina Mountains. The relative 
humidity is lowest in the arid fore-summer, although it is sometimes 
nearly as low for brief periods in the arid after-summer. During the 
two rainy seasons the humidity is extremely variable and may fluctuate 
through a daily range of as much as 70 per cent. The daily curve of 
humidity is extremely uniform during the cloudless days of April, May, 
and June, falling rapidly during the early forenoon to mid-day values 
as low as 5 and 10 per cent, and rising slowly through the late afternoon 
and more rapidly during the night to a daily maximum of 20 to 30 per 
cent just before sunrise. Cloudy days in the arid seasons cause a higher 
minimum but seldom raise the maximum above 40 per cent unless 
there is a trace of rainfall. 

The humidities of the mountain, varying with altitude and with the 
seasons, possess their greatest importance for vegetation in their role 
as joint determinants of the rate of evaporation. The altitudinal 
gradient of humidity has, therefore, been most satisfactorily investi- 
gated when it has been measured together with temperature and wind 
in the collective effect of these climatic factors upon the evaporative 
power of the air. It is not without interest, nevertheless, to know 
something of the relative humidities which are prevalent at the moun- 
tain altitudes and are partially responsible for the rates of evaporation 
encountered there. 

In spite of the pronounced altitudinal changes of vegetation and of 
climatic conditions which have been discussed (or are yet to be treated), 
there are so many features of the Encinal and Forest vegetation that 
strongly suggest the Desert (see p. 36) that it seemed particularly 
desirable to secure readings of relative humidity at the forested alti- 
tudes in the arid fore-summer. The few figures to be given here were 
secured with a sling psychrometer and converted to percentages by 
the use of Marvin's tables. 


On May 22, 1911, in the east fork of Sabino Basin, at 3,400 feet 
elevation, the 'humidity at 3 h 30 m p. m. was 6 per cent, at 6 p. m. it 
was 8 per cent, and at 8 h 30 m p. m. it was 12 per cent. At 4 h 30 m a. m., 
on the following day, the humidity had risen to 24 per cent. These 
figures show the prevalence of desert humidities at a locality which is 
low in elevation but is well in the heart of the mountain mass as a whole. 
At Marshall Gulch, at 7,600 feet in the Forest region, on May 20, 1911, 
the humidity at ll h 30 m a. m. was 10 per cent, and it was the same at 
4 h 30 m p. m. At 9 h 15 m a. m. on the following day the humidity was 
16 per cent, and at 3 p. m. it was 11 per cent. Although these figures 
are roughly twice those of the readings in Sabino Basin, they are never- 
theless indicative of a low humidity for a forested locality and show 
that in the arid fore-summer there are days on which the humidity is 
nearly as low as it is on the Desert. 

A number of humidity readings were taken in June 1911, but none 
of them showed as low values as those just mentioned. In Bear Canon, 
at 6,100 feet elevation, on June 21 (a dull and intermittently cloudy 
day), the humidity was 46 per cent at 1 p. m. and 42 per cent at 3 p. m., 
falling to 41 per cent at 7 p. m. In Marshall Gulch on June 23 (a nearly 
cloudless day), the humidity at 5 a. m. was 33 per cent and it fell 
steadily to 22 per cent at 12 noon, with a temporary rise during an 
interval of cloudiness at 10 a. m. In the afternoon the percentages 
rose from 25 per cent at 3 p. m. to 29 per cent at 6 p. m., but fell again 
to 26 per cent at 8 p. m. The highest humidity observed at Marshall 
Gulch was 48 per cent at 4 h 30 m p. m. on June 8, 1911, after the summit 
of the mountain had been covered several hours with cumulus clouds. 

Continuous records of relative humidity have been secured in yellow 
pine forest at the Fort Valley Experiment Station at 7,200 feet eleva- 
tion, near Flagstaff, Arizona, by Pearson.* The monthly mean values 
for May and June (1909-1912) are 38 and 34.9 per cent respectively. 
Some of the lowest extremes involved in the composition of these means 
have been kindly communicated to the writer by Pearson. The number 
of days in June on which the humidity fell to 16 per cent or less was 
as follows: 8 days in 1909; 11 days in 1910; 6 days in 1911. The lowest 
humidities were a single occurrence of 10 per cent and two occurrences 
of 11 per cent. Humidities as low as 11 per cent also occur in July, 
and values as low as 16 per cent occur between May and October. 

These figures for the Coconino Plateau are in agreement with the 
lowest figures secured at Marshall Gulch in May 1911, and show that 
desert humidities are of frequent occurrence in the arid fore-summer, 
both on isolated mountains, such as the Santa Catalinas, and on ex- 
tended plateaus in the midst of the arid region. On the days that 
exhibit such low humidities at higher elevations there is practically a 

* Pearson, G. A. A Meteorological Study of Parks and Timbered Areas in the Western 
Yellow Pine Forests of Arizona and New Mexico. Mo. Weather Rev., 41: 1615-1629, 1914. 


flat altitudinal gradient of humidity. The difference between the 
observed humidities of 6 per cent in Sabino Basin and 12 per cent at 
Marshall Gulch is a very small one, and would doubtless register very 
small differences on the rate of evaporation under otherwise identical 
conditions. The differences in evaporation actually found to exist 
between the base and summit of the mountain are to be ascribed to the 
nocturnal humidities, which are higher in the Forest than on the Desert, 
to the greater frequency of cloudiness at higher elevations in the arid 
fore-summer, to the lower temperatures at higher altitudes (especially 
at night), and to the wind protection afforded by the forest cover itself. 


The investigation of temperature on the Santa Catalinas has been 
carried on with a view to determining the decrease in length of the 
frostless season which accompanies increase of altitude, the normal 
decrease of temperature with increasing altitude, and the departures 
from the normal gradient of decrease which are due to the nature of 
the topographic relief and to other causes. The results secured afford 
an outline of the major temperature features which are capable of 
influencing the distribution or seasonal activities of the plants of the 
Desert, Encinal, and Forest regions. 

The character of the temperature conditions, and their relation to 
altitude and topography, in an isolated desert mountain is not without 
complexities which make it impossible to predict the conditions for 
vegetation in a given locality through a knowledge of its altitude 
and of general meteorological theory. The relative smallness of the 
entire mountain mass and its position in the midst of arid plains make 
its temperature conditions very different from those of extensive 
plateaus of the same elevation. The currents of warm air which ascend 
by day and the streams of cold air which descend by night serve to 
increase the diurnal amplitude of temperature in certain situations and 
to give striking differences within very short distances. Differences 
of slope exposure bring about differences of diurnal warming and 
nocturnal cooling of the soil, and these differences affect the general 
temperature conditions and also directly influence the vegetation. 
The differences of diurnal warming and nocturnal cooling which exist 
between the relatively bare soils of the Desert and Encinal regions 
and those of the Forest, with their heavy cover of vegetation, their 
litter of leaves and high humus content, are also considerable and tend 
to lessen the importance of topography at the higher elevations. 

Temperature readings have been secured at two localities, at ele- 
vations of 5,300 and 7,600 feet, respectively, since the early summer of 
1908. Since 1911 a series of thermometers has been exposed at the 
rainfall situations at 4,000, 6,000, and 7,000 feet, and during 1913 and 
1914 a complete series of thermometers was maintained at 1,000-foot 


intervals from 4,000 to 9,000 feet. All of the instruments in this series 
were located on the summits of ridges, so as to give comparable readings 
from similar topographic situations. In addition to the six instruments 
in this series there were also thermometers in the bottoms of canons at 
5,000, 6,000, and 7,600 feet; a thermometer was exposed on the top 
of the fire tower of the Forest Service on Mount Lemmon, the actual 
elevation of the instrument being 9,225 feet, and thermometers were 
buried in the topmost layer of soil at 6,000 and 8,000 feet. 

Alcoholic minimum thermometers were used in the earlier years of 
these observations, but were replaced by mercuric Six's thermometers 
in 1913 and 1914. Various types of thermometer have been used at 
the station at 7,600 feet, and as many as three instruments have been 
exposed simultaneously at that place. All thermometers have been 
calibrated before use and have been verified in place from time to time 
by comparison with a portable thermometer of known error. The 
readings of the thermometers have been taken at irregular intervals, 
as opportunity afforded, and most of the figures secured are for periods 
of several weeks, or for the several months which elapse between the 
last visit in the autumn and the first in the spring. Only at the 7,600- 
and 9,000-foot stations has it been possible to expose the thermometers 
in such a manner as to secure reliable maxima; at all other stations 
the only data secured have been the absolute minima for the intervals 
between visits. The placing of the thermometers in small boxes, with 
numerous perforations, has made possible the securing of good minima, 
but no record has been made of the maxima secured under such con- 
ditions of exposure. The conspicuousness of adequate instrument 
shelters would have invited human interference with the thermometers 
which would have been productive of errors. 

A few records of temperature from the same locality for a number 
of consecutive days have been secured by Professor J. G. Brown and by 
Dr. H. A. Spoehr, as well as by the writer. A large number of single 
observations of minima and of current temperatures have been made 
by the writer at various localities, and it has been possible to use these 
in connection with data from the regular stations in determining the 
normal gradient of temperature decrease and in ascertaining the verti- 
cal shortening of the f restless season. 


It has been impossible, for the most part, to make direct observations 
of the dates of last vernal and first autumnal occurrence of a tempera- 
ture of 32 F. at the several stations on the Santa Catalinas. The 
dates at which visits were made to the mountain were occasionally 
such as to establish the dates exactly for one of the stations, and in 
several cases visits were made at such frequent intervals as to place 
the date within a week or two. In the majority of cases, however, 



the frost dates which limited the growing season fell between the last 
visit of autumn and the first one of spring. This has particularly been 
the case with all of the lower stations. These circumstances have 
made it necessary to resort to an indirect method of determining the 
dates, which is as follows: A series of graphs was drawn showing the 
march of the weekly absolute minimum temperatures at the Desert 
Laboratory, as registered by thermograph, for the years covered by 
the mountain records. Each reading of minimum temperature for a 
given station was then compared with the minimum for the same 
period at the Desert Laboratory, and the total number of such differ- 
ences was averaged. In this manner it was possible to secure the 
figures given in tables 10 and 11. 

TABLE 10. Allitudinal shortening of the frostless season in the Santa Catalina Mountains, 
as shown by the dates of the last spring occurrence and the first autumn occurrence of a 
temperature of 32 at 3 altitudes in 1908, 1909, and 1910. 

Station, etc. 


Last 32 
in spring. 

First 32 
in autumn. 

Desert Laboratory; elevation, 2,663 
feet; length of frostless season, 285 


Feb. 20 
Mar. 15 
Feb. 25 

Nov. 30 
Nov. 30 
Dec. 31 

Average dates 

Mar. 1 

Dec. 10 

Xero-Montane Garden; elevation, 
5,300 feet; length of frostless season, 
195 days. 


May 10 
Apr. 12 
Apr. 20 

Oct. 10 
Nov. 10 
Nov. 26 

Average dates 

Apr. 24 

Nov. 5 

Montane Garden, Marshall Gulch; 
elevation, 7,600 feet ; length of frost- 
less season, 126 days. 


June 15 
May 31 
May 9 

Sept. 25 
Sept. 26 
Oct. 17(7) 

Average dates 

May 29 

Oct. 2 

With a knowledge of the average difference between the minimum 
temperature at the Desert Laboratory and at a given station, and with 
the graph showing the march of minima at the Laboratory, it was 
possible to locate the approximate date of the last and first occurrence 
of 32 at the mountain station. Such a graph for 1911 is given in 
figure 17, together with the graph of march of minimum temperatures 
at the Montane Plantation in Marshall Gulch, at 7,600 feet. It will 
be noted that the graph for the Laboratory rises by several pronounced 
jumps during March, April, and May, and falls by precipitate stages 
during September, October, and November. The relatively sudden 
advent of summer and of winter is an invariable annual occurrence, 
and it has helped to make more accurate the estimation of the limiting 
dates for the mountain stations. In the cases in which a minimum 



temperature of 32 or less was registered at a station during an interval 
of two or three weeks, the date of such minimum could be determined 
by finding the exact date of the lowest temperature for the same period 
at the Desert Laboratory, and such determinations undoubtedly have 
a very slight possibility of error. Taking into account the number of 
direct observations and the larger number of estimations, the limiting 
dates of the frostless season, given in tables 10 and 11, may contain 
errors of as much as 7 to 10 days. The swamping of these errors by 
averaging the dates for the 6 years of observation reduces the probable 
error to about 5 days. 

TABLE 11. The altititdinal shortening of the frostless season in the Santa Catalina Mountains, 
as shown by the dates of the last spring occurrence and the first autumn occurrence of a 
temperature of 32 at 5 altitudes in 1911, 1912, and 1913. 

[Data for 9,000 feet are partially interpolated.) 



Last 32 
in spring. 

First 32 
in autumn. 

Desert Laboratory; elevation, 2,663 
feet; length of frostless season, 282 


Feb. 20 
Feb. 26 
Mar. 31 

Dec. 25 
Dec. 9 
Dec. 8 

Mar. 7 

Dec. 14 

Climatological Station; elevation, 5,000 
feet; length of frostless season, 248 


Feb. 27 
Apr. 15 
Apr. 14 

Dec. 4 
Dec. 9 
Nov. 21 

Mar. 30 

Dec. 1 

Climatological Station ; elevation, 7,000 
feet; length of frostless season, 187 


Apr. 3 
May 13 
May 5 

Oct. 30 
Nov. 18 
Oct. 13 

Average dates 

Apr. 27 

Oct. 31 

Marshall Gulch; elevation, 7,600 feet; 
length of frostless season, 148 days. 


May 16 
May 18 
May 9 

Oct. 30 
Oct. 2 
Sept. 26 

Average dates 

May 14 

Oct. 9 

Mount Lemmon; elevation, 9,000 feet; 
length of frostless season, 122 days. 


May 29 
May 20 
June 16 

Oct. 16 
Oct. 7 
Sept. 8 

Average dates 

Sept. 30 

In figure 2 are shown separately the curves for altitudinal shortening 
of the frostless season for the years 1908, 1909, and 1910, and the years 
1911, 1912, and 1913. In the latter group of years the advent of spring 
was nearly a month earlier at the middle altitudes than it was during 
the former years, and the advent of winter was correspondingly later, 
in spite of the fact that the frostless season was of approximately the 
same length at the Desert Laboratory during the two periods. A con- 



sideration of the two sets of curves is more fruitful than the possession 
of their average, as it shows the extent of a fluctuation which must 
be a common and normal feature of the climatology of the mountain, 
just as it is of every locality, regardless of its topographic location. 

