Relative resistance of tree seedlings to excessive heat

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Washington, D. C. September 3, 1924

RELATIVE RESISTANCE OF TREE SEEDLINGS TO EXCESSIVE HEAT

By Cartos G. Bates, Silviculturist, and JAacoB Roeser, Jr., Forest Examiner,
Fremont Forest Experiment Station, Forest Service

CONTENTS.

: Page Page

het problems! oy ds fe ee 1 | Description of experiments—Contd.

Raterature available. 2 2 Results in dry air 46, 64, and
Description of experiments_________ D 90 to 92 days after sowing__ 9
BRrelimimary., tests.__..__ += rer + 4 5 Influence of age of seedlings___ 12

Blan of fests in 1922° "ees 5 Temperature scale for each spe-
Results in moist air 30 days CICS SE ey a a a a 12
EEEEy SOW 4S ey mat Conelisions ate) eo ee 13
intteratureeited =e. ee si Fe Eee 16

THE PROBLEM.

The studies of forest types and of the limiting factors in the dis-
tribution of the Rocky Mountain forest trees, which have been car-
ried on at the Fremont Forest Experiment Station near Manitou,
Colo., and elsewhere in the central Rocky Mountain region since
1910, indicate that excessive temperatures due to direct insolation
may often be a deciding factor as between one species and another.

In nature, however, and under the usual experimental conditions,
it is almost impossible to attain an intense heat or an injurious tem-
perature without rapid and usually thorough drying of the sur-
face soil. For this reason it has been very difficult to draw safe con-
clusions as to whether seedlings which succumb under direct insola-
tion and extreme heating of the surface soil were in fact first in-
jured by the temperature or by this dry condition.

With a moist condition, the first injury under intense heat may
be to the leaves. But in direct sunlight the surface of the soil, un-
less it is very moist, reaches a higher temperature than the leaves
of the seedlings. If, therefore, the high temperature is approached
gradually with opportunity for soil drying and heating, the first in-

_ jury may be to the stem of the seedling, which wilts at the ground line

and causes the seedling to collapse. This constitutes a “ border-
line” case between heat injury and moisture shortage. More pro-
longed heating and drying will cause a general wilting, plainly due

91204°—24

2 BULLETIN 1263, U. S. DEPARTMENT OF AGRICULTURE

to the inability of the seedling to supply moisture equal to the
transpiration. Of course, this loss of water may be accompanied
by direct heat injury as well; but, in general, if the soil drying has
advanced very far, the result should be thought of as due primarily
to lack of water.

In the present study the effort has been made to devise tests which
would show the ability of seedlings to withstand high temperatures,
as distinguished from a lack of water in the stems or leaves. The
general assumption has been that if the protoplasm is heated to a
temperature of 140° F. or 60° C., direct injury, from which re-
covery is not probable, will ensue almost immediately. It seems im-
probable that the qualities of protoplasm differ appreciably in this
respect as between the several species. On the other hand, the ex-
tent to which the cells of the plant remain below the air tempera-
ture, or fail to be heated by radiation penetrating to their interior,
must vary considerably according to the absorptive nature of the
cell walls and plastids, and the rate at which the heat is being dis-
sipated by vaporization and transpiration. Information has been
particularly sought to answer the questions: Are the seedlings of
different species protected in different degrees by their varying
transpiration rates? Is this the basis for their apparently dif-
ferent heat requirements, heat tolerances, and light tolerances?

LITERATURE AVAILABLE.

There is little information in the scientific literature on direct
heat injury to plants growing under natural conditions. That ex-
cessive temperatures from direct insolation may be a deciding factor
between one species and another was claimed by Bates in a recent
article (2), and is brought out again in his more elaborate dis-
cussion (3) of the physiological requirements of the Rocky Moun-
tain trees and in his extensive data on climatic and soil conditions
just published (24). In the second citation it is clearly shown that
the common belief that spruce requires more water than pine is not
well founded, and is, in fact, definitely controverted by transpira-
tion measurements. Engelmann spruce (P2cea engelmannii), Doug-
las fir (Pseudotsuga taxifolia), lodgepole pine (Pinus contorta),
and western yellow pine (2. ponderosa), require increasing amounts
of water in the order named, either on a basis of the increments or
the leaf-exposure area.” :

The concept is therefore reached that since spruce transpires less
water than pine it may be more sensitive to heat, primarily because
the incident energy is not so largely utilized in the evaporation proc-
ess. It is, however, admitted by Bates (2) that since spruce has
long been accustomed to seek cool places, where alone its seedlings
may survive, it has acquired the habit of slow, shallow rooting, be-
cause these cool places are rarely subjected to rapid drying or to
extreme drying of the surface soil. Conversely, western yellow pine,
since it needs much heat for its proper functioning and develop-
ment, has become accustomed to soils which are directly exposed to

—

1 The numbers in italics in parentheses refer to “ Literature Cited” at the end of this

bulletin. , F
2Reference to citation (2) will show that this arrangement depends on several condi-

tions. The arrangement given is the first deduction from transpiration tests in 1917.

—. » os

oe ee SS ae

RELATIVE RESISTANCE OF TREE SEEDLINGS TO EXCESSIVE HEAT 8

the sun and which are quickly and frequently dried out, at least in
the superficial layer. The pine, therefore, has the quick, deep-root-
ing habit and the large seeds which make such rooting possible even
before the cotyledons have unfolded. Pinon pine (P. edulis), which
grows in even warmer situations than yellow pine, has this habit
developed to a high degree, opening its cotyledons and shedding the
seed coat with the greatest deliberation while a strong tap- -root is
being formed.

It is, therefore, easily seen that in investigating the present reac-
tions of seedlings of the different species, not only the fundamental
physiological requirements must be taken into account, but adapta-
tions of form and habit which fit the trees for certain com! pinations
of conditions commonly occurring in nature. Western yellow pine
and Douglas fir, as shown by the large size oi their seeds and the
deep rooting of the seedlings, are clearly adapted to sites whose
moisture supply changes rapidly, whatever their total requirements
for moisture may be. Lodgepole pine and Engelmann spruce seed-
lings come from much smaller seed, and are “about equally frail,
shallow rooting, and unadapted to quick drying of the surface.

The fact that the few references on heat resistance that have been
found are taken from observations in the more arid portions of the
United States, seems to indicate the close dependence of excessive
temperatures upon a dry condition of the atmosphere, and a con-
sequent aridity of the soil surface. The problem, as understood in
this discussion, arises from the great intensity and heating power
of sunlight, due both to the aridity and high altitude of much of
the Rocky Mountain region. In moister regions the problem is no
doubt less serious.

The first specific evidence of direct heat injury to tree seedlings
appears to have been noted by Hartley (5) and Bates (7) in 1909,
in the conduct of planting experiments in the Nebraska sandhills,
where the occurrence in a single day’s observation of a soil surface
temperature of about 60° C. led Bates to the belief that the losses in
plantations on sowth exposures, where much higher temperatures
doubtless occur at times, might be primarily due to ythis factor. This
appeared especially probable i in cases where the small trees had been
partly or completely buried by moving sand; but it was also noted at
this time that many trees, which had not been covered and did not
die, had considerable enlargements of their stems at the grou nd line.

Hartley (5) in 1918 published his observations of heat injury
noted at about the same time in Nebraska and elsewhere. He con-
cluded that lesions which were common on the stems of very young
seedlings in the nursery were undoubtedly due to excessive “heating
of the surface soil, while similar lesions on the stems of trees as
much as 4 years old were probably due to the same factor. Ex-
periments were conducted in the laboratcry which showed that stem
injury very similar to that noted in nursery beds could be produced
by heat applied in several ways, but more readily above than below
the surface of the soil.

Elsewhere it has been frequently noted in the handling of nursery
stock that trees growing close tcgether had normal stems, while those
spaced widely enough to permit much sunlight to reach the ground
showed lesions on the stems. Korstian and Fetherolf (6), for ex-

ample, found that by inclining spruce seedlings slightly to the south

4 BULLETIN 1263, U. S. DEPARTMENT OF AGRICULTURE

when transplanting them the ground was sufficiently shaded by the
es oe practically eliminate the injury which they termed “ stem-
girdle.”

That the optimum temperature for assimiliation and other chemi-
cal activities within the plant may be considerably below the maxi-
mum temperatures experienced, has of course long been recognized.
Cieslar (4), for example, discussing the experiments of Lubimenko
to determine the effect of light and temperature upon assimilation of
Abies, Picea, Pinus, Larix, Taxus, Tilia, Robinia, and Betula, states
that “with medium light intensity, the assimilation rises to a tem-
perature optimum, varying with species, then sinks (with higher
temperatures), excepting with the larch, which continues to increase
its assimiliation to the highest temperature used. Similar results
were secured under the strongest light.” Cieslar concluded that
“a close relation between light and temperature influence exists and
that the (shade) tolerant species behave differently from the intoler-
ant. ‘The former decrease in (assimilative) energy very rapidly after
the maximum; the latter very little, or not at all. So it may be
said that the foliage of intolerant species is less sensitive to heat
than that of tolerant.” The results of the experiments reported
here substantiate this last statement.

MacDougal and Working (8), experimenting with Opuntia at
the Desert Laboratory, found that “ joints of this plant maintained
a fair rate of enlargement at a temperature of 56.5° C., the air sur-
rounding them being 58° C. (136° F.), and the growth of young
joints of the same plant stopped and shrinkage ensued when the
temperature rose to 144° F. in an air temperature of 146° F., but
growth was resumed when their temperature fell below 122° F.” Of
special significance was the finding that “ data from observations in
which temperatures were taken from the air have but little value in
any estimation of the working temperature of active protoplasm,
by reason of the abnormal hydration and transpiration conditions
introduced. ‘These conditions as well as the proportions and state
of the main colloidal components must determine the temperature
effects.” It is very likely that the transpiration conditions involved
in the above study were considerably more severe than those involved
in the present consideration of the central Rocky Mountain conifers.
In a comparative study of species, however, we are not more con-
cerned with absolute plant temperatures than with the recording
of air temperatures ‘and the different reactions to specific external
conditions.

The usually close connection between heat injury ‘and excessive
transpiration is likewise illustrated in a discussion by Munns (7)
of the effect of the Santa Ana wind on various eucalypts. This wind
is a hot current which occurs in southern California in the vicinity
of the mountains. While the effect of the wind on the foliage of
eucalypts is described as a “burning,” the conclusion is that this

°Qne of the factors that has no doubt retarded the study of possible injuries from
excessive heat has been the lack of suitable thermometers for recording the plant tem-

peratures closely enough to establish the connection between them and the injury. While
the apparatus has not been used in the present study, some of the difficulties of tem-
perature measurements have been solved by E. Shreve (9). The apparatus described con-

sists of a comparatively simple and small thermocouple outfit, the sensitive junction of
which may be pressed against the surface of a leaf, obtaining a_ quick response to its
temperature and therefore not seriously interfering with the conditions which most di-
rectly influence that temperature,

RELATIVE RESISTANCE OF TREE SEEDLINGS TO EXCESSIVE HEAT 5

was in the nature of a rapid drying, and that the various degrees
of injury were controlled more by soil differences than by any other
variable in the habitat or in the species. Trees on sandy soils of
relatively small water-holding capacity were most injured, and those
on loamy soils of relatively high capacity were best able to with-
stand the unusual demand for water at the leaves. Therefore, the
conclusion seems justified that this instance does not give an illustra-
tion of heat injury, in the strict sense of the term, and it is difficult
to conceive of any temperature recorded in naturally heated moving
air being high enough to produce heat injury, at least so long as the
fohage retained any moisture whose evaporation might have a
cooling influence.