The average length of the frostless season at the Desert Laboratory 
is about 38 weeks, at 5,000 feet in the Santa Catalinas it is about 
30 weeks, while at 8,000 feet it is about 17 weeks. The period of safe 
plant activity is, therefore, less than half as long in the Forest region 
of the mountain as it is in the Desert of the Santa Cruz Valley. The 
altitudinal abbreviation of the frostless season is of primary importance 
to vegetation, especially as it is accompanied by a series of inseparable 
features of temperature, such as the lower range of the entire daily 
curve of temperature, the attainment of lower minima, the more 
































Flo. IS. Schematic representation of length of frostless season at different altitudes in Arizona, 
together with curves for limits of growing season at successive altitudes in the Santa Cata- 
linas for 1909 to 1914 inclusive. 

frequent occurrence, and the longer duration of freezing temperatures. 
One of the cardinal features of importance in the altitudinal shortening 
of the growing season is the concomitant shortening of the arid fore- 
summer. The rising temperatures of spring call the vegetation of the 
Desert into activity at a tune of the year when extremely arid condi- 
tions are bound to prevail for from 14 to 16 weeks. At the summit oi 
the mountain, however, the advent of spring is only 5 or 6 weeks in 
advance of the earliest of the mid-summer rains. In other words, the 
most trying season of the year is shortened on the mountain by the 
inhibitory effects of low temperatures, so that the arid fore-summer is 
only one-third as long in the Forest as in the Desert. These relations 
are graphically represented in figure 2, which shows both the curves 
of the frost dates and the incidence of the rainy seasons. More will 
be said in regard to this subject in a succeeding section (see p. 93). 



In order to compare the altitudinal shortening of the frostless season 
on the Santa Catalinas with the same datum for a series of valley 
stations located at progressive altitudes, figures were collected which 
are shown in table 12. These figures are based on the last vernal and 
first autumnal occurrence of a temperature of 32, for 1903 to 1912 
inclusive, without regard to the reports of frost made by the voluntary 
observers at these stations. In figure 15 the length of the frostless 
season at the several stations is graphically shown by horizontal lines, 
and the limits of the frostless season for the Santa Catalinas (for 1909 
to 1914) are shown by oblique lines. 

Spring opens at an earlier date at 3,000 and 4,000 feet on the Santa 
Catalinas than it does at Tucson and Benson, but at 4,000 and 5,000 
feet it does not open at so early a date as it does at Cochise and Fort 
Huachuca. At all four of the elevations mentioned the close of the 

TABLE 12. The altitudinal shortening of the frostless season in southeastern Arizona, as 
shown by the dates of the last spring occurrence and the first autumn occurrence of a tempera- 
ture of 32 at eight stations at graduated altitudes, in the decade of 1903 to 1912. 


Last 32 
in spring. 

First 32 
in autumn. 

Length of 
season (days). 

Jan. 28 

Dec. 18 


Phoenix 1 108 feet . 

Feb. 1 

Dec. 18 


Tucson, 2,390 feet 

Mar. 15 

Nov. 19 


Benson 3 523 feet 

Mar. 26 

Nov. 7 


Cochise, 4 219 feet 

Mar. 10 

Nov. 6 


Fort Huachuca 5 100 feet 

Mar. 26 

Nov. 8 


Flagstaff 6 907 feet 

June 11 

Sept. 24 


Chlarson's Mill, 7,200 feet * 

May 13 f 

Oct. 19 f 


* The elevation of Chlarson's Mill is reputed by the proprietor to be 8,000 feet, and it is so 
stated in the publications of the Weather Bureau. Several aneroid determinations by the writer 
indicate that it is approximately 7,200 feet. 

t These dates are based on an incomplete record. 

growing season comes sooner at the valley stations than it does on the 
mountains. The length of the frostless season at Flagstaff is notably 
shorter than it is at the same elevation in the mountains. 

The advent of spring is retarded at Tucson and Benson by the cold- 
air drainage of the Santa Cruz and San Pedro rivers respectively. 
Cochise is situated in the middle of the eastern bajada of the Dragoon 
Mountains and Fort Huachuca at the top of the northern bajada of 
the Huachuca Mountains. Each of these stations is therefore removed 
from the operation of cold-air drainage, as is manifested by the failure 
of their greater altitude to impose upon them shorter frostless seasons 
than those of Tucson and Benson. 

The length of the frostless season at Marshall Gulch and at the 
similarly situated mountain station at Chlarson's Mill (Pinaleno Moun- 
tains) is greater than at Flagstaff, which is at a slightly lower altitude. 
This is to be attributed partly to the higher latitude of Flagstaff, but 



chiefly to its location on an extensive elevated plateau and to its 
proximity to the cold-air flow from the San Francisco Peaks and other 
neighboring elevations. 

It may be said, in general, that the frostless season is longer on the 
ridges of an isolated mountain than it is in adjacent valleys at the 
same elevations. Although the advent of spring at Cochise and Fort 
Huachuca is earlier than on the mountains, the arrival of autumn is 
earlier also, so that these two stations show an equality in length of 
frostless season with the mountain ridges without a correspondence 
with them in the dates of commencement and close. 


The temperatures which have been secured on the Santa Catalinas 
do not form an altogether satisfactory basis for the determination of 

TABLE 13. Average differences between all observed minimum temperatures at stations situ- 
ated on ridges in the Santa Catalina Mountains and the minima for the same days or 
periods at the Desert Laboratory. 







4,000 feet... 
5,000 feet... 


+ .1 

8 1 

+ 1.9 
8 1 

6,000 feet... 
7,000 feet... 
8,000 feet... 



20 1 

20 1 

9,000 feet... 




[The plus sign indicates a higher temperature at the mountain station.] 

the normal altitudinal gradient, a datum which should be derived from 
extended series of mean temperatures. However, in the absence of an 
ideal collection of records from the several altitudes these imperfect 














Fio. 16. Graph showing altitudinal fall in temperature in the Santa Catalina Mountains 
(A), and lines showing rate of fall in free air (E) on Pike's Peak (C), on the Sierra 
Nevada (D), and average rate for 17 extra-tropical mountains (B). 

data have been used as the basis of an approximate determination of 
the gradient of fall of temperature with increase of altitude. The 



minimum temperatures at the several stations are the only ones that 
have been used, and they have been in all cases compared with the 
minimum for the same period at the Desert Laboratory. The average 
differences between the minima of the mountain stations and those of 
the Laboratory are shown in table 13, and it is these differences that 

TABLE 14. Daily maximum and minimum temperatures at Marshall Gulch (7,600 feet) and 
at the Desert Laboratory for June, July, and August 1911. 












D. L. 


D. L. 


D. L. 


D. L. 


D. L. 


D. L. 



















5th. . . . 





















































































Average ) 

21 days, 30.5 

20 days, 25.0 

20 days, 24.1 

21 days, 29.4 

23 days, 26.1 

17 days, 27.3 

have been used in the construction of the graph in figure 16, which 
shows the gradient for the Santa Catalinas, the average gradient for 
17 mountain ranges in different portions of the world (according to 
Hann), the gradient for Pike's Peak, the Sierra Nevada, and also the 
gradient in the free air, as determined at the Blue Hill Observatory. 



Some estimate of the error involved in basing the gradient only on 
minimum readings may be made by the figures presented in tables 
14 and 15. These tables exhibit the only daily records of maximum and 
minimum temperatures for the Catalinas for any period longer than 
a few days. The average maxima and minima for Marshall Gulch for 
the months of June, July, and August have been contrasted in table 14, 
with the average maxima and minima for the Laboratory. During 
June the apartness of the minima was 30.5, of the maxima 25.0. In 
July this relation was reversed, the apartness of the minima being 
24.1, that of the maxima 29.4, while for August the two were more 
nearly the same, the apartness of maxima being 26.1, of maxima 27.3. 
The facts that the minimum temperatures of valley and mountain 
were further apart than the maxima were during June, and not so 
far apart in July and August, may be connected in some manner with 
the clear dry weather of June and the rainy, cloudy character of July 
and August. However, the data in table 15, showing the maximum 

TABLE 15. Daily record of maximum and minimum temperatures for a portion of June 
2, at summit of Mount Lemmon, with corresponding data for the Desert Laboratory. 

Day of mouth. 


M aximuiQ . 

Day of month. 



M. L. 

D. L. 


D. L. 


D. L. 


D. L. 

40 5 




June 13 










10. . . 










Average difference of maxima, 33.9; of minima, 31.1. 

and minimum temperatures on Mount Lemmon in June 1912, indi- 
cate that there was a greater apartness of maxima than of minima 
when these data are averaged and contrasted with those for the Desert 

It would require a much fuller mass of figures than are in hand to 
make any attempt at an explanation of the differences that exist in 
the apartness of desert and mountain maxima and minima in different 
localities and different months. For the present purpose it is instruc- 
tive to average the entire set of apartnesses for Marshall Gulch for 
June, July, and August 1911, which gives an average difference of 
minima of 80.7, and of maxima of 81.7. In other words, throughout 
a series of several months there is doubtless a swamping of the irregu- 
larity of the apartnesses for individual months. If, then, there is an 
average equality between the apartness of maxima and minima which 
is to say that there is an equality of daily mean range of temperature 
between desert and mountain it would indicate that the minima are 



just as good data on which to base an estimation of the gradient as 
mean temperatures would be. Since the figures used in constructing 
the gradient were secured in all months from April to October, in 
several years, and in a wide diversity of localities all outside the 
influence of cold-air drainage it is probable that the gradient here 
presented, figure 16, is within one or two degrees of the same measure 
of accuracy that could be secured by a long series of consecutive read- 
ings of maximum and minimum. 

Using the elevation of the Desert Laboratory (2,663 feet) as a base, 
the actual fall of temperature between that locality and the 9,000-foot 
station on the Santa Catalinas is at the rate of 4.11 per 1,000 feet. 
The gradients between the several mountain stations are indicated in 

FIG. 17. Graphs showing march of weekly minimum temperature at 
Desert Laboratory (upper) and weekly or other minimum tempera- 
ture at Marshall Gulch for 1911 (lower). 

figure 16, on which it will be seen that there is a negative gradient 
between the Desert Laboratory and the 4,000-foot station, and that 
the gradient between 4,000 and 5,000 feet is at the rate of 10 per 1,000 
feet. From 5,000 feet upward the gradients are more uniform and 
more nearly equal between the successive stations. 

Figures are given by Hann * for the gradients of temperature on a 
number of mountains in different parts of the world. The average 
values for 17 extra-tropical mountains is 3.13 F. per 1,000 feet (0.57 C. 
per 100 m.). The only mountains in the western United States for 
which Hann gives figures are Pike's Peak and Sierra Nevada (Colfax, 
Placer County). The gradient of the former is 3.46 per 1,000 feet 

* Hann, J. Handbook of Climatology. Translation by R. DeC. Ward, pp., 243-246. 
York, 1903. 



(0.63 C. per 100 m.), and for the latter 4.12 per 1,000 feet (0.75 C. 
per 100 m.). The gradient for Colfax is seen to be almost exactly 
coincident with the entire gradient for the Santa Catalinas. It is 
interesting to note, in this connection, that the fall of temperature in 
the free air has been determined at the Blue Hill Observatory to be 
2.5 F. per 1,000 feet, which is far more gradual than any of the moun- 
tain gradients that have been cited. The Blue Hill data apply only 
to low elevations, but are in substantial agreement with figures more 
recently secured in the free air at Avalon, California.* Seven balloon 
ascensions from Avalon to elevations of 18 km. and higher showed a 
mean gradient of fall in the first 3 km. (9,842 ft.) of 2.2 per 1,000 feet. 
These two determinations of the free-air gradient indicate a conserva- 
tism of temperature change in the lower atmosphere as compared 
with the changes on the slopes of mountains. 

While the normal temperature gradient is of profound interest from 
the standpoint of pure climatology, it is nevertheless of subsidiary 
value in the study of climate in relation to vegetation. Its chief value 
is as a basis with which to compare the differentiation of temperature 
conditions originating in the irregularities of topography and other 
causes. In later pages the subsidiary influences upon the temperature 
gradient will be discussed. 


The absolute minimum temperature of the winter was secured at 
5,300 feet and at 7,600 feet for four winters, and during the winter of 
1912 and 1913 was secured at four stations, and during the succeeding 
winter at 10 stations, differing both in altitude and in topographic 

The winter of 1912 and 1913 was one of exceptional severity at 
Tucson in fact throughout the extreme southwestern United States 
while the winter of 1913 and 1914 was one of the customary modera- 
tion. The data for these two winters are calculated, therefore, to 
exhibit the extreme and the average conditions of winter temperature 
for stations in Arizona. 

The minimum temperature readings at the mountain stations are 
given in table 16; and in table 17 are given the minima for December, 
January, and February of the same years for a selected series of stations 
in Arizona. The lowest temperature recorded on the Santa Catalinas 
in 1912-13 was 6 at 6,000 feet, while the lowest temperature at 
the highest station, at 7,600 feet, was 2. This figure should be con- 
trasted both with the absolute minimum at the Arizona Experiment 
Station, 6, and with that at the office of the Desert Laboratory, 1, 
as well as with that for Flagstaff, 23, situated in northern central 

* Blair, William R. Free-Air Data in Southern California, July and August, 1913. Mo. 
Weather Rev., 42: 410-426, 1914. 



Arizona at an elevation 700 feet lower than that of the 7,600-foot 
Station in Marshall Gulch. The extremely low temperatures at the 
Arizona Experiment Station and the office of the Desert Laboratory, 
as contrasted with the minimum temperature of 17 at the Desert 

TABLE 16. Absolute minimum temperatures of two winters at stations in the Santa Catalina 

Mountains and at Tucson. 


Location of station. 



Arizona Experiment Station 
Office of Desert Laboratory 

Near bottom of Santa Cruz Valley . 
Edge of flood-plain of Santa Cruz 




Slope of bill, 335 feet above Santa 

Cruz Valley 



4 000 feet 

Ridge, in Lower Desert Region .... 