DESCRIPTION OF EXPERIMENTS.

PRELIMINARY TESTS.

The first actual experiments in heat resistance made by the present
writers, in 1919, appear to have given what have been described
above as “ border-line ” cases, involving drying injury quite as much
as or more than direct heat injury. These tests have been described
and reported by Bates (7) and, therefore, need be only briefly
mentioned to give the reader a full understanding of the problem.
Seedlings of the four Rocky Mountain species which have been
mentioned, as well as the eastern white pine, were grown in a
number of pans, which after germination had taken place were
kept at different moisture contents, varying from 8 to 14 per cent.
The pans were heated by sunlight in a greenhouse, where the air
temperatures were sometimes raised by artificial heat. Maximum
temperatures of about 130° F. were recorded by blackened ther-
mometers resting on the surface of the soil. The pans were watered
early each day so that, by the time the maximum temperature was
attained, they had had considerable opportunity to dry. The most
heavily watered pans were never in a dry condition, however, and
the moistest soil did not permit any loss that could possibly be
ascribed to heat injury. The losses increased quite definitely with
decreasing moisture, so that the influence of this factor is evident.
Most of the injury took the form of stem wilting, similar to that
described by Hartley (5). Douglas fir and yellow pine showed the
lowest percentages of injury, followed by white pine, Engelmann
spruce, and lodgepole pine. The severe injury to lodgepole was
ascribed in part to its slow germination, so that the seedlings were
_ younger and more frail than those of other species when the first
severe exposures were given. It seems quite certain that the poor
showing of both the lodgepole and spruce seedlings was due to their
frailty, small mass to resist drying, and relatively small root systems
to supply moisture.

PLAN OF TESTS IN 1922.

The second series of experiments was planned to test the resistance
to heat injury exclusively of the various species at the ages of 30,
45, 60, and 90 days. It was conducted in the greenhouse and labora-
tory at the Fremont Forest Experiment Station.

6 BULLETIN 1263, U. S. DEPARTMENT OF AGRICULTURE

On June 30, 1922, enough seed* of lodgepole pine, Engelmann
spruce, Douglas fir, and western yellow pine was sown in four of
the seed-testing tills, to secure approximately 100 seedlings of each
Species in each till. The contents of one till were used for each pe-
riodic test. The soil was coarse and derived by sifting from the
granitic soil of the region. In order to secure the best germination
the tills were kept under the regular seed-testing conditions, which
include a daily maximum temperature of about 85° F. at the soil
surface. They were watered daily. Germination was prompt, all
species starting in from 7 to 15 days, and all seeds but these of lodge-
pole pine germinated fully within about 30 days.

The first test was made July 30, when the seedlings were heated
in the electric oven, the temperature of which is readily regulated
and controlled by a thermostat. The atmosphere within was kept
saturated by means of a pan of water placed over the heating coil,
the object being to reduce the drying of the seedlings to the lowest
possible point. The procedure was as follows:

For each subtest, five seedlings of each species, selected at ran-
dom, were so placed between wet blotters that the root portion of
each was protected and well supplied with water, while the aerial
portion was exposed. The blotters were held together with clips,
and the whole was placed in the oven in a horizontal position, with
the Jeaves of the seedlings not in contact with any solid object. A
maximum-registering thermometer was placed with its bulb in the
same plane with the leaves of the seedlings. At the expiration of
the allotted time, the packet was promptly removed from the oven,
the thermometer was read, the time noted, and the blotters again
moistened and placed so that the seedlings projected over the edge of
a table. As the seedlings wilted, they were removed and the time
at which wilting took place was noted. After the immediate obser-
vation each set was removed to the side, kept moist, and observations
made for wilting at intervals of a few minutes for seven hours.

For the three later tests; made on August 15, September 2, and
September 28 and 30, an entirely different plan was followed. The
purpose was to produce a heating and drying effect similar to that
produced by sunlight. A commercial electric heater was used as the
source of heat. The heating coil, which uses about 550 amperes, is
23 inches long, and 14 inches in diameter, and the reflector 8? inches
in diameter.

For the second test the seedlings from one of the tills were trans-
planted about August 5, after being hardened by continuous ex-
posure to the sun for several days. Ten deep 6-inch galvanized pots
were prepared, each containing six seedlings of each species placed in
rows. These pots were exposed to the sun five days prior to the
time of the test and given water each day until the day before the
test. The soil was decidedly moist at the time of testing.

One pot was used for each subtest. To attain various temperature
effects, the electric coil was placed at distances varying from 1 to 2.5
feet from the center of the top of the pots at angles of 45° to 75° with
the surface of the soil. Two thermometers were used, one with the

4The source and quality of the seed employed is a matter of record, and for present
purposes need not be described.

ea

RELATIVE RESISTANCE OF TREE SEEDLINGS TO EXCESSIVE HEAT 7

bulb partly covered by the soil, the other suspended in the air with
the bulb in the same plane as the “crowns” of the seedlings. Ex-
posures were for five minutes or more, but in all cases so short
as to cause little drying of the soil. In consequence, the thermometer
exposed in the air and nearer to the heating coil than the soil ther-
mometer always recorded the higher maximum temperature. Since
the injury was almost wholly to the leaves of the seedlings, it is this
temperature of the air thermometer, receiving the same direct radia-
ticn as the seedlings, which will be referred to. The temperatures
were observed throughout the period of test, and the maxima, which
ordinarily were obtained near the end of each test, were recorded. In
the tables all temperatures noted are the maxima, in degrees Fahren-
heit. :

After each subtest the pot was watered and kept shaded for a
half day or so to note any immediate results, after which it was trans-
ferred to the seed-testing table in the greenhouse, where it attained a

_ daily maximum temperature of approximately 77° F. at a depth of 1

inch in the soil, or about 85° F. at a depth of one-fourth inch. Daily
observations were made to note wilting.

The third test, made 64 days after sowing, employed four pots
which had been used in the second test and had lost no seedlings in
the exposures of that test. The seedlings were subjected to the direct
radiation for periods of 6, 8, 10, and 12 minutes, at such distance
from the coil as to produce in all cases maximum temperatures of
about 150° F.

For the fourth and final test, 90 to 92 days after sowing, the same
procedure was employed as for the second test; except that, instead
of placing the seedlings in rows, the four species were scattered pro-
miscuously, using as before six seedlings of each species in each pot.
The pots were exposed several days before testing to make certain
that the seedlings were well established, and the final tests used the
electric coil in conjunction with solar heat. The period of time
varied from 12 to 128 minutes, and the thermometer recorded air tem-
peratures from 110.84° to 161.06° F.

After the completion of the fourth test on September 30, the pots
were placed under observation until November 17, since the seedlings
by this time had become hardened to the extent that many of them
survived for several days. The immediate results in each test were
noted for each species; such as, two killed, four badly cooked, one
slightly injured. The judgment formed at this time as to the extent
of injury, however, had no bearing on the final results. The seedlings
were not removed from the pots until obviously dead, and none were
removed until the second day. At the expiration of the periods of
observation in each of the three tests, the seedlings were classified as
dead; injured, will eventually die; injured, will recover; and unin-
aa RESULTS IN MOIST AIR 30 DAYS AFTER SOWING.

In test 1, the seedlings were subjected to hot-air temperatures in a
saturated atmosphere, ranging from 101° F. for two minutes, which
eaused no loss, to 150.5° F. for four minutes, which caused the death
ot all seedlings. The results are shown in Table 1, where the subtests
are arranged in the order cf increasing temperatures.

8 BULLETIN 1263, U. S. DEPARTMENT OF AGRICULTURE

TABLE 1.—First test of heat resistance in moist, hot air, 30 days after sowing,
July 30, 1922.

[Five seedlings of each species used in each test.]

Number of seedlings wilting after exposure, classified by
species and by elapsed time.!

Subtest No. temper- sure
ature. time Western Lodgepole Douglas Engelmann
yellow pine. pine. fir. spruce.
ag Min

2 OTE AO Se ANE ME 2 111.0 6 2-0-0-1-0-0-0 1-0-1-1-0-1-1 0-0-0-1-0-2-0 0-0-2-1-0-0-0
ae aa Se a ee eee 122. 0 10 3-0-0-0-0-0-1 2-0-1-0-0-0-0 | 4-0-0-0-0-1-0 2-0-0-1-0-0-0
(Fale cient A ew eal lat 124. 0 2 0-0-0-0-1-2-0 |} 0-1-0-0-3-0-1] 0-0-0-0-1-0-1 0-0-0-0-0-2-1
1 ee ee ide 5s 5 0-2-1-0-0-0-0 | 3-0-1-0-0-0-0 | 0-0-1-1-0-0-0 1-1-0-1-0-0-0
Sa ee gt Riray 128. 0 5 3-0-0-0-0-0-1 | 3-1-0-0-0-0-0 | 3-0-0-0-0-0-0 2-2-0-0-0-0-0
~! ae es eae 28 8 ee ae 125 4 3-0-0-0-0-0-0 | 1-0-1-0-i-0-0 | 1-0-0-0-1-1-0 1-0-0-0-1-1-0
Cece ea oe Od 134. 5 3 1-0-0-1-1-0-0 } 2-0-0-2-0-1-0} 06-0-0-2-0-1-0 0-1-0-1-0-1-0
tere ee Se TS 135. 0 2 0-0-0-1-1-0-3 | 0-0-0-0-2-1-2 |} 0-0-1-0-1-0-3 0-0-0-1-0-2-2
of Ne Se hy ee 137. 0 3 3-0-2-0-0-0-0 | 2-0-3-0-0-0-0 | 4-0-1-0-0-0-0 2-0-0-0-0-1-2
TKN) Seateelite lip eed onan en Seg 138. 0 5 5-0-0-0-0-0-0 | 3-0-0-0-2-0-0 | 2-0-0-0-3-0-0 1-0-0-1-3-0-0
7 fle. eae OB ae ed 139. 0 2 0-0-0-2-1-0-0 | 0-0-0-2-0-1-0 ; 0-0-0-2-0-0-0 0-0-0-1-0-1-1
LG eee = eae 143. 3 2.5 | 40-0-1-0-0-0} 3-0-0-1-1-0-0} 1-0-0-1-2-0-1 1-0-0-1-0-0-3
ih Gene So wee ELEY, 148. 0 1 0-0-0-0-5-0-0 |} O-0-0-0-3-0-2} 0-0-0-0-40-1 0-0-0-0-1-0+4
iA epee t= Ries hs 149.7 3 4-0-1-0-0-0-0 | 2-0-2-0-1-0-0 2-0-1-0-0-0)-2
TRY Be aS ae sy 150. 5 4 5-0-0-0-0-0-0 | 5-0-0-0-0-0-0 5-0-0-0-0-1-0
MS OG alee et eee altel el oi Lee ee 34-2-3-6-9-2-5 |29-2-8-6-12-4-5 |22-0—-5-7-13-4-6 | 17-4-3-8-5-8-15

_

Grand total ae oe ee ae a 61 67 57 60

Per contiSis.. ti: rn 76.2 83. 8 71.2 | 75.0

1 The seven divisions under each species correspond to minutes of elapsed time after removal from oven,
in the following order: 0-4-10-20-60-180- over 180. ee
2 Subtest No. 2 was made at 101°, with a 2-minute exposure, no seedlings wilting.