5 000 R 

Slope, 125 feet above floor of Soldier 



5 000 V 

Floor of Soldier Cafion, in Lower 


6 000 R 

Ridge in Upper Encinal 



6 000 V 

Floor of Bear Cafion 


7 000 

Ridge, north rim of Bear Cafion, in 



7 gOO 

Bottom of Marshall Gulch, in Fir 


15 5 


Ridge, north rim of Marshall Gulch, 



North slope, below summit of Mount 

9 225 

Lemmon in heavy Fir Forest .... 
Top of fire-tower on summit of 



Laboratory, are to be accounted for through the operation of cold-air 
drainage. On the coldest night in January 1913 there was a difference 
of only 3 between the temperatures of the Santa Cruz Valley and 
Marshall Gulch, 5,400 vertical feet apart. It is desired here not so 

TABLE 17. Absolute minimum temperatures of winter months for two writers at selected 
stations in Arizona, together with the absolute winter minima at 7,600 feet in the Santa 
Catalina Mountains. 










Santa Catalinas, 7,600 feet 


15 5 

Tucson, 2,390 feet 


- 7 
- 4 






Chlarson's Mill, 7,200 feet 

Flagstaff, 6,907 feet 

- 6 

- 8 

- 7 

- 5 

- 1 


~ 4 

Fort Valley Experiment Station, 7,300 feet 

Fort Apache, 5,200 feet 

Fort Huachuca, 5,100 feet 

Snowflake, 5,644 feet 

much to lay emphasis on the exceptional coldness of the Santa Cruz 
valley, but on the exceptional warmness of the summit of the Santa 
Catalinas, as contrasted with similar elevations in Arizona which are 


very dissimilar in their topographic location. In table 17 it will be 
seen that the winter minimum at Flagstaff, in northern central Arizona, 
was 21 below the minimum for Marshall Gulch, although Flagstaff is 
located 700 feet lower. This relation is similar to that which exists 
between the length of frostless season at Marshall Gulch and at Flag- 
staff, and is due to the facts mentioned in that connection on page 74. 
At the Fort Valley Experiment Station, located in the vicinity of 
Flagstaff and nearer to the San Francisco Peaks, the winter minimum 
was 4 lower than at Flagstaff. At Snowflake, on the extensive Mogol- 
lon Plateau, and at Fort Apache, in the cold-air drainage of one of the 
main forks of the Salt River, there were also minima which were 
respectively 9 and 3 lower than at Marshall Gulch, although these 
stations are respectively 2,000 and 2,400 feet lower than Marshall 
Gulch. Fort Huachuca is located at the base of the Huachuca Moun- 
tains in such a manner as to escape cold-air drainage from any of the 
large canons of that range of mountains, and its absolute minimum was 
6 higher than that of Fort Apache, which is of approximately the same 
elevation. Chlarson's Mill is situated in Frye Canon in the Pinaleno 
(Graham) Mountains, surrounded by heavily forested slopes. Its loca- 
tion is analogous to that of Marshall Gulch, being similarly situated 
in an isolated desert mountain and surrounded by heavily forested 
slopes. The single monthly minimum available for Chlarson's Mill 
is 15, for a month in which the minimum at Tucson was 18, while 
it was 7 for the Fort Valley Experiment Station, at almost the same 
elevation as Chlarson's Mill. 

An inspection of the absolute minima for 1913-14, in table 17, will 
show that the same relations hold true between the several stations 
that have just been discussed. The winter minimum for Tucson, 26, 
was much higher than in the preceding winter, and so was that for 
Marshall Gulch, 15.5, although the absolute minimum in the new 
station on the fire tower at Mount Lemmon was 3, and in the heavy 
timber on the north face of Mount Lemmon was 5. 

The data just discussed amply bear out the statement that the lowest 
temperatures of winter are less severe on the Santa Catalinas than 
they are at the same elevation on the plateau of north-central Arizona, 
and even less severe than they are in many situations of lower altitude. 
The fragmentary records of earlier years at Chlarson's Mill, which 
are not given here but are available in publications of the Weather 
Bureau, show that it is likewise favored by lower winter temperatures 
than are the plateau stations of the same elevation in Arizona. The 
length of the frostless season has just been shown to be less in the Santa 
Catalinas and in the Pinaleno Mountains than at Flagstaff. In short, 
the indications are very strong that all phases of winter temperature 
conditions are less severe on the small and isolated desert mountains 
than on the plateaus and highlands of the same elevations and of nearly 


the same latitude. The radiation of the desert valleys and the diurnal 
convection currents of warm air are not without a strong ameliorating 
influence on the climate of elevated but small masses of land. 


The ideal conditions of uniform decrease of temperature with increase 
of altitude are seldom actually encountered in nature, at least not in 
mountains of small size and rugged topography. The principal factor 
which brings about departures from the normal or ideal gradient of 
fall is the operation of cold-air drainage, or inversion of temperatures. 
This is a phenomenon which has long been known and has frequently 
been discussed with respect to its influence on vegetation. In an 
earlier paper * the writer has described some observations of tempera- 
ture inversions at the Desert Laboratory and in the Santa Catalinas, 
and has pointed out the causes involved in making cold-air drainage 
much more pronounced in deserts than it is in humid and forested regions. 

The scanty vegetation of the desert subjects its soil, rocks, and sands 
to full insolation and to a pronounced heating throughout the day. 
The dark rocks of Tumamoc and other volcanic hills in its vicinity 
become so hot during the long clear days of May and June that it is 
impossible to hold one's hand on them without pain. During the day 
there is a constant and active radiation of heat from the rocks and soil, 
which warms the lowest layers of air and causes a convectional heating 
of the lowest portion of the atmosphere. Immediately after sunset 
the warmed surfaces become rapidly cooler and the rate of radiation 
is quickly reduced. The air nearest the cooling rocks and soil becomes 
cooler than the air above it, and consequently begins to fall by gravity 
before there is opportunity for it to mix with the warmer air above. 
This cooled air descends from hillsides and even from gentle slopes 
and soon collects in valleys and depressions, where it results in a slowly 
or rapidly moving mass of air which is appreciably cooler to the senses 
than is the air of the slopes or hillsides. The inversion of temperature 
thus caused usually reaches its maximum during the first half of the 
night, although this is determined in great measure by the size of the 
drainage area. 

It is only on clear and still nights that cold-air drainage operates 
in the most pronounced manner. A high wind will disturb the flow 
or completely eliminate it. Heavy cloudiness will cause the rate of 
radiation to lag so that there is time for an admixture of cool and warm 
air, thereby preventing the flow of cold air or greatly reducing it. 

On clear nights which follow heavy rains the inversion of temperature 
will be reduced to a negligible amount, because of the increase of the 
specific heat of the soil brought about by its becoming wet. 

Shreve, Forrest. Cold Air Drainage. The Plant World, 15: 110-115, 1912. 



It has been shown in the paper to which reference has been made 
that the Desert Laboratory is situated well above the level of the 
cold-air flow of the Santa Cruz Valley, 335 feet below it. The greatest 
observed difference of minimum temperature in a single night between 
the Laboratory and the Valley was 24, and the greatest difference 
between the mean monthly minima of the two localities was 17.8 for 
May. During the humid mid-summer the mean monthly difference 
falls to 8 and 9 for these stations. 

TABLE 18. Minimum temperature records to show the operation of cold-air drainage in the 
open vegetation of Soldier Canon and Bear Canon and its abeyance in the heavy forest of 
Marshall Gulch. 

In each case one record is from the floor of the cafion and the other from its slopes or rim. The 
minus differences indicate a higher temperature on the floor and the absence of cold-air 


Slope or 



Soldier Cafion, floor at 4,900 feet, slope at 5,025 feet: 
Sept. 27, 1913, to May 16, 1914 



May 17 to 19, 1914 




May 20 1914 . 




May 21 to July 22, 1914 . 




July 23 to 28, 1914 



July 29 to Aug 8 1914 . 




Aug. 9 to Oct. 10, 1914 



Bear Canon, floor at 6,000 feet, rim at 7,000 feet: 
Sept. 24 to Sept. 26, 1913 . . . 




Sept 27 1913 to May 17 1914 




May 18 to 19. 1914 . 




May 20 to July 23, 1914 




July 24 to 27 1914 




Marshall Gulch, bottom at 7,600 feet, rim at 8,000 feet: 
Sept. 25, 1913 . 

34 5 



Sept. 26, 1913 

30 5 



Sept. 27, 1913, to May 17, 1914 




May 18 to July 24, 1914 

33 5 



July 25, 1914.. . ... ... 




July 26, 1914 



- 2 

July 27, 1914 

51 5 

51 5 

July 28 to Oct. 11, 1914 . . . 




The vigor of cold-air drainage is determined not only by the condi- 
tions of cloudiness and wind but also by the size and nature of the area 
from which the cold air is derived and by the character of the valley 
bottom through which it moves. In the Santa Cruz Valley cold air 
is derived from an area of more than 1,000 square miles, resulting in 
the pronounced low temperatures shown in tables 16 and 18. The 
broad level trough of the valley is conducive to a slow movement of 
the air, and the nocturnal minimum is usually reached during the last 
hours of darkness. The valleys of the Salt and Gila Rivers are larger 
than the valley of the Santa Cruz, and they have their sources in still 
higher mountains, but they do not seem to possess a well-marked 


cold-air drainage, to judge by the minimum temperature records for 
the towns on the lower courses of these rivers, as Florence and Phoenix. 
It is possible that in traveling long distances at nearly the same altitude 
the cold air is gradually warmed by mixture with the warm air above it. 
During the winter of 1913-14 and the summer of 1914 thermometers 
were exposed in the Santa Catalinas so as to give a basis for comparing 
the cold-air drainages of the mountain with the drainage of the Santa 
Cruz Valley as investigated at the Desert Laboratory. Table 18 shows 
the readings of instruments placed so as to reveal the differences of 
temperature due to cold-air drainage, at three localities of different 
elevation. The first locality is in Soldier Canon, in the open Encinal, 
where readings were taken on the floor of the canon, at 4,900 feet and 
on its slope at 5,025 feet. The lowest minima of the winter were 
identical at the two stations, which can be accounted for only on the 
possibility of the stream of cold air having become so deep as to reach 
the upper thermometer, or else on the possibility that the lowest mini- 
mum of the winter occurred on a cloudy or very windy night. During 
three intervals in the arid fore-summer the depression of the temperature 
in the floor of the canon was 6.5, 8, and 6 respectively, whereas through- 
out the humid mid-summer the depression was absent or negligible. 

The regular 7,000-foot station is located on the rim of Bear Canon, 
and the data from it may be compared with those from an instrument 
placed in the floor of the canon 1,000 feet below. This station may- 
further be compared with the 6,000-foot station located on the summit 
of Manzanita Ridge. In spite of the difference of 1,000 feet in the 
elevation of the two thermometers in Bear Canon, the lowest tempera- 
ture of the winter was 6 in the Canon and 12 on the rim. A two-night 
interval in the autumn of 1913 gave a difference of 5 between these 
stations, due to air drainage, and during the arid fore-summer of 1914 
differences of 9 and 4 were obtained. During the three particularly 
cloudy and rainy nights in July there was an actual reversal of the 
conditions of cold-air drainage and a manifestation of the true tempera- 
ture conditions to be expected from altitude alone, the rim having a 
minimum 2 lower than the floor. Although the winter minimum of 
1913-14 was 6 in the floor of Bear Canon at 6,000 feet, it was 18 on 
Manzanita Ridge at 6,000 feet. This difference of 12 between two 
stations at the same altitude is as great as should be expected, under 
the operation of the normal gradient of temperature, between localities 
3,468 vertical feet apart (12 -^3.46, the rate of fall per 1,000 feet). 

Although there are some small bodies of forest on the walls of Bear 
Canon and many scattered trees, nevertheless the surface of its sides 
is largely occupied by cliffs and boulders (see plate 21), and these are 
responsible for the acute operation of the drainage phenomenon. 

Several preliminary tests had shown an extremely weak manifesta- 
tion of cold-air drainage at the heavily forested elevations of the Santa 


Catalinas. The data presented in table 18 for the rim of Marshall 
Gulch at 8,000 feet, and for the Montane Garden at 7,600 feet, bear 
out the results of the preliminary tests. The rim was 10 colder than 
the bottom of the gulch in the over- winter period and 18 colder in 
the period from May 18 to July 24. The 5 one-night readings and the 
readings for the period from July 28 to October 11 all show an equality 
or a slight difference, more often a difference, with the temperature 
more commonly higher in the bottom of the Gulch than on the rim. 
In other words, cold-air drainage is in abeyance at this locality. 
Whether this is invariably the case can only be stated after further 
instrumentation and after complete assurance that the readings have 
not been influenced by the character of the weather during the nights 
of lowest temperature. The heavy vegetation of the Forest region, 
together with the high humus content of the soil and the litter of leaves 
by which it is covered, all militate against the rapid terrestrial radia- 
tion in which cold-air drainage has its origin. It will not be surprising, 
therefore, to find that the phenomenon is either very weak or absent 
above the elevation of 7,000 feet in such portions of the mountain as 
are forested. The elimination of cold-air drainage by a forest cover 
can take place only in small mountains which are forested to the sum- 
mit. A large mountain mass, an extremely steep mountain side, or 
an extensive area lying above timber line will cause a flow of cold air 
down through forested areas below. This is exemplified at the San 
Francisco Peaks, Arizona. 

The case mentioned in which cold-air drainage occasioned a differ- 
ence of temperature at the same altitude, which was the equivalent of 
nearly 3,500 feet, probably represents its maximum effect. The differ- 
ence of 8 in Soldier Canon is the equivalent of an altitudinal difference 
of about 2,200 feet. 

The influence which cold-air drainage exerts on vegetation is regis- 
tered chiefly in the shortening of the season of vegetative activity on 
the floor of a canon as contrasted with its sides. This effect has been 
repeatedly observed in Bear Canon, where the oaks on the floor of 
the canon are always far behind the individuals on the canon wall in 
the advancement of their new foliage in the spring. Likewise in the 
autumn the frost-killing of herbaceous perennials and of the leaves of 
Prunus, Rhus, and Populus jamesii takes place in the floor of the canon, 
while the herbaceous plants of the slopes are still green and active. 
The plants on the canon floor are, in other words, subjected to a grow- 
ing season similar to that usually found at a much higher altitude. 