The reactions of all species were much alike; in fact, no species
can be said to be decidedly more resistant than any of the others
under conditions which tend to eliminate all transpiration, and
therefore to put the species on a basis of equality. Lodgepole pine
suffered most, losing 100 per cent by wilting at a temperature of
124° F. for 2 minutes; 40 per cent, however, survived 139° F. in a
different subtest for the same period. Ina third instance all species
withstood 139° F. for 2 minutes, while in another group all lost 100
per cent. when submitted to 185° F. for 2 minutes. The explanation
of these variations is not readily discernible. It does not seem prob-
able that the seedlings of all species should have been stronger in one
instance than in another. The variations are more likely to have
been due to differences in humidity, which near the critical tempera-
ture is probably the decisive factor. All trees were destroyed at
148° F. with a 1-minute exposure, and from study of Table 1 it seems
that approximately 141° F. for a period of 1 minute may be con-
‘naa ed critical for all species. A total of 80 seedlings of each species

ras used. The totals lost in all tests, which may “be indicative of
meine resistance, were as follows: Douglas fir, 57; Engelmann
spruce, 60; western yellow pine, 61; and lodgepole eae 67.

‘The pines exhibited the disastrous effect of too much heat by wilt-
ing more promptly than spruce or fir, but suffered no more severely
after longer periods. The pines are more succulent at this early age,
and their morphological structure is not so strong in relation to
their size as that of spruce and fir. Sixty-nine per cent of the
yellow pine, 66 per cent of the lodgepole, 56 per cent of the fir, and

OS ee
Si

RELATIVE RESISTANCE OF TREE SEEDLINGS TO EXCESSIVE HEAT 9

50 per cent of the spruce wilted within the first 15 minutes. In prac-
tically all instances the injurious effect of high temperatures was
shown first by the needle tips, which, because of their small masses,
would naturally become heated more “quickly.

It is believed that this test of the 30-day seedlings more nearly
furnishes an expression of the effect of injurious temperatures upon
the protoplasm itself than do the tests upon the older seedlings, be-
cause of the fact that transpiration was of minor consequence. In the
saturated atmosphere maintained throughout, the pines could not
take advantage of their power of transpiring more readily than the
others and thereby keep down internal temperatures. The great
difficulty in any tests of this nature hes in the inability to separate
the transpiration factor from the heating factor which might more
immediately cause the coagulation of the protoplasm. Therefore, by
eliminating transpiration it was to be expected that all species would
react alike to high temperatures, assuming all protoplasms to be sub-
ject to coagulation at the same temperatures. It should be noted,
however, that while test 1 comes nearest to eliminating transpiration,
it 1s conversely farthest from simulating actual field conditions.

RESULTS IN DRY AIR 46, 64, AND 90 TO 92 DAYS AFTER SOWING.

Since these three tests were performed in a similar manner, and the
results obtained were much alike, they will be considered together.
The results at the ages of 46, 64, and 90 to 92 days, respectively, are
shown in Tables 2, 3, and 4. The subtests are arranged in each table
in the order of maximum temperatures recorded.

TABLE 2.—Second test of heat resistance. Exposure to radiation from elec-
tric coil 46 days after sowing, seedlings transplanted in sand.

[Six seedlings of each species used in each test.]

| F
: Number of seedlings dead, mortally injured,
Maxi- Maxi- | slightly injured, and uninjured, by species.!
y. t et aibenlibaeouel. 1Db.<
Pot No. Sra tempera-| posure. l
lies ture. Western | Lodgepole | Douglas | Engelmann
Saige | yellow Pine, pine. fir. | spruce.
OF oi Min.
Tes cae a aoe 90. 5 78.7 10 0-0-0-6 0—0-0-6 0-0-0-6 0-0-0-6
FN SR as i iii ae ae 106. 7 88. 7 10 0-0-0-6 2 0-0-0-5 0-0-0-6 0-0-0-6
Migr en ney sc es ON 116. 6 98. 2 14 0-0-0-6 0-0-0-6 1-0-1-4 0-0-0-6
A eed Reine epee SHUN Se 82)h fu 137.8 112. 3 13 0-0-0-6 0-0-0-6 3 0-0-0-6 0-0-0-6
Li Ss 915 SEO 8 eg 154. 8 118. 3 10 1-0-0-5 0-0-0-6 3 5-1-0-0 3 1—1-1-3
iS ok Metacafe 8 165. 2 134. 2 10 4 3-0-2-1 2-0-1-3 5 6-0-0-0 > 6-0-0-0
p 8 parse a ee aes Sa ae 172. 4 IPG 8 6 2-1-2-1 § 2-0-1-3 5 6-0-0-0 6 4-0-9-0
TE ect EPR ae a, SS 176. 4 117. 6 5 0-0-1-5 1-0-0-5 3 6-0-0-0 3 6-0-0-0
(he 2 GO Oe ae oe 176. 4 118. 3 6 3-0-1-2 0-0-0-6 3 §6-0-0-0 | 3 6-0-0-0
Ree ee en 181.8 122. 4 7 621-1-2} 61005| 5600-0| *5-0-1-0
Averages - -a--..- 147.9 111.0 9.3 |
yf ame Bie We apla. ies lee Rll sana 11-2-7-40 | 6-0-2-51 | 36-1-1-22| 28-1-4-27
| List A RMN I STE FS }
Reem COCA ene 5 ee 2 ee ee eee See 81-4-14-140

1 The four divisions under each species refer, respectively, to the following degrees of injury: Dead
seedlings; injured seedlings that will die; injured seedlings that will live; uninjured seedlings.