The influence of cold-air drainage in determining the distribution of 
plants is likewise marked. It is not wholly responsible for the fact 
that mountain species extend down the canons to lower altitudes than 
they assume on slopes or ridges, for the influence of ground water and 
soil moisture is very potent in this connection. The occurrence of the 



highest individuals of every species other than those of aquatic or 
streamside habitat on or near the summits of ridges, and their invari- 
able absence from the bottoms of canons at these higher elevations, 
are to be attributed to the absence of cold-air drainage from the ridges 
and higher slopes, together with the influences grouped in the "factor" 
of slope exposure. 


In the autumn of 1913 instruments were placed at the 6,000- and 8,000- 
foot stations to secure the absolute winter minimum temperature of 
the soil at a depth of 3 cm., and the thermometers were maintained in 

TABLE 19. Minimum temperatures of the soil and of the air at 3 elevations in the 
Santa Catalina Mountains for irregular periods. 






At 6,000 feet: 
Sept 23 to Sept 27, 1913 



+ 5 

Sept. 28, 1913, to May 16, 1914 
May 17 to 19 



+ 10 
- 2 

May 20 to July 22 



+ 1 

July 23 to 27 



+ 2 

July 28 to Oct 10 . 



+ 2 

At 8,000 feet: 
Sept 25 1913 



+ 6.5 

Sept 26 1913 



+ 6.5 

Sept. 27, 1913, to May 17, 1914 
May 18 



+ 3.5 

May 19 to July 24 



+ 7.5 

July 25 




July 26 



+ 9.5 

July 27 .. 



+ 5.5 

July 28 to Oct 11 

29 5 


+ 9.5 

At 7,600 feet: 
Sept 25 1913 



+ 9 

place and read at irregular intervals during the summer of 1914. The 
object in placing the thermometers at so slight a depth was to obtain 
a measure of the activity of terrestrial radiation by a comparison of the 
superficial minima of the soil and the atmospheric minima. The ordi- 
nary type of Six's thermometer was used, buried in a wooden box and 
covered with earth. The readings secured in this manner and the 
readings of atmospheric minima for the corresponding periods at the 
same stations are given in table 19. 

At the 6,000-foot station the soil minima are higher than the air 
minima in every case except one, the over-winter difference being 10. 
At the 8,000-foot station all of the 9 readings secured show a higher 
minimum for the soil. The over-winter period shows a difference of 
25, and the night of July 25 shows a difference of 18.5. The readings 


show that the soil temperatures at 6,000 feet were cooler in general, 
in terms of the air temperature, than were those of the 8,000-foot 
station. This difference is not to be attributed to the difference of 
elevation so much as to the naked and stony character of the soil at 
the 6,000-foot station and the relatively abundant humus and litter in 
the surface soil at 8,000 feet. In short, the radiation from the soil 
surfaces in the Encinal is greater than it is in the Forest, as has been 
already discovered from the difference in the behavior of cold-air 
drainage in these two regions. There is also a slight indication that 
the differences between the air and soil minima are least in the dry 
seasons of May and September, which is again in keeping with the 
greater radiation exhibited in dry soils as compared with wet ones. 

On the night of September 25, 1913, the difference between the air 
and soil temperatures was simultaneously determined on the rim of 
Marshall Gulch at the 8,000-foot station and in a thicket of young fir 
trees in the bottom of the gulch. The soil remained 6.5 warmer than 
the air on the rim of the gulch and 9 warmer in the fir thicket, showing 
the degree to which a heavy cover of vegetation retards radiation and 
conserves the warmth of the soil. On this night the air temperature 
in the bottom of the gulch was 1 lower than that on the run (see 
table 18). 

One of the most striking features of the soil minima is the fact that 
although the air temperature at 8,000 feet fell to 5 in the winter of 
1913-14, the soil temperature fell only to 30. This means that in the open 
forest on the rim of Marshall Gulch the soil must have been very slightly 
if at all frozen in the winter in question, which was apparently a winter 
of about average severity. During the same winter a lower absolute 
minimum of the soil was recorded at 6,000 feet than at 8,000 feet. In 
shaded situations and on north slopes in the Fir Forest the soil undoubt- 
edly freezes to a slight depth. Inasmuch as no soil temperatures have 
been secured with the bulb of the thermometer in contact with the 
soil, and no readings have been secured at a greater depth than 3 cm., 
the further discussion of soil temperature conditions in these moun- 
tains should await further investigation. 




The earlier chapters of the present paper have described the salient 
features of the vertical distribution of vegetation in the Santa Catalinas, 
and also some of the principal gradients of climatic change. Both the 
vegetation and the climate have been shown to exhibit progressive 
changes with increase of altitude, and these changes have been found 
to undergo hastening or retardation under the influence of topographic 
irregularities. It will be the object of the following pages to correlate, 
in so far as possible, the altitudinal changes of vegetation and climate, 
in an effort to determine roughly some of the physical factors which 
are of critical importance in limiting the vertical ranges of the types 
of vegetation and of their characteristic species. 


Any attempt to ascribe vertical limits to the Desert, Encinal, and 
Forest, or to state the vertical limits of individual species, is met at 
once by the omnipresent importance of slope exposure in determining 
these limits. The altitudinal range of vegetations and species may be 
determined by examining only slopes of south exposure, or only those 
of north exposure, and the two examinations would agree closely as 
respects the vertical ranges, but would disagree by approximately 
1,000 feet with respect to the upper and lower limits of the vegetations 
or species. It is impossible to determine the normal character of vege- 
tation at a given altitude by seeking level ground, for it will be found 
only in the flood-plains, subject to the influence of a high soil moisture, 
or on a ridge, subject to equally special conditions. It is also impos- 
sible to visit adjacent valleys or plateaus lying at the same elevation 
and to find on them vegetation which is subject to the same climatic 
and soil conditions. For some purposes it is desirable to consider the 
vertical stages of vegetation under ideal conditions, as affected by 
altitude without the complications due to topographic features. It 
is then possible to hypothecate a norm of vertical stages of vegetation 
by averaging the altitude of any given limit as separately determined on 
north and south slopes, or it is possible to take into consideration only 
the altitudinal changes of south slopes or of north slopes, taken alone. 

It has been shown that the influence of topography on the vegetation 
is chiefly (sometimes solely) to carry the common types of vegetation 
above or below the elevations at which they are universal. The 
influence of topography on the gradients of climate is of the same char- 
acter; the topographic relief causes no wholly new factors to come into 
play, but serves merely to carry the physical conditions of the Desert 
into the Encinal, for example, or to bring the conditions of the Forest 


down into the Encinal. The physical factors which underlie the effects 
of topography are, then, to be considered simply as special cases of the 
same influences that are grouped in the effects of altitude itself. It 
is desirable, nevertheless, in studying the correlation of climate and 
vegetation to consider separately the normal gradient of vegetation 
and the departures from the normal gradient. 


The student of vegetation too often loses sight of the fact that vege- 
tation is composed of individual species of plants and that the behavior 
of the vegetation is a function of the behaviors of these species. After 
our review of the vegetation of the Santa Catalinas, and in connection 
with the discussion of its control by climatic factors, it is necessary 
to consider the vertical distribution of the individual species in relation 
to the physical conditions of the mountain. 

There are no species of plants which grow spontaneously both at 
the base and the summit of the Santa Catalina Mountains, except a 
few palustrine forms of Carex and Juncus. The total range of physical 
conditions through the 6,000 feet of elevation here involved is so great 
that no native plant possesses the power of accommodation to the 
complete gamut of Desert, Encinal, and Forest. Indeed, very few 
plants range through half of the entire gradient of conditions, in any 
portion of it. The species which exhibit the widest belts of vertical 
distribution are to be found in the most dissimilar habitats at the lower 
and upper edges of their ranges, which indicates that these species are 
not really capable of existence through 2,000 or 3,000 vertical feet of 
the climatic gradient under the same conditions of topographic loca- 
tion, slope exposure, and insolation. In fact, a close analysis of the 
habitats occupied by characteristic plants, in connection with their 
vertical ranges, indicates that, below 6,000 or 7,000 feet, no plants 
outside the desert succulents and semi-succulents range through more 
than 1,000 to 1,500 feet in habitats of the same topographic character. 
At Vcrher elevations a number of common plants extend more than 
1,500 feet in situations of the same character, as for example Pinus 
arizonica, which ranges through nearly twice that altitude on dry 
southern slopes. 

A vertical range of 4,700 feet is exhibited by Vitis arizonica, which 
occurs in several arroyos and canons at 3,000 feet and is found in the 
same habitat throughout the Desert and Encinal regions of the moun- 
tain, reaching its highest observed station at 7,700 feet in a steep dry 
arroyo in the Pine Forest. Although the habitat of Vitis is superficially 
identical throughout its range, it is found at 3,000 to 5,000 feet only 
in the largest arroyos, in which it is able to draw upon much greater 
and more constant supplies of soil moisture than are available in the 
small arroyos to which it is confined at the upper edge of its range. 


Robinia neomexicana ranges through 3,800 feet, from its lowest occur- 
rence on flood-plains near constant water at 5,300 feet, to its highest 
occurrence on dry ridges near the summit of Mount Lemmon at 9,100 
feet. On the higher mountains of southern Arizona this species ascends 
to over 10,000 feet. Amorpha californica, after the manner of Vitis, 
ranges 3,500 feet from moist arroyos at 4,200 feet to dry ones at 7,700 
feet. Agave palmeri ranges from dry slopes of east or west exposure at 
3,200 feet to open ridges and crevices of rock at 7,400 feet, a belt of 
4,200 feet. Among the species which reach neither the Bajada nor 
the top of the mountain there are no others with vertical extensions 
of more than 4,000 feet. An extreme range of 3,700 feet is possessed 
by Juniperus pachyphlcea, from northern slopes at 4,200 feet to ridges 
at 7,900 feet. Nolina microcarpa extends from 3,750 feet on north 
slopes to 7,200 feet in open pine forest, a range of 3,450 feet, and Dasy- 
lirion wheeleri from 3,600 feet to 6,600 feet, on opposing slopes, a range 
of 3,000 feet. 

Among other plants which occur chiefly in the Encinal Region there 
are none with vertical ranges in excess of 3,000 feet, few in fact approach 
that range. Pinus cembroides extends from north slopes at 5,000 feet 
to open rocky ridges at 7,800 feet, a range of 2,800 feet; Agave schottii 
ranges from 3,700 to 6,000 feet, a belt of 2,300 feet; Garrya wrightii 
ranges from 4,300 to 6,500 feet, an extent of 2,200 feet; and Quercus 
emoryi extends from north slopes at 4,300 feet to south slopes at 6,200 
feet, a vertical range of 1,900 feet. 

Among the plants which have their lowest occurrence in the flood- 
plains of the Encinal and their principal range through the Forest 
Region, a number have vertical belts of occurrence of more than 3,000 
feet. Pinus arizonica itself is found through 3,300 feet and its upper 
limit is determined only by the height of the mountain. Pseudotsuga 
mucronata is found through 3,100 feet, and is also terminated by the 
summit of the mountain. Quercus hypokuca and Quercus reticulata are 
found through nearly 3,000 feet, and this extent of vertical range is 
attained by a large number of herbaceous perennials of the Upper 
Encinal and Forest. 

The Desert species which are encountered at the foot of the mountain 
do not begin their vertical ranges at that point, and statements of the 
elevations which they reach on the mountain are not to be compared 
with figures for the ranges of Encinal and Forest plants. Mamillaria 
grahami reaches the attenuated limit of its occurrence at 7,000 feet, 
Echinocadus wislizeni at 5,600 feet, Fouquieria spkndens at 5,600 feet, 
Opuntia versicolor at 5,500 feet, and Carnegiea gigantea at 5,100 feet. 
Very few other species of the bajadas and desert hills are found above 
4,700 feet. 

Among the most restricted vertical ranges of any plants which reach 
neither the foot nor the summit of the mountain are those of Quercus 


oblongifolia and Vauquelinia califomica. If we except the occurrence 
of the former in the beds of Sabino and Ventana canons at 3,000 to 
3,200 feet, its lowest occurrence on slopes is at 3,900 feet and its highest 
at 5,600, a range of 1,700 feet; while Vauquelinia ranges from 3,900 to 
5,500 feet, a vertical range of only 1,600 feet. These limits also apply 
very nearly for Erythrinaflabelliformis, Ingenhousia triloba, and several 
shrubs and shrublets, and are only slightly exceeded by the range of 
Quercus emoryi, which has already been stated to be 1,900 feet. 

Certain species of plants are confined to arroyos throughout their 
vertical ranges, as are Vitis arizonica, Amorpha califomica, Platanus 
wrightii, and Juglans rupestris; or are found chiefly in arroyos, as 
Cupressus arizonica and Acer interior. The great majority of trees, 
shrubs, and shrublets, as well as the semi-succulents (such as Agave, 
Yucca, Nolina, and Dasylirion), are found on slopes and ridges in at 
least some portions of their ranges, or are chiefly found there. The 
oaks, the deciduous trees, and most of the shrubs may be found along 
arroyos, or in flood-plains at elevations from 500 to 1,000 feet below 
the level at which they become common components of the slope vege- 
tation. The semi-succulents, like the succulents, are rarely found in 
arroyos, although they may grow very close to them or may be found 
in dry flood-plains. Of all species not confined to arroyos, their lowest 
occurrences are generally to be sought on north slopes or in arroyos 
at even lower elevations, and their highest occurrences are to be sought 
on south or southwestern slopes or (particularly in the case of cacti) 
on rocky ridges. At the vertical center of the distributional range of 
these species they may be found, as a rule, on slopes of every exposure, 
and perhaps in flood-plains as well, particularly in the case of the ever- 
green oaks. The exceptions to the rule are Quercus oblongifolia, which 
is commoner on south slopes than on north ones at all parts of its 
vertical range except the very lowest, and Pinus chihuahuana, which 
is rarely found on north slopes at any part of its range, even its lowest 
occurrences being on south slopes or on an approximate level. 


In order to overlook for the moment all of the subsidiary influences 
which cause local disturbance of the vegetistic gradient let us consider 
that the southern slopes at all elevations are representative of the 
normal altitudinal changes of vegetation, and let us then consider 
some of the differences of physical conditions that accompany the 
ascent from 3,000 to 9,000 feet. The differentiations of vegetation 
which we are accustomed to designate as "due to altitude" are actually 
due to three groups of physical factors: (a) moisture factors, (6) 
temperature factors, (c) light factors. It has been customary to regard 
atmospheric pressure as a negligible agency in relation to plants, but 


there is no work known to the writer which proves or disproves this 
view. The light factors have been but little investigated and may 
prove to occupy a r61e of great importance. In spite of the funda- 
mental physiological r61e of light, it is more than probable, however, 
that this factor plays a less important part in influencing the distri- 
bution of plants than do moisture and temperature. 


In the case of a mountain which arises from an arid region to a 
considerable height, the moisture factors are of critical importance in 
controlling the vertical distribution of plants. This group of factors 
may be defined as those which have to do with the maintenance of a 
close degree of equality between the daily intake and outgo of water 
through the plant. The description of rainfall and soil moisture con- 
ditions in the Santa Catalinas has indicated the nature of the water 
supply for plants, and the data on atmospheric evaporation have shown 
the collective force of the chief of those ultimate external factors 
which determine the water loss of plants. During the humid seasons 
the ratio of water available to water lost is such as to make conditions 
favorable for all plants. During the most acute periods of aridity the 
value of this ratio becomes an item of the first moment. 

The soil moisture data given in an earlier section are from too slight 
a depth to indicate the possible supplies for trees and the largest shrubs. 
They are nevertheless from a depth which is freely exploited by the 
roots of perennial plants, and it is more than likely that they bear a 
rather definite ratio to the moisture at greater depths. 

Since the arid fore-summer is the portion of the year in which the 
maintenance of an equilibrium between intake and outgo of water is 
most difficult, it is instructive to determine their relation for this season 
at the different altitudes. This may be done by determining the ratio 
of evaporation (in terms of cubic centimeters per day) to soil moisture 
(in percentage of dry weight), using the average daily evaporation of 
the arid fore-summer, and the average soil moisture of the arid fore- 
summer at 15 cm. These ratios are exhibited in table 20. The approx- 
imate ranges of the ratios are 1 to 25 for the Forest, 20 to 35 for the 
Encinal, and 35 to 50 for the Desert. If evaporation data had been 
secured at 9,000 feet the value of the ratio for that elevation would 
have been less than unity. 

The values of the ratio of evaporation to soil moisture afford a 
concise expression of the major conditions which affect the water 
relations of plants, and they demonstrate the wide divergence of these 
conditions in the desert valleys and on the forested mountain summits 
during the arid fore-summer. The average daily evaporation rate 
has been shown (fig. 12) to fall during the humid mid-summer to half 
the amount during the arid fore-summer. The soil moisture is like- 



wise increased in the former season to an amount that would greatly 
reduce the values of the ratio if determined for the humid mid-summer. 
The ratio of evaporation to soil moisture is not in itself a full index 
of the comparative aridity of Desert, Encinal, and Forest, for the con- 
ditions expressed by the ratio are of much longer duration at 3,000 
feet than at 8,000 feet. The shortening of the arid fore-summer from 
16 weeks at 3,000 feet to 7 weeks at 8,000 feet (see fig. 2) signifies that 
the most severe drought conditions of the year are more than twice 
as prolonged at the lowest elevation as compared with the uppermost. 
It is necessary here to bear in mind that the effects of drought on plants 
are cumulative, and that, for example, a period with a given set of 
conditions of increasing aridity which endures for 16 weeks may be 
twice as fatal or deleterious as a period that lasts for 14 weeks. For 
purposes of general climatic description, however, the values of the 
ratio of evaporation to soil moisture multiplied by the duration of the 
arid fore-summer may be taken as an index of the aridity of the several 
elevations (see table 20). 

TABLE 20. Average daily evaporation (E) and the moisture of the soil (SM), together with 

/ w \ 

the ratio of evaporation to soil moisture [ ^-TJ J for north and south exposures at six eleva- 
tions in the Santa Catalina Mountains for the arid fore-summer of 1911. 






S M 

Duration of 

3,000 feet . 


101 1 


50 5 


4,000 feet . . 


80 4 


40 2 

4 000 feet . 


82 7 

2 5 

33 1 


5,000 feet . 


61 7 

3 1 

19 9 

5 000 feet 


74 4 

3 5 

21 3 


6 000 feet . 


59 4 

1 8 


6,000 feet . 



56 1 

3 5 



7,000 feet.. 



62 8 

2 6 

24 1 

7,000 feet 



49 9 

5 5 

9 1 


8,000 feet . 


29 3 

7 4 

3 9 

8,000 feet . . . 



29 4 

11 3 

2 6 


The ratio of evaporation to soil moisture comprises a measurement 
of all the external factors which affect the water relations of plants, 
except the influence of radiant energy on transpiration and the possible 
effects of soil temperature on this function. It is accordingly unneces- 
sary to give further consideration to rainfall, which is not in itself a 
factor for vegetation, at least in such a region as Arizona. If any 
differences existed between the seasonal distribution of rainfall at 
different elevations in the Santa Catalinas the fact would be of great 
importance to the vegetation, but only in the effect it would have on 
the annual march of the soil moisture conditions. The evidences of 
observation and instrumentation have shown that the major drought 


periods of the Desert are rarely broken on the mountain by rainfall 
of significant amount. 

The records of rainfall for 8 years show that the summer rain at 
8,000 feet may be from 1.9 to 3.5 times as great as that at 3,000 feet 
(see table 4). The average of the 8 years shows the summer rain at 
8,000 feet to be about 2.4 times that on the Desert. The average of 
a longer series of years will probably approximate this amount and 
the securing of the winter precipitation would probably make little 
difference in the proportion. 

The very conditions of low evaporation which favor the water rela- 
tions of the plants in the Forest region are also favorable to the preser- 
vation of the moisture of the soil. The effect of the winter rains upon 
the soil moisture of the Forest is accordingly carried forward many 
weeks (see table 7 for soil moisture at 9,000 feet after 6 weeks without 
rain). The slow melting of snow on north slopes still further prolongs 
the effect of winter precipitation. These causes underlie the vernal 
activity of herbaceous plants in the Forest region (see p. 29) and the 
growth by trees of the Forest region during the arid fore-summer. 


The r61e played by temperature in differentiating the vegetation of 
the various altitudes of the Santa Catalinas is by no means so simple 
a matter as that played by moisture conditions, and is far from being 
capable of expression in a concise mathematical form. It has already 
been shown that the frostless season decreases from a length of 40 
weeks on the Desert to a length of 19 weeks in the Forest at 7,600 feet 
(see tables 10 and 11, and fig. 2), and that the temperature has an 
average apartness of 26 between Desert and Forest (see table 13). 
No instrumentation has been carried on which would establish the 
quantitative nature of the difference between other phases of the tem- 
perature conditions. The shortening of the growing season with in- 
crease of altitude, and the concomitant lowering of the temperatures 
of this season, are factors of great moment to the vegetation, but no 
work has been done to establish the precise temperature and tempera- 
ture-duration requirements of any species of plants. The shortness of 
the growing season in the Forest and the coldness of the nights of 
mid-summer (40 to 50, see table 14) are both hostile to growth 
activity and may well be limiting factors in the upward distribution 
of many species of the Encinal. 

The temperature conditions of winter are equally important with 
those of summer in underlying the limitation of species and vegetations, 
and their r61e may be played independently from that of the summer 
temperature conditions, or the two may play conjointly upon the 
same species at the same altitude. Among the various phases of winter 
temperature conditions are the length of the period subject to frost, 


the number of days with freezing temperature, the number of consecu- 
tive days or hours of freezing, and the absolute minimum reached. 
Any two or more of these phases may operate conjointly to influence 
a plant, and the temperature preceding a particular constellation of 
conditions may enhance the harmful effects of those conditions. In 
fact the general weather conditions accompanying or following a given 
phase of temperature may determine the full effect of the temperature. 

In those cases in which plants are killed by the action of low tem- 
peratures the most important factor to be considered is the actual 
duration of the period during which the plant is subjected to tempera- 
tures below 32. Secondary to this are the considerations of the amount 
of precooling received by the plant, the actual minimum temperature 
to which it was taken, the condition of the soil and the atmosphere 
during the freezing, and the nature of the weather subsequent to it. 
In a previous paper* the writer has called attention to the manner in 
which the most critical phase of low temperature conditions increases 
in severity with increase of altitude. Even on the coldest winter days 
the temperature on the Desert never fails to rise above 32 during the 
mid-day. The lowest temperatures of winter are invariably accom- 
panied by a clear sky, and the days preceding and following very cold 
nights are clear. A cloudy or rainy period is always accompanied by 
more moderate temperatures, as is shown by the rarity of snow on the 
Desert itself. The longest duration of a shade temperature below 
freezing, in the ten-year records of the Desert Laboratory, is 19 hours. 
It frequently happens that a duration of 6 hours is the greatest for 
an entire winter. On ascending the mountains the length of the most 
prolonged period of freezing becomes greater until an altitude is 
reached at which there are occasional winter days when the air 
temperature does not rise above freezing. At this altitude there is 
a sudden increase in the maximum number of hours of frost from 
22 or 23 hours to a length of 40 to 45 hours. Such a sudden in- 
tensification in the duration of a critical climatic condition causes 
this condition to operate more sharply in the limitation of plant 
distribution than is the case with conditions that exhibit the usual 
form of slowly graduated change. 

No winter thermograph records have been secured in the Santa 
Catalinas, and it is therefore impossible to state the exact altitude at 
which this sudden intensification of the frost factor becomes manifest. 
It is probable that it lies at about 4,500 feet. The exposure of plants 
to insolation may often save them from the effects of an air tempera- 
ture of less than 32, and in the case of succulents the temperature to 
which their tissue is raised during the insolation of the preceding day 
will shorten the period of freezing for them. These subsidiary matters 

* Shreve, Forrest. The Influence of Low Temperatures on the Distribution of the Giant 
Cactus. The Plant World, 14: 136-146, 1911. 


affect the exact altitude at which the maximum number of freezing 
hours may operate in the limitation of a given species; and the exact 
topographic character of the location of an individual plant may also 
affect the operation of this factor. 

The experimental work of the writer has shown that a duration of 
more than 18 hours of freezing temperature is fatal to Carnegiea and 
that Opuntia versicolor and Echinocereus polyacanthos are capable of 
withstanding durations of 66 hours. The limitation of Carnegiea is 
apparently due to the operation of this factor. Its occurrence becomes 
confined to south slopes at 4,000 feet and it becomes less and less 
abundant from that elevation up to 4,500 feet. One of the highest 
individuals at the latter elevation is protected by a rock on its north 
side, above the summit of which the cactus now projects for 8 inches. 
This projecting top was badly frosted on its north side in the severe 
winter of 1912-13, while the north side of the plant below the summit 
of the rock was uninjured. A small Carnegiea (18 inches high) has 
been discovered in Soldier Canon at 5,100 feet. It grows on the south 
side of a low rock, and its location is on the steep south slope which 
terminates a long ridge between two main branches of the canon. The 
plant is here well protected from the cold-air flow of the canon and is 
subjected to the full insolation of the short winter days. It showed 
some slight effects from the exceptionally cold winter just referred to, 
but succeeded in recovering from them. In the early arid fore-summer 
of 1911 the writer transplanted a young Carnegiea 3 inches high from 
the base of the mountain to the vicinity of the 6,000-foot station on 
Manzanita Ridge. The cactus was placed on the southwest side of a 
rock, with a large plant of Arctostaphylos northeast of it, and occupied 
a location near the summit of the ridge. The plant was watered 
several times in order to help it to become established, but was not 
assisted after the commencement of the summer rains. It success- 
fully passed the winter of 1911-12; it made gains in turgidity in the 
summer of 1912, but no measurable growth; in the spring of 1913, 
after the winter in which the minimum temperature at that locality 
was 6, the plant was found to be dead. Although the rainfall 
at Manzanita Ridge in the summer of 1912 was 8.68 inches as com- 
pared with 5.61 inches at the location from which the cactus was 
taken (near the 3,000-foot station), it was not able to seize the 
advantage. This fact itself involves the factor of summer temperature, 
which doubtless determines the rate of growth of the roots and their 
power for the intake of water. 

The evidence which shows Carnegiea to be limited in its upward 
distribution by the greatest number of freezing hours is probably 
applicable to a large number of desert plants, non-succulent as well 
as succulent forms, which find their limitation at about the same 



The normal or ideal gradient of vegetation is disturbed by three 
sets of topographic influences: (a) that of slope exposure, (6) that of 
the surface flow or underflow of streams and arroyos and the high 
soil moisture of flood-plains, and (c) that of location with respect to 
ridges, slopes, or valley bottoms, which may be designated briefly as 
the influence of topographic relief. These three sets of topographic 
features do not bring into operation any factors, nor any intensities 
of the common factors, which are not involved in the normal vertical 
gradients of physical factors, although in some cases they bring about 
new combinations of factors not exactly duplicated at other elevations 
under the conditions of the hypothetical normal gradient. In the 
description of the vegetation there have been frequent allusions to 
these three sets of departures from the ideal gradient of vegetation. 
Instrumentation has also been described which throws light upon the 
operation of slope exposure and of topographic relief. The influence 
of streams has been very obvious in its nature and has not been investi- 
gated instrumentally. 


The importance of slope exposure in determining the vertical limits 
of species, and in thereby determining the vertical range of types of 
vegetation, has been a matter of observation and comment among 
almost all writers on the vegetation of the western United States. 
Although the phenomenon is of universal occurrence throughout the 
extra-tropical portions of the globe it is rendered particularly striking 
in regions where there are transitions from desert or grassland into 
forested country. In any region like the Santa Catalina Mountains, 
with their steep climatic gradient and varied topography, the opera- 
tion of the factors involved in slope exposure is such as to present an 
alternation of vegetistic regions, causing constant departures of the 
vegetation from the theoretical norm to the norm of higher or lower 
portions of the mountain. 

Slope exposure is a "factor" in differentiating the vegetation of 
opposed slopes at all elevations. Even at altitudes between 2,000 and 
3,000 feet among the volcanic hills of the Tucson region, there are 
conspicuous differences between the south slopes, with their heavy 
stands of Carnegiea, Encelia farinosa, and Opuntia bigelovii, and the 
north slopes with their abundant individuals of Parkinsonia microphylla 
and Lippia wrightii and their heavier growth of perennial grasses.* 
The difference between northern and southern exposures is most con- 
spicuous between 4,000 and 5,000 feet, where the former have orchard- 

* See Spalding, V. M. Distribution and Movements of Desert Plants. Carnegie last., Wash., 
Pub. 113. 1909. 


like stands of evergreen oaks and the latter are treeless (see plate 9s). 
Almost equally striking, however, is the contrast between the open 
pine forests on south slopes at 9,000 feet and the heavy, deep-shaded 
stands of fir on north slopes at the same elevation (compare plate 
29A and plate 35). 