2 Only five lodgepoles transplanted into this pot.

3 All seedlings in pot slightly cooked.

4 Three of the seedlings cooked during exposure.

5 All seedlings cooked.

6 One seedling cooked.

10 BULLETIN 1263, U. S. DEPARTMENT OF AGRICULTURE

Reactions in these tests were dissimilar to those in the first, because
a dry atmosphere introduced the factor of transpiration. In all cases
the soil was moist enough so that wilting could not be ascribed to
Jack of soil moisture. There was no constant relation between air
and soil temperatures, the ratio of soil to air temperature increasing
with the period of exposure. For the critical air temperatures above
approximately 140° F., the soil temperatures were from 15° to 50°
less. In only one instance did the soil temperature exceed 140°;
144.8° was secured with an exposure of 30 minutes, which produced
an air temperature of 161° F. Under ordinary field conditions,
where the soil is not commonly so moist as under these experimental
conditions, a higher temperature is likely to be recorded by a ther-
mometer at the surface of the ground than in the air above the
ground; and the seedlings are subject to wilting, not only in the
leaves but also in the stem where it comes in contact with the surface
of the ground.

TABLE 3.—Third test of heat resistance: Exposure of seedlings that survived
second test to radiation from electric coil 64 days after sowing.

[Six seedlings of each species used in each pot.]

Number of seedlings dead, mortally injured,
Maxi- Maxi- slightly injured, and uninjured, by species.!
mum mum
Pot No. temper- soil Brgy: |
= > Ji 5 |
sae peeing ee Lodgepole | Douglas Engelmann
: pine. fir. } spruce.
pine.
| | eo rah
°F °F Min |
| : : : er f A (6) A (6)
iis ee aie eA Se | 147. 2 102.0 6 eau, 0-0-0-6 \ +3 0-0 ; 1-0-3-2
| \ P 6
Deed ick sare gee 150.8] 110.0 st PSB} ro005 , BOL sore
B (2) \ yee: { B (8) B (4)
Bagge Sates Sac Soe oa etl | eae ee ~ { MEG oes a 3-2-1-0 | 2-0-2-2
BG B( B (6) \
4__--------------------- | 150. 8 106. 3 12 { 3-0-2-1 0-1-2-3 3 4-0-0-1 | 1-14
ANGI AGC. 2 ces ae 150. 4 106. 3 9 AO
Lio) bea oe Sa a es | cee ene | 2 ee eee [az see Sees 5-3-10-6 0-2-3-18 14-§-2-1 | 6-1-6-11
Grand totals=- 23) 2. 2B eee EE Ee | te / 25-19-21-36

——— ———,

1 Capital letters indicate immediate condition of seedlings following exposure; figures in parentheses
indicate seedlings so affected, as follows: A=slightly cooked; B=seriously cooked. ‘The four numbers
under each species indicate the final condition, in the following order: Dead seedlings; injured seedlings
that will die; injured seedlings that willlive; uninjured seedlings.

2 Only five lodgepoles transplanted in this pot.

3 Of five firs in pot 4, the one escaping injury was very short stemmed.

Throughout the tests at high temperatures, the fir was most visibly
affected. The needles appeared to be both shriveled and blistered,
for which superficial effect the word “ cooked” has been used. The
spruce was affected in a similar manner, but less markedly. The
pines apparently took care of themselves by excessive transpiration
up to the limit of their ability to supply water to the leaves. In
view of the fact that the yellow pine seedlings were considerably
taller than the others, and thus more exposed, they resisted well.
When the danger point was reached their needle tips shriveled first.
Lodgepole may have received some protection from the taller seed-
lings; but its general high resistance in all the tests discounts this
factor, especially since the spruce seedlings of approximately the
same height suffered much more severely.

—————w

RELATIVE RESISTANCE OF TREE SEEDLINGS TO EXCESSIVE HEAT J]

TABLE 4.—Fourth test of heat resistance: Exposure to radiation from electric
coil 90 to 92 days after sowing, seedlings potted in sand.

[Six seedlings used in each pot, except where noted.]

Number of seedlings dead, mortally injured,

Maxi- Maxi- slightly injured, and uninjured, by species.
mum mum eee
Pot No. | tempera- soil ioe ‘ l
tae 6 ee esate Lodgepole | Douglas /Engelinann
“pine. pine. Ldaye spruce.
RT NE WN ARI A onsite ad i tial Theos a
A a SKS Min
Jac scocsscesaeeee sees 110.8 107.0 128 0-0-0-6 0-0-0-6 0-0-0-6 0-0-0-6
i) Si tie ep 111.4] 1020 78 0-006] 0006| 0006) coos
Tay SS Aen ee eee aia 111.4] 105.5 30 0006} oo0c| oco24] c006
2 p= S55 snes ee sles anne 115. 7 105. 5 87 0-0-0-6 0-0-0-6 0-0-0-6 0-0-0-6
oh S 2 ae oo ea ee 118.0 112. 1 30 0-0-0-6 0-0-0-6 0-0-1-5 0-0-0-5
po nithighe As fytrr: 133. 3 119. 5 30 0-0-0-6 0-0-0-6 0-0-2-4 6-0-0-6
; i Sil A C (1) E (2) F (1)
Oger tae Seopa Lp oy MR ne 1 00-3-3| 1005| i032 2-0-1-3
AG eee matress * Lars 142.7 119. 0 15 0-0-1-5 0-0-1-5 0-0-5-1 10-14
Dy » KFQAC2| Ba | F@Cc@ B (1)
crear gy <a vatpae aia Ea esa a i 0-2-1sb 2320-048 | 3-129-0 2-1-1-2
o | E (4) E (1)
Rae eer ia 146.8| 123.0 12 HOG 22) 10002) ON ones
= A E(2) C(8) B (1)
“(Sag he 0 Wiles aie 152.2} 121.5 15 i ara 0-024 { FQ C@ 38D.
5 A oe E (4) B (2)
(OU ee ao EES 154.4] 126.8 12 k aay \ 0-0-1-5 { aS. Baad
eee E (5) DQ) | E(S)4CQ) C (2)
6----------------------- 161.1 144.8 30 { 2E 1-1 3-0-1-2 6-0-0-0 2-3-0-1
A VGFORG. 20521 obi 133. 5 119. 9 42.5 SSS =
UNG LS ha Re SE Ee ee pares Cee Ce 8-2-15-51 | 7-05-62 | 17-4-20-36 1i-#-6-52
—— a eT
Gfarid tqigl 9k | +055 .02 53 | i pk ew 2 Pps Seeere as 43-12-46-201
_————
Average 5_________ 149. 0 122. 6 13.5 | 129-10] 004-16] 7-3-10-3 | 5-2-4-10
aD Tp aS ae oe et Oe eee re Je ©). oe Bese eee ]
ah ee
(Sratid. fatals. si. boes oe < 2 is] eee 4 |e et pe rate Th | 13-7-27-39