Although the influences of slope exposure are operative at all eleva- 
tions they acquire added power with increase of altitude.* In the 
Desert region and the Lower Encinal the uppermost limits of species 
on north slopes and on south slopes are from 600 to 1,000 feet apart 
(Carnegiea, Echinocactus, Quercus emoryi), while in the Forest region 
the upper limits on opposed slopes differ by 1,000 to 2,000 feet (Quercus 
hypokuca, Juniperus pachyphlcea, Arbutus arizonica). Another test 
of the same fact may be had by comparing a north slope at 3,000 feet 
with a south slope at 6,000, and by then carrying the comparison up 
3,000 feet. Between the north slope at 3,000 feet and the south slope 
at 6,000 feet is the strong contrast of Desert and closed Encinal, with 
only a few xerophilous ferns and one small cactus in common. Between 
the north slope at 6,000 and the south slope at 9,000 feet is the very 
close resemblance of two stands of Pine Forest, in one of which are still 
to be seen a few Encinal forms that have disappeared from the other and 
higher one. 

The increased influence of slope exposure at higher elevations is not 
to be attributed to the fact that the species of the Upper Encinal and 
Forest range through greater elevations than do the species of the 
Desert and the Lower Encinal. The number of feet through which 
a species ranges on south slopes or on north slopes has no necessary 
connection with the difference between its upper limits on north and 
on south slopes. The ability of a large number of plants to range 
through a greater vertical distance in the Upper Encinal and Forest 
than it is possible for the plants of the lower vegetations to do may be 
owing to the ability of the plants of the upper portion of the mountain 
to withstand a greater gamut of conditions than the plants of the basal 
vegetations can. It would, in any event, not be due to the existence 
of more gradual gradients of climatic change at the higher elevations, 
since in every case of the measurement of these gradients they have 
been shown to grow steeper between 6,000 and 9,000 feet than below 
6,000 feet. The increase in the effects of slope exposure with increase 
of altitude can only be ascribed to an increasing differentiation of the 
climatic conditions between north and south slopes at higher elevations. 
An examination of the curves of evaporation and of soil moisture (figs. 
10 and 14) will show that the readings for the highest stations exhibit 
the greatest apartness, at least with respect to the intensities involved. 

* Merriam has illustrated this fact in a diagrammatic profile of San Francisco Peak, but has 
not mentioned it in the text of his paper. See Merriam, C. Hart. Biological Survey of the San 
Francisco Mountain Region, Arizona. U. S. Dept. Agric., North Amer., Fauna No. 3, 1890, pi. 1. 


It is obvious that the importance of slope exposure lies in the topo- 
graphic control of the physical factors which form the environment of 
the plants concerned. It is possible to know, on purely a priori grounds, 
that two slopes of the same inclination, which lie in opposed positions 
so that one faces north and the other south, will present to plants two 
environments differing in almost every essential physical feature. The 
temperature of the air on two such slopes might be identical as deter- 
mined by the thermometers of a carefully established meteorological 
station, but they are distinctly different as they affect the vegetation, 
for the plants not only receive the direct rays of the sun but receive 
very different amounts of heat through diurnal terrestrial radiation. 
This circumstance is of small importance to full-grown trees and large 
plants, but is of great importance to young plants and seedlings. The 
soil temperatures of opposed slopes are also widely unlike, even in the 
presence of the undisturbed cover of natural vegetation. The two 
opposed slopes would in all likelihood receive the same rainfall, al- 
though this is not necessarily the case. An equal amount of rain might 
effect an equal elevation of the soil moisture on the two slopes, and to 
the same depth, but the soil evaporation of the south slope would 
greatly exceed that of the north slope, and a lower moisture would soon 
prevail in the soil of the former. Greater or less differences may thus 
be shown to obtain between the opposed slopes with respect to the 
most vital features of plant environment. Any attempt to explain the 
importance of slope exposure in determining plant distribution is there- 
fore incomplete unless it takes into account every possible environ- 
mental difference between the slopes. Some of these differences are 
undoubtedly of far greater importance than others, but the question 
of their relative importance is always one that must be asked with 
respect to a particular species of plant. To make a thoroughgoing 
answer as to the importance of slope exposure for a single species is in 
itself a very great undertaking. 

The universal occurrence of a large number of species of plants in 
the vegetation of the Santa Catalinas, their commonness within their 
ranges, and the consistency of their distribution with respect to slope 
exposure, all indicate that there has been ample time in the history of 
the mountain for all of these species to attain as wide a distribution as 
it is possible for them to have under existing climatic conditions. It 
is difficult to conceive of any upward or downward movement being 
possible for any of the common species of plants, inasmuch as thousands 
of years have already given an opportunity for such extensions of 
range. In view of the steep climatic gradient of the mountain it is 
easy to believe that all of the common species have reached upper and 
lower limits beyond which their survival is prevented by definite fea- 
tures of the physical environment. The present vertical limit of a 
species, whether upper or lower, must be looked upon as the average 


point at which some particular feature of its physiological activities 
is met by some particular environmental condition that is preventive 
or unduly inhibitory to it. The minor fluctuations of climate, which 
have their minimal and maximal values within periods that are as 
brief as the normal life of a perennial plant, are registered in the infre- 
quency of every species as it approaches its distributional limit and 
in the scattered individuals which lie farthest out from the main area 
of occurrence. The secular changes of climate which have their maxi- 
mal and minimal points many centuries apart are registered in slight 
movements of the limits of species, the marginal region of scattered 
occurrence being, of course, the first affected by such movements. 

The writer has seen no evidence indicating that competition between 
plants is at any place in the Santa Catalinas responsible for the limi- 
tation of any species. There is, of course, competition such as that 
between seedling pines in heavy stands of 10 to 40 years in age, and 
such competition as occurs between individuals of the same or different 
species of herbaceous plants in small areas of moist flood-plain. While 
competition may thus determine the surviving individuals of a stand 
of young trees, or may determine the composition of a small community 
of ephemeral or root perennial plants, it is not responsible for the find- 
ing of a plant in one habitat rather than in another, and is not respon- 
sible for the exclusion of a species from an area in which it might find 
favorable conditions. 

It is only consistent with our knowledge of the diversified physical 
requirements of plants that there should be such great diversity in 
the location of the belts of altitude occupied by different species, and 
it accords with our knowledge of the distribution of plants in general 
that these belts should be wider in some cases than in others. It is 
possible, however, to pick out groups of plants the limits of which 
correspond roughly with the limits of the Desert, Encinal, and Forest 
types of vegetation respectively. Even the plants of equatorial regions, 
in which there is a notable constancy of climate, both daily and annual, 
are able to endure small ranges of climate, or occasionally to endure 
changes in individual factors which are many times as great as the 
normal fluctuations of their native climates. In addition to the 
fluctuations of climate from month to month or from year to year which 
must be endured by any plant, there are often even greater differences 
which must be simultaneously endured by the most remotely separated 
individuals of the same specific stock. For example, Asclepias tuberosa 
is found from Maine to Minnesota in the north and from Florida to 
Texas in the south, and thence sporadically in the mountains westward 
to Arizona. In view of the prodigious range of this somewhat poly- 
morphous plant it is surprising not to find it reaching a greater elevation 
than 6,500 feet in the Santa Catalinas. With regard to possible differ- 
ences in the physiological behavior of the most remotely separated 
individuals of a plant stock of such wide range, we know little. 


To find a plant growing only on a north slope at 5,000 feet, only on 
a south slope at 7,000 feet, and on both at 6,000 feet, as is the case with 
Pinus cembroides, for example, means that there is much in common 
between the physical conditions on the north slope at 5,000 feet and 
the south one at 7,000. If such reversals of habitat in relation to slope 
were rare it would only be warrantable to state that there were some 
physical features in common between the opposed slopes, but, as 
already stated, there are only two common plants (aside from those 
of the streamways) regarding which a similar statement could not be 
made. It is obvious, therefore, that if we compare separately the alti- 
tudinal gradients of climatic change for the south slopes and for the 
north slopes of these mountains, the two gradients will be similar in 
character and will be closely related. Their relationship will consist 
in the fact that a given intensity or value on one of the gradients will 
be found on the other at a lower or higher elevation, unless barred by 
the base or summit of the mountain. In the curve showing the alti- 
tudinal gradients of evaporation on north and on south slopes (fig. 14) 
it will be seen that the rate on the south slope at 6,000 feet is exactly 
the same as the rate on the north slope at 5,000 feet. The rate on the 
south slope at 8,000 feet, however, is far less than the rate on the north 
slope at 7,000 feet. The latter rate is found on south slopes at about 
7,300 feet, according to the evidence of the curve. In spite of differ- 
ences in the pitch of the climatic gradients at different elevations, it 
is always possible to find a slope which exhibits the same intensity of 
a given factor as that which has already been found on an opposed 
slope, but it is necessary to go up or down the mountain from 500 to 
1,500 feet to do so. It might be possible to find two spots on opposed 
slopes in which there was very nearly the same complex of all environ- 
mental conditions, although the rinding of two slopes with identical 
ones would be rendered almost impossible by the necessity of seeking 
these spots at different altitudes. 

Even if a series of considerable differences were found between the 
north slope at 5,000 feet and the south slope at 7,000 feet, on both of 
which Pinus cembroides is growing, nevertheless such differences would 
be little greater than those which are met by this species as it grows 
on both north and south slopes at 6,000 feet or those that exist between 
the north slopes at 5,000 and 6,000, or the south slopes at 6,000 and 
7,000 feet. 

The physical factors which underlie the influence of slope exposure 
are simply a special case, for the most part, of the same factors which 
cause the altitudinal differentiation of the vegetation of the entire 
mountain. The only instrumentation carried out with a view to secur- 
ing a measure of the influence of slope exposure is comprised in the data 
on soil moisture and on evaporation for north and south exposures at 7 
and 5 elevations respectively (see tables 7 and 9 and figs. 10, 13, and 14). 



~i 1 r 


6.0CO N.and 5 

. 7.000 a 

Neither the data for soil moisture nor those for evaporation show 
the exact alternation exhibited by the vegetation itself, by virtue of 
which a given north slope is similar in vegetation to a south slope 
about 1,000 feet above it, and a given south slope is similar to a north 
slope about 1,000 feet below it (with the exception of the highest 
altitudes). The conditions of evaporation found through the range 
of Pinus cembroides, which has been used as an example of the effects 
of slope exposure, are indicated in figure 18, where curves are given 
showing the seasonal march of evaporation on a north slope at 5,000 
feet, the average of the evaporation on north and south slopes at 6,000 
feet, and the amounts on the south slope at 7,000 feet. These curves 
follow a course which is parallel and indicate evaporation conditions 
which are remarkably 
similar for the lower, cen- 
tral, and upper individ- 
uals of this pine, except 
for the higher rate at 
5,000 feet during the arid 

The ratios of soil mois- 
ture to evaporation at 
different altitudes have 
been worked out sepa- 
rately for the north and 
south exposures at the 6 
stations (see table 20). 
Since these ratios are an 
expression of the conditions of the arid fore-summer they must be 
taken as elucidating only those phases of slope exposure which are 
themselves due to the climate of that season. Any subsidiary in- 
fluence of temperature in affecting the slope exposure phenomena of 
vegetation in the Santa Catalinas still awaits a full investigation. 

The comparative conditions of the lower, central, and upper habitats 
of Pinus cembroides may be again investigated in the light of the ratios, 
which are as follows: North slope at 5,000 feet 21.3, average of north 
and south slopes at 6,000 feet 24.5, south slope at 7,000 feet 24.1. These 
figures indicate a still more remarkable similarity of the water con- 
ditions in the three habitats than the evaporation figures do. To 
compare a plant of lower range we may take Agave schottii, which 
encounters conditions expressed by the following ratios: north slope at 
4,000 feet 33.1, average of north and south slopes at 5,000 feet 20.6, 
south slope at 6,000 feet 33.0. These figures fail to show as close 
agreement, but indicate a close similarity of the water conditions at 
the lowest and uppermost habitats, and a more favorable set of con- 
ditions in the central habitat. To make a similar comparison for a 

Fio. 18. Graphs showing seasonal march of rate of evap- 
oration for a north slope at 5,000 feet (dotted line), the 
average for a north and a south slope at 6,000 feet (solid 
line), and for a south slope at 7,000 feet (broken line). 


plant of higher range than Pinus cembroides we may take Quercus 
hypoleuca, which ranges through 3,000 feet, with the usual alternation 
at the top and bottom of its range. The ratios for its habitats are as 
follows: North slope at 6,000 feet 16.0, average of north and south 
slopes at 7,000 feet 16.6, south slope at 8,000 feet 3.9. Here is close 
agreement of the ratios for the lower and central portions of the range, 
with a much lower value for the top, indicating that in spite of the 
ability of Quercus hypoleuca to withstand the conditions expressed by 
the ratio of 16, it is likewise capable of withstanding the more favorable 
water conditions indicated by the ratio of 3.9. Here, in other words, 
is a typical Encinal plant, accompanied throughout its range by many 
others, which is able to extend up to an elevation at which the water 
conditions are much more favorable than they are in the lower part 
of its range. This is a thing which the Desert plants do not do, and 
the reason is undoubtedly that the plants of the Desert encounter 
unfavorable temperature conditions at the same elevations at which 
they begin to encounter more favorable water conditions, while such 
a plant as Quercus hypoleuca is capable of withstanding the rigorous 
temperatures of 8,000 feet and is thereby enabled to range upward 
into a region of more favorable water conditions. 

Allusion has been made to the more pronounced character of the 
effects of slope exposure at higher elevations. It is of interest in that 
connection to contrast the ratios of evaporation to soil moisture for 
similarly located pairs of habitats at low and at high altitudes. For 
example, the north slope at 4,000 feet has a ratio of 33.1, the south 
slope at 6,000 has a value of 33.0. To carry the comparison up 2,000 
feet: the north slope at 6,000 feet has a ratio of 16.0, the south slope 
at 8,000 feet has one of 3.9. The greater similarity of the ratios for 
the two lower habitats is in accord with the evidences from the vege- 
tation (see p. 98), which indicate an altitudinal increase in the potency 
of slope exposure in the determining of the vegetation. 