_ 1 Capital letters indicate immediate condition of seedlings following exposure; figures in parenthesis
indicate number of seedlings so affected, as follows:
pea echa needles cooked; B=Slightly injured; C=Seriously injured; D=Badly burned; E=Cooked;

= ed.
_ The four numbers under each species indicate the final condition, in the following order: Dead seed-
lings; injured seedlings that will die; injured seedlings that will live; uninjured seedlings.

* Second exposures given these pots, after two days’ observation showed no apparent injury from first
exposure.

3 Owing to mishap after the exposure, only 4 yellow pine, 2 lodgepoles, 5 firs, and 3 spruce were leftin
pot 8 for final observation.

* This injured seedling plainly was protected by the leaves of a taller seedling, but died eventually.

® These averages and totals are for pots 10, 8, 7, and 4 apart, whose period of exposure makes possible a
comparison with earlier tests.

Of 156 lodgepole seedlings used in the three tests, 15, or 10 per
cent, died or were injured so severely that death was only a matter
of time; 31, or 19 per cent, of the 160 western yellow-pine seedlings;
53, or 33 per cent, of the Engelmann spruce seedlings; and 78, or
49 per cent, of the Douglas fir suffered likewise.

The point clearly brought out by these tests was the great re-
sistance of lodgepole pine, which surpassed even yellow pine. AI-
though it is a well-known fact that lodgepole reproduction is most

easily secured and thrives best on open burned areas, where extremes

12 BULLETIN 1263, U. S. DEPARTMENT OF AGRICULTURE

of temperature are likely to occur, its natural range at high eleva-
tions greatly reduces the probability of its having to resist tempera-
tures as high as those experienced by yellow pine, and its total heat
requirements are apparently less. Its preference for bare mineral
soils as a germinating bed is an expression of its need to secure
readily available soil moisture. In the lhght of results given here,
a failure of lodgepole seedlings under excessive temperatures must
be construed as being due to relatively short root length and corre-
sponding inability to secure necessary moisture.

Douglas-fir seedlings suffered most severely under the high tem-
per atures, considerably more so than the spruce seedlings of frailer
structure, and far more than the yellow pines. This reaction corre-
sponds to some extent with common occurrences noted in the open.
Douglas-fir seedlings require, to a decided degree, some form of
protection during the first two or three years of their existence, or
until safely established. It has been observed where the two species
occur together and the soil is reasonably moist that spruce seedlings
are more often found without protection than those of fir.

Very few fir seedlings recovered when visibly injured. It ap-
peared on even casual observation that lodgepole and spruce seed-
lings recovered from slight injury much more rapidly than_the
fir. Western yellow pine was intermediate in this respect. Where
no visible “ cooking” was observed immediately, but later observa-
tion showed that the seedling was injured, a portion of the stem
or the bases of the needles, rather than the tips, were likely to
be affected; and where such injury existed, the seedling usually
succumbed.

INFLUENCE OF AGE OF SEEDLINGS.

A portion of the data in Table 4 is summarized in the last three
lines so that the results of this test may be compared in part with
the earlier tests on a basis of similar time and temperatures. Thirty-
five and six-tenths per cent of the seedlings were killed or severely
injured at 46 days of age, 39.4 per cent at 64 days of age, and 23. 3
per cent at 92 days of age. The mean temperatures recorded in the
subtests, which are thus compared, were 147.9° F. for 9.3 minutes,
150.4° F. for 9 minutes, and 149° F. for 13.5 minutes. The higher
loss in the second period i is fully accounted for by the 2.5° additional
temperature; and there may, of course, have been variations in
atmospheric conditions or in the arrangement of the apparatus which
made the recorded temperature more effective at one time than at
another. However, the average result for the third period shows
clearly that the seedlings become more resistant with age. This
seems to be true of all the species considered, but is possibly true in
a slightly greater degree for the spruce and lodgepole seedlings,
which at the outset are extr emely small and frail.

TEMPERATURE SCALE FOR EACH SPECIES.

It is evident that some individual seedlings offer much greater re-
sistance to heat injury than do others of the same species, age, and
general origin. Such variations are to be expected with all organ-
isms. Owing to the comparatively small number of seedlings tested
for each age, period, and temperature, these variations make it very

RELATIVE RESISTANCE OF TREE SEEDLINGS TO EXCESSIVE HEAT 13

difficult to determine with any precision either the effects of different
periods of exposure under the same temperature conditions or of
different temperatures for similar exposures.

Regarding the effects of different periods of exposure, as many
comparable data as are available have been examined, and it is found

that beyond a. period of five or six minutes, which was the shortest

employed in the radiation tests, there is not so much additional in-
jury with added time as might be expected. ‘This confirms the idea,
which has been previously emphasized, that in nature it is probably
the momentary maximum temperature which determines the degree
of injury. It is also in agreement with results obtained in 1919,
which showed that seedlings surviving one severe exposure were in no
danger from a repetition of the same thing. However, it is entirely
conceivable that transpiration which affords protection may tem-
porarily attain a rate which can not. be maintained indefinitely, and
that the protection will therefore break down.