The fundamental causes differentiating the conditions on opposed 
slopes are only partly comprised in the evaporation soil-moisture ratios. 
The differences of evaporation rate on north and south slopes are 
largely due to the dry, warm winds which ascend the mountain during 
the day and partly to the differences of air temperature. The humidity 
of the air shows only slight differences on opposed slopes. The soil 
moisture on north slopes is higher than on south ones because of the 
more direct insolation on south slopes, and because of the higher soil 
temperature and increased soil evaporation which are due to this. In 
addition to the differentiating features which are expressed in the 
ratios, we have the differences of soil temperature, due to the direction 
of slope, and the differences of air temperature, which are only partially 
registered in their effect upon the evaporation rate. The increased 
insolation on slopes as compared with level ground has been worked 


out by Hall,* who shows that the amount of radiant energy reaching 
a south slope of 45, with the sun 45 above the horizon, is 1.4 times 
as great as that reaching a level piece of ground through an aperture 
of the same size. It is through this difference, which is still greater 
between south and north slopes, that the soil is given a higher tempera- 
ture, that the air is warmed to a higher degree through radiation, and 
the soil dried more rapidly on all south-facing exposures. 


In the description of the vegetation of the Santa Catalinas constant 
allusion has been made to the distinctive plant communities of springs, 
streams, flood-plains, and arroyos. The contrast between the vege- 
tation of these moist or relatively moist situations and that of the 
mountain slopes is very striking at the mouths of the larger canons, 
and throughout the Desert and Encinal regions. At the higher alti- 
tudes, and particularly in the Fir Forest, the moist habitats are not 
only less striking to the casual observer, but their vegetation actually 
comprises a great many species which are frequently found away from 
proximity to streams. 

The influence of streams and flood-plains consists, in brief, in bring- 
ing components of the upland vegetation of each altitude down along 
the streamways of the altitudes just below. In this manner the Encinal 
is traversed by bands of Forest, and the Desert slopes are traversed 
by bands of Encinal. Furthermore, the streams and springs of the 
mountain afford the sole habitats for a number of species of aquatic 
and palustrine plants which do not appear on the upland at any 

The mechanical agencies of gravity, sheet-floods, and stream flow 
are all capable of aiding in the downward dissemination of the seeds 
of all mountain plants and these mechanical agencies should assure 
the occurrence of all mountain plants in all situations at lower altitudes 
in which they are capable of survival. The number of seeds which 
are borne down by streams is, of course, enormous, and the number of 
resulting germinations is probably very large. The number of sur- 
vivals, however, is controlled by the physical conditions of the new 
low-altitude habitat, and in a manner to be further considered. 

In the discussion of slope exposure no account has been taken of the 
occurrence of plants along streamways at elevations below their lowest 
upland occurrence, since the individuals along the arroyos and streams 
are subjected to a very different set of environic controls from those 
that determine the location of the upland individuals. In the earlier 
discussion of the vertical limits of species the streamway occurrences 
were taken into account. 

* Hall, H. M. A Botanical Survey of San Jacinto Mountain. Univ. Cal. Pubn. Bot., vol. i, 
pp. 1-140, 1902. 


Among the palustrine plants which occur along streams at 7,000 to 
8,000 feet are two species of Juncus and two of Carex which also occur 
in Sabino Canon, under the most favorable conditions of moisture 
supply, at 3,000 to 3,200 feet elevation. The perennial composite 
Tagetes lemmoni grows along the drier arroyos of the Pine Forest down 
to 6,000 feet, and is found in lower Sabino Canon growing along the 
margin of the stream at 3,200 feet. Other palustrine plants of the 
Forest region are found from time to time at low elevations along the 
largest streams, but no others have been observed to become thoroughly 
established there. 

The well-known cosmopolitanism of many aquatic plants would 
cause us to expect such behavior as is exhibited by Juncus, Carex, and 
Tagetes in the Santa Catalinas. There are several species of Scirpus 
and Eryngium, and at least one woody plant (Cephalanthus occidentalis) 
which range from the Gulf of Mexico across the southwestern boundary 
of the United States to California. The individuals of these species 
are subjected to a wide diversity of atmospheric humidities, but are 
all found under conditions of closely equivalent high soil moisture. 

A greater interest attaches, in the present connection, to the cases 
of low streamside occurrence of plants which grow typically in upland 
situations. Mention has already been made of the trees of Quercus 
arizonica and Quercus oblongifolia which grow along the Sabino Creek 
at 2,800 feet, about 1,200 feet below their lowest occurrence on north 
slopes. Small plants of Quercus hypoleuca have been found growing in 
deep shade in the bed of Sabino Canon at 3,200 feet, which is 2,700 
feet below the lowest north slope occurrence of this tree. The first- 
named oaks have descended no further than many other upland plants 
have done, but the last-named oak shows the most pronounced de- 
pression of range that has been detected. 

At the mouth of Soldier Canon, at 3,000 feet, the writer has found 
one or two individuals each of Dasylirion wheeleri, Mimosa biundfera, 
Erythrina coralloides, and Asclepias linifolia. At an elevation of 4,500 
feet Dasylirion and Asckpias have begun to appear on slopes of south 
exposure, and at 5,000 feet Erythrina and Mimosa have also left the 

At 4,900 feet Ceanothus fendkri is found in the shade of oaks on the 
flood-plain of Soldier Canon. It occurs also at 5,300 feet in similar 
situations at the head of Soldier Canon, and becomes frequent in the 
Upper Encinal at 6,000 feet. Similarly Quercus submollis occurs near 
the constant water at Horse Camp, in Bear Canon, at 6,100 feet and 
is of increasing frequence along streams up to 7,200 feet. At that 
elevation and up to the uppermost limit of Pine Forest it is common 
on slopes as well as near streams. Robinia neomexicana is found in 
the flood-plain of Soldier Canon, near a spring, at 5,300 feet, and first 
becomes a frequent upland shrub of the Pine Forest at about 7,500 feet. 


It is possible to say, in brief, that the conditions presented by stream- 
sides and flood-plains are such as to depress the ranges of very many 
plants by as much as 1,000 feet, and of a few plants by amounts as 
great as 2,000 feet. A depression of as much as 2,700 feet, found in 
the case of Quercus hypoleuca, does not represent the lowest occurrence 
of established plants, but rather a chance survival at an elevation in 
which it would doubtless be impossible for the tree to reach maturity. 
It can at least be said that throughout the entire length of Sabino 
Canon, from the mouth to the Basin, there are no known occurrences 
of full-grown trees or even shrubs of Qi^rcus hypoleuca. 

The extent to which the types of vegetation are depressed in their 
ranges by the influence of streams and flood-plains is about the same 
as the average depression of the individual species, that is to say about 
1,000 feet. In the case of the occurrence of a closed Encinal in the 
Basin of Sabino Canon there has been a depression of 1,500 feet in 
the limit of this type of vegetation from 5,500 to 4,000 feet. 

Some evidence has already been given leading to the view that the 
lower limits of all Encinal and Forest plants are determined by those 
features of the environment which in turn determine the water relation 
of plants. The facts of the depression of vertical ranges by streams 
form an additional evidence of this view. So far as concerns atmos- 
pheric water-demand the plants growing beside streams are subjected 
to the same conditions as plants of the nearby upland, but the conditions 
of water supply are infinitely better for them. In other words, in the 


ratio of evaporation to soil moisture, S] ^, the numerator is the same 
for stream-side and upland plants and the denominator is greatly 
increased for the latter, thereby lowering the values for the ratio. In 
the cases alluded to in which the lowest individuals of a species not 
only grow in a flood-plain but in the shade of larger vegetation, the 
plants are under ameliorated conditions with respect to the numerator 
as well as the denominator in the ratio. 

A number of mountain plants are able to survive when taken down 
to the Desert provided they are placed under conditions in which one 
or both of the sets of conditions indicated by the above-mentioned 
ratio are ameliorated. Parthenocissus from 6,000 feet survives with 
irrigation and partial shade; Echinocereus polyacanthos, from 5,000 to 
7,000 feet, survives with occasional irrigations during the arid fore- 
summer; Zauschneria calif ornica, Aquilegia chrysantha, and Sedum stelli- 
forme, all ranging from 5,500 to 7,500 feet, are capable of survival at 
Tucson from year to year when grown in complete shade with frequent 
irrigation during the arid fore-summer. These facts point to the ability 
of such plants to withstand at least the shade temperatures of the 
Desert, provided the moisture supply of the soil and the moisture 
requirement of the air are made more nearly like those conditions in 
the mountain habitats of the plants. Many introduced plants have 


shown themselves incapable of withstanding the atmospheric aridity 
at Tucson even when grown under the most liberal irrigation. The 
inability of a plant to pass water on to its transpiring tissues as rapidly 
as it is withdrawn by a desert atmosphere is undoubtedly a feature 
common to very many mesophilous plants, and it is apparently the 
cause which prevents a greater number of palustrine mountain plants 
from descending the large streamways to the Desert, and it doubtless 
prevents a lower descent upon the part of many Forest species which 
reach the flood-plains of the Lower Encinal. 

It might be argued that the low occurrence of Forest along the 
streams of the Encinal and the descent of the Encinal into the Desert 
Slopes are due to the influence of cold-air drainage rather than to the 
effects of soil moisture, or that cold-air drainage is at least an important 
contributory factor. It is difficult to believe that low temperatures, 
especially those of the winter months, should be a favoring factor for 
plants which are subjected during the day to just as high temperatures as 
are the plants of the upland. During the summer months the low noctur- 
nal temperatures might be of some slight importance, but such importance 
would reside solely in aiding the plant to recover from the excessive 
transpiration of the preceding day and to build up a reserve of water 
against the transpiration of the following day, as has been shown by 
Edith B. Shreve to occur in Parkinsonia microphylla.* The facts that 
it is the highest diurnal temperatures that are apt to be deleterious 
to low-ranging mountain plants and that their effect can be only 
indirectly and slightly offset by the lowest nocturnal temperatures 
make it appear that cold-air drainage has at least a very minor r61e 
in this connection as compared with the moisture conditions. 


Each of the leading types of vegetation in the Santa Catalinas 
reaches the uppermost limit of its occurrence on ridges and high south- 
facing slopes. This carries the Desert upward into the Encinal and 
carries the Encinal up into the Forest in such a manner that there is 
an interdigitation of the vegetistic regions throughout the portions of 
the mountain in which the topography is mature enough for it to be 
manifest. This appearance of interdigitation is partly brought about by 
the influence of streams (which has just been discussed) and is some- 
times merged with the influence of slope exposure. These facts do not 
in the least obscure the high range of each type of vegetation on the 
narrow ridges which point due south or north and are therefore free 
from the influence of slope exposure. 

On the ridges which lie between the tributaries of Soldier Canon 
have been found the highest individuals of all of the characteristic 

* Shreve, Edith B. The Daily March of Transpiration in a Desert Perennial. Carnegie Inst. 
Wash., Pub. 194, 1914. 


species of the Desert (for elevations see page 37). On a high ridge 
tributary to Bear Canon have been found the highest individuals of 
Opuntia sp., the highest species of that genus on the mountain, and 
Mamillaria grahami, the highest-ranging plant of the Desert. The 
individuals which most nearly approach these highest stations for 
Opuntia and Mamillaria have been found on south exposures about 
600 feet lower, in the Bear Canon drainage. 

On an exposed ridge, with a considerable inclination to the south, 
at 7,800 feet are found the highest individuals of Pinus cembroides, 
Juniperus pachyphlcea (with one known exception), Yucca schottii, 
Echinocereus polyacanthus, and Arctostaphylos pungens. In this station 
the influences of slope exposure and of topographic relief are combined, 
thereby bringing about the pronounced conditions that are expressed 
in the highest occurrence of 5 species of the Upper Encinal. On the 
ridges above Marshall Gulch are found the highest occurrences of 
Quercus hypoleuca and Quercus reticulata, both of which forms extend 
further down the south faces of these east-and-west ridges than they 
do down the north faces. 

When Desert plants are found on the ridges of the Encinal region 
they fail to appear on the south-facing slopes just below these ridges. 
When the plants of the Encinal are found at their highest locations 
on ridges of the Forest Region they are also absent on the south-facing 
slopes just below the ridges. This does not appear to be the case with 
respect to the highest occurrences of plants which are believed to have 
their true climatic limit just below the summit of Mount Lemmon, 
such as Quercus hypoleuca and Quercus reticulata. 

The extent by which the highest individuals on ridges exceed the 
highest individuals on south slopes is never more than 500 to 600 feet, 
except in the case of Pinus cembroides, in which it is about 700 feet. 
Opuntia sp. and Mamillaria grahami, which have their upper limit in 
the vicinity of 7,000 feet, agree in this respect with Opuntia versicolor, 
Echinocactus wislizeni, and Fouquieria splendens, which have their 
limit in the vicinity of 5,500 feet. 

Perhaps the most common explanation of the highest occurrence of 
species on ridges is that the soil is driest in such situations and therefore 
offers to plants from lower elevations a habitat more like that in which 
they are abundant. The principal objection to such an explanation 
is the unquestionable fact that a somewhat more moist soil is not 
inimical to the plants of the Desert nor to the plants of the Encinal. 
Neither is there a sufficient difference between the soil moisture at 
the bottom of a slope and on the ridge at the top of the slope, in the 
arid seasons, to cause a differentiation of the vegetation. 

The explanation of the phenomenon may be sought partly in the 
existence of cold-air drainage, which is at least responsible for the 
absence of Desert and Encinal plants from the bottoms of canons at 


the highest elevations to which they attain. The streams of cold air 
are not more than 75 to 100 feet deep, however, and can not, therefore 
be functional in preventing the occurrence of plants on the middle 
and upper slopes of canons. An apparently valid explanation of the 
high occurrences on ridges is in accordance with the theory already 
mentioned, that the upper limits of the Desert species, and possibly 
of the Encinal species also, are set by winter temperature conditions. 
The ridges are obviously the localities which receive the fullest and 
longest insolation on the short winter days with low sun. This cir- 
cumstance would not only warm the plants themselves but would 
warm the soil and rocks in a manner such as to lessen the severity of 
the coldest nights. With the pronounced low temperatures in the 
canons, due to cold-air drainage, and with the favorable conditions 
of the ridges for a pre-warming of both plant and habitat, it may be 
expected that there will be great differences between the vertical 
limits of species in canon bottoms and on ridges. 


The desert mountain ranges of the southwestern United States stand 
in the midst of a region which presents severe conditions for plants. 
The relative richness of the vegetation in this region is due chiefly to 
the occurrence of two yearly seasons of rainfall. The entire annual 
vegetational behavior is related primarily to the moisture seasons and 
much less pronouncedly to the thermal seasons. The perennial plants 
lead an existence which permits of rapid growth during the warm 
humid season, together with an extremely low ebb of activity during 
the arid seasons, and with the possible loss through drought-death of 
much of the growth that has just taken place. 

The severe conditions of the desert environment cause the vegetation 
to exhibit a high degree of sensitiveness to slight topographic and 
edaphic differences. Wherever the character of the soil or the topo- 
graphic location is such as to present a degree of soil moisture slightly 
above that of the general surroundings, or as to maintain it for a longer 
time in the periods of extreme aridity; or in whatever locations plants 
are protected from the most extreme conditions of transpiration in 
such places are to be found heavier stands of vegetation or else particular 
species of plants. 

The higher mountains of the desert region exhibit strong gradients 
of change in climate and in vegetation. Both of these gradients are 
much more pronounced than those of mountains of equal elevation 
in more humid regions. They lead from arid to humid, or at least 
semi-humid, conditions of moisture, and from sub-tropical to tem- 
perate conditions of temperature; from low, open microphyllous and 
succulent desert, through a sclerophyllous semi-forest to heavy conif- 
erous forest. 


The sensitiveness which desert vegetation exhibits to slight environ- 
mental differences is even more pronounced with respect to the climatic 
gradients of the mountains. Throughout a vertical range of 6,000 
feet there is not only a very striking gradient of vegetation, but a 
very nice adjustment of vegetation to the physical conditions. In the 
Desert and Encinal regions, and to a great extent in the Forest as well, 
this is chiefly an adjustment of plant to environment and scarcely at 
all an adjustment of plant to plant. Every juvenile individual in the 
open Desert and Encinal regions is a pioneer, and on reaching maturity 
this individual is part of an ultimate stable community. 

The principal features of altitudinal climatic change are: the short- 
ening of the frostless season, the lowering of the daily curve of tempera- 
ture throughout the frostless season, the increasing of the intensity 
and duration of all critical phases of low temperature during the frost 
season, the shortening of the arid fore-summer (the critical season of 
aridity), the increasing of precipitation and therefore of soil moisture, 
and the decreasing of evaporation. 

On a mountain having the form of a smooth cone it would be possible 
to observe the ideal manner in which these climatic gradients would 
collectively control the vertical distribution of the vegetation. The 
occurrence in nature of irregularities of relief is responsible, however, 
for local departures from the ideal vertical gradients of climate and 
also from the ideal altitudinal distribution of vegetation which would 
be anticipated on a geometrically constructed mountain. It is possible, 
nevertheless, to correlate the climatic and vegetational gradients in 
spite of the local irregularities of each of them, and in fact the study 
of these departures from the ideal has aided in the interpretation of 
the correlations. 

The vertical distribution of vegetation on the Santa Catalina Moun- 
tains has been found to be due to the interaction of two sets of controls 
which are nearly distinct. One of these controls has its seat in the 
moisture conditions, the other in the temperature conditions. The 
temperature control has been studied experimentally only with respect 
to three species of plants, but it is believed on this evidence (as well 
as the evidence of the departures from the normal gradient of vegeta- 
tion, correlated with instrumentation) to be the control which limits 
the upward distribution of the Desert species and perhaps of some 
species of the Encinal. The moisture control has not been studied 
experimentally in connection with the present investigation, but its 
operation is well known, and the instrumental study of soil moisture 
and evaporation at successive altitudes, with due attention to the 
departures from the normal gradient of vegetation, has indicated that 
the ratio of the latter factor to the former affords a concise expression 
of the control which limits the downward distribution of Forest and 
Encinal plants. 


The principal departures of the vegetation from the ideal gradient 
that would be found on a geometrical cone are expressed in the irregu- 
larity of the upper or lower limits of vegetations or of individual species 
as observed in different habitats. The chief departure is that due to 
slope exposure, by virtue of which the vegetation of north-facing and 
south-facing slopes at the same elevation shows striking differences. 
A second departure is that due to the influence of streams and the 
high moisture content of the soil of arroyos and flood-plains, by reason 
of which the plants of all altitudes are carried below their normal 
lowest occurrences on slopes. Another departure is due to the influence 
of ridges, on which the plants of all elevations (and particularly those 
of the Desert) find their highest occurrences. These departures seldom 
result in the occurrence of distinctive plant communities, but are 
operative rather in the carrying of the usual and widespread communi- 
ties into elevations at which they are exceptional. The effect of slope 
exposure is to carry the normal vegetation of a given elevation both 
up and down the mountain, so that its lowest occurrences are on north 
slopes and its highest on south slopes. The effect of streamways is 
to carry either the normal or the streamside vegetation down the moun- 
tain, so that the extreme lowest occurrences of almost all Encinal and 
Forest plants may be sought along the streamways. The effect of 
ridges is to carry the vegetation (or more particularly individual 
species and small groups of species) up the mountain, so that all highest 
occurrences of Desert and Encinal species are to be found on narrow 
ridges the highest occurrences of Forest plants are not reached on 
the Santa Catalina Mountains, and they are controlled by a very 
dissimilar group of factors. 

It is impossible to study the distribution of vegetation in a region 
where pronounced differences may be found within short distances 
without being impressed with the independence which each species 
exhibits in its allocation. Plants which are associated on the Lower 
Desert Slopes, for example, range to very different maximum altitudes, 
and plants which are associated in the Upper Encinal are found to be 
in part at the upper edges of their ranges, in part at the lower edges, 
and also in part rather closely restricted to that region. It is nowhere 
possible to pick out a group of plants which may be thought of as 
associates without being able to find other localities in which the asso- 
ciation has been dissolved. Certain plants may be thought of as having 
closely identical physical requirements because of their associated 
occurrence in the same spot. Nevertheless, the fact that the vertical 
ranges and habitat characteristics of these species will reveal more 
or less pronounced differences goes to show that each of them has 
survived in a particular section of the climatic gradient. It is true in 
the Santa Catalina Mountains, as it is true in all other places, that the 
associated members of a plant community are not able to follow each 


other to a common geographical and habital limit. The physical 
requirements of plants are so varied and so elastic that the composition 
of a series of communities occupying similar habitats in widely sepa- 
rated places shows the constant overlapping of the ranges of individual 
species which is due to the physiological inequivalence of these species. 

It is particularly true of the plant communities of arid and semi- 
arid regions that the most closely associated individuals are not alike 
in their life requirements, and this is true to a less pronounced extent 
in all plant communities. The members of the many diverse biological 
types, or growth forms, which are found together in Desert and Encinal 
find their soil water at different levels, procure it at different seasons, 
and lose it through dissimilar foliar organs, at the same time that they 
react differently to the same temperature conditions. In brief, these 
associated plants are not living in the same climate but are living in 
different sections of the same climate, the demarcation of these sections 
being either temporal or spatial. 

The use of the physical characteristics of the habitat as a criterion 
in the definition of a plant community does something to give a greater 
rigidity and a wider applicability to the definition. On the other hand 
it confuses cause with effect and makes it impossible to investigate 
the relation of physical conditions to a community defined in that 
manner without reopening the whole question as to the nature and 
identity of the community. There is much strong logic to support 
the view that all necessary definitions and classifications of vegetation 
should be made on the basis of the vegetation alone. When units of 
vegetation are thus defined they lend themselves to the further study 
of their life requirements, and it is such study applied to individual 
species as well as to vegetation that affords the most promising and 
important field for ecological activity. 

The distribution of vegetation in the Santa Catalina Mountains is 
strongly controlled by a steep climatic gradient; the vegetation itself 
is diversified in its display of growth forms; and the secular changes 
of vegetation due to physiographic phenomena, and to the reaction 
of the plant upon its habitat, are in almost complete abeyance. These 
circumstances have made it possible to give a delineation of the vege- 
tation upon purely vegetational characteristics, without regard to the 
secular changes which are taking place in very restricted areas, and 
with particular emphasis upon the individualism of behavior among the 
characteristic species. The same circumstances have also made it 
possible to lay side by side the facts respecting the vegetational gradient 
and those respecting the climatic gradient in such manner as to reveal 
the correlations between the two and to indicate some of the physical 
controls which operate in the limitation of the activities and of the 
ranges of species and of vegetations. 


Plate 1 


Plate 2 

A. South face of Santa Catalina Mountains viewed 7 miles from their base. Mount Lcmmon is on right center. 
In foreground is bajada vegetation of Covillea tridentata, Opuntia spinosior, and Isocoma hartwegi. 

B. Extreme southwestern ridge of Santa Catalinas viewed from the north. In foreground is the bed of the Canada 
del Oro, with individuals of Hymenoclea monogyra and a marginal fringe of Prosopis velutina and Chil- 
opsis linearis. 


Plate 3 

A. Typical Low Bajada, with pure stand of ('arillca fridctttata and summer carpet of B&uteloua 

B. Looking southwest from Upi>er Bajada near mouth of Soldier Canon. In the arroyo in foreground are 
Camei/iea yiganlca, Parkinsonia microphylla, Acacia yregyii, and Fouquieria splendens. Distant 
hills are relict toes of ancient liajadas. 


Plate 4 


Plate 5 

A. Desert Slopes near mouth of Pima Canon. In foreground, from the left ;ire Parkinxonia microphylla, 
Simmortdsia californica, ('arncuiva gigantea, Opuntia tountcifi, and Lifdum twrlandieri. 

B. Roeky Desert Slopes in same vicinity as A. Against the sky are Fouquieria splendens, Prosopia 
velutina, and Alomitia pallifla; in foreground Sptw-ralcea pedata, Lippia wri'jhtii, and Opuntia 


Plate 6 


Plate 7 


Plate 8 


Plate 9 

A. Looking northeast along Lowest Slo])es of the Lower Eneinal at 4,300 feet. At left Arctntstaphylos pungens, at 
right Qutrcus obloiu/ifolia, below it Dasyliriun irheelcri, to left of the latter \olina microcarpa. 

B. Looking southeast in Soldier Canon. On right the treeless slopes of the Upper Desert, on left open Eneinal 

of Quercus oblongifolia and Quercus arizonica. 


Plate 10 

A. Flood-plain of Soldier Canon at 4,200 feet. The predominant plant is Baccharis sarolhroides. 

B. Lower Slopes and Flood-plain of Soldier Canon at 4,900 feet. The predominant trees are Quercus emoryi 
and Juniperus pachyphlaea, the shrubs Arctostaphylos pungens and Garrya wrighlii. 


Plate 11 

A. Kncinal in Soldier Canon at .5, (100 fret, with Qui-rcns emoryi, Jiini/irrii.-- piiiii i/ phlim, Garrya wrijhtil, 
Yftrcn i/iai'ruairpa, and .\olina microcarpa. At lower left i.s Kouttiotia rothrockii. 

B. Quercus oblongifolia in Flood-plain of Soldier Canon. At left Quercus emoryi, at right Nolina microcarpa. 


Plate 12 


Plate 13 

A. Heavy Carpet of Summer Herbaceous Vegetation on a Flood-plain at 5.000 feet. Soliilayo sparsiflora 
var. subcinerea., Mottarfla pectinata, Crotalaria litpitlina, and Sphceralcea sp. 

B. Agate schotHi in the Lower Encinal. Extensive areas between 4,200 and 5,400 feet are 

covered by this plant. 


Plate 14 

A. Oymnopteria hispida occupying a ledge of rock in the Lower Eneimil ut 5,200 feet. 

B. Mats of Selaftinella sp. among rocks at 5,300 feet. 


Plate IS 


Plate 16 


Plate 17 


Plate 18 


Plate 19 

A. Flood-plain in Bear Canon at 0,000 feet, with Pimis arizonica, Populun jamesii, an open carpet of 
summer-active herbaceous perennials, and Sporolxtlus con/usim. 

B. Streamway in Bear Canon at 6,000 feet, with Pinus arizonica, Juglans rupestris, and Vitis arizonica. 


Plate 21 

Open stand of Pinus ariionica, Finus chihuahuana, and Juniperus pachyphl&a near floor of Bear Canon at 0,100 
feet. In background are rocky slopes of north wall of Bear Canon. 


Plate 22 


Plate 23 


Plate 24 


Plate 25 

A. Looking toward .south face of Mount Lommon from crest of Marshall Gulch, near site of 8,000-foot 
climatological station. Pinits arizonica and .scrub of Quercua reticulnta. 

B. Looking southwest into Marshall Gulch. The open area in the forest is a thicket of Populus tremuloides. 


Plate 26 


Plate 27 

A. Typical heavy stand of l j i>ius arizunica on a bench in Marshall Gulch at 7,800 feet. 

B. Looking east along south-facing slope of Marshall Gulch at 7,700 feet. The bunch-grass is 


Plate 28 


Plate 29 

A. Open Forest on Steep Soutli Slopes of Main Rifl^e at S,o()0 feet. The shrubs are (^uercua reticulata. 

B. Stream and Narrow Flood-plain in Marshall Gulch near Montane Garden. Alnus acuminata, Acer 

brachypterum, and Abies concolor. 


Plate 30 


Plate 31 


Plate 32 

A. Looking northwest from main ridge toward Samaniego Ridge. 

B. Looking east along main ridge of Santa Catalinas from a point east of Mount Lenimon at about 8,600 feet. 


Plate 33 


'* ?w 

A. Open Forest on summit of Mount Lemmon, at 9,000 feet, with gtxnl reproduction of Pinus arizonica and 

a close stancl of Duyaldia hoopcsii. 

B. Mature thicket of Populus tremuloides on summit of Mount Lemmon at 9,100 feet. In foreground are 
Pteris aquilina var. pubescens and flowering plants of Frasera speciosa. 


Plate 34 


Plate 33 

An alluvial flat in Fir Forest on north slopes of Mount Lemmon at 8,600 feet. Pinuit arizonica, Abies concolor, 

and Populits trenndoides. 


Plate 36 

A. Santa Catalinas viewed from north, showing grassy plains at elevation of 4,200 feet in the vicinity of 
Oracle. At right Prosopis velutina, at left Yucca alata. 

B. At north base of Santa Catalinas looking toward San Pedro River, at 4,500 feet. At right Quercus emoryi 

and Nolina microcarpa, at left Yuuca alata. 

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