To arrive at. an' approximate measure of the effects of different
temperatures, it has been necessary to counteract the large number
of variable factors by averaging the results in three temperature
groups without regard to the time factor. The temperature groups
cover respectively all the tests below 140° F. which caused any injury,
those between 140 and 160° F., and those above 160° F. In order to
obtain for each test a summation of all the degrees of injury, the
classes of injury shown in Tables 2, 3, and 4 have been rated as
follows: One seedling killed, 162 per cent injury; one seedling in-
jured, will die, 125 per cent injury; one seedling injured, will live,
41 per cent injury. This rating is based on six seedlings in a group,
and is increased proportionately for smaller numbers, so that the pos-
sible sum for a group is 100 per cent.

By averaging the temperature and degrees of injury for all of the
tests in each temperature group, three ratings have been established
for each species. The lower temperature groups show greater aver-
age age of seedlings, but also longer exposures; and it is believed
that these two factors will be just about compensating, so that the
effects of different temperatures are brought out well enough for
comparative purposes.

The curves drawn through the three points for each species are
shown in Figure 1. The marked differences between the four species
are clearly brought out. While too much significance should not
be attached to the shapes of the several curves, it 1s apparent that
Douglas fir is much more sensitive than the other species at com-
paratively low temperatures. At about 150° F. the spruce shows a
more marked sensitivity. ‘The shapes of their curves suggest that
the pines might resist a very high temperature before all individuals
would be killed; but there is every reason to believe that at a slightly
higher temperature than any recorded in these tests, first the yellow
pine, and soon thereafter the lodgepole, would show complete break-

downs.
CONCLUSIONS.

In considering the general application of these experiments with
possible forms of heat injury to seedlings of the four most important
trees of the central Rocky Mountains, the following observations are
of value:

14 BULLETIN 1263, U. S. DEPARTMENT OF AGRICULTURE

1. Under laboratory conditions approaching natura! field condi-
tions, the wilting of seedlings of all species was mainly in the nature
of a collapse of the stem at the ground line. In this particular, the
small, frarl, shallow-rooted seedlings of Engelmann spruce and lodge-
pole pine were much more susceptible to injury than the larger,
deeper-rooted and firmer-stemmed seedlings of western yellow pine
and Douglas fir.

2. Seedlings removed from the soil at the age of about 30 days,
and subjected to heating in air practically saturated with water
vapor, showed considerable injury after a 6-minute exposure at
111° F., and increasing injury with higher temperatures, so that it
may be estimated that an exposure of one minute at approximately
141° F. would cause critical injury to all seedlings of the four species.
The injury at the higher temperatures took the form of almost im-
- mediate drooping or wilting of the entire aerial portion, and little

Number tests 5 Number tests 10 _ Number tests 6
Average temperature 1226F. (11)4-135") Average tempersture 149.45 F (1822-1548) Average temperature [7222°F. (ié1.1-181.8 )
Average period of exposure 28 (/4-35) Average period of exposure 15 (6-50) Average period of exposure // (5-30)

Average age al/ seedings Ebdays (4%-9/) Average agea// seed /ings 75 7elays (45-3)) Average age,a// seedlings A days (46-9)

a

HR RA
lca lath beeline basen
COCOA eee
eH ee er

a E URE RFR Dd. AT LEV. FE LEAVES
OL | Ee 1 Frelenariyne econoao Ar ueret 04

0 :
foo 105 HO WS 20 125 180 185 0 445 150 ISS 160° KS 4170 175 180 185 190
DEGREES FAHRENHEIT

PECENTAGE OF INJURY

20

/0

lig. 1.—Average extent of injury at various temperatures under direct radiation.

or no difference between the species was brought out. It thus appears
that conditions designed to heat the entire plant to a temperature
of about 140° F., without time or opportunity for appreciable water
loss from the leaves, are immediately fatal.

3. Direct radiation from an electric heating coil with reflector sup-
plying greater heating power and less luminous intensity than sun-
light, applied to the seedlings at the ages of 46, 64, 90, and 92 days,
served to create a condition of internal heating of the leaves similar
to that produced by sunlight, but in greater degree and acting more
promptly, so that the factors of water loss and water supply would
not materially affect the results. The greater sensitiveness of
Douglas fir under these conditions, followed closely by spruce
seedlings, at least at the higher temperatures, was very apparent.
The seedlings of lodgepole pine showed the least injury at all
temperatures. |

+.

;

Peat es ee
F

RELATIVE RESISTANCE OF TREE SEEDLINGS TO EXCESSIVE HEAT 15

While some injury was noted when the recorded temperatures were
as low as 120° to 130° F., this was only after long exposures, and
the fact that many seedlings lived through exposures of 170° to 180°
F. for from 6 to 8 minutes shows clearly, by comparison with the
results in moist air, that under ordinary atmospheric conditions the
internal temperatures of the leaves must be markedly kept down by
aa as has been suggested by Bates (2, 3), in considering
the relation of transpiration rates to heat requirements. Since the
order of susceptibility of the species closely approximates that as-
sumed by Bates, it becomes fairly certain that a free rate of trans-
piration, as compared with a more restricted rate, offers a measure of
protection from the direct effects of very intense insolation.

Under natural conditions, on exposed southerly Be in the cen-
tral Rocky Mountains, maximum temperatures at the surface of the
soil as high as 154° F. have been recorded, and this as late in the sea-
son as September 21. Temperatures as high as 160° F. may not be
uncommon. In contrast, the maximum temperatures at the soil sur-
face on north exposures are little higher than the maximum air tem-
peratures, or from 100° to 110° F. From the mere fact of the occur-
rence of temperatures as high as 150° F., 1 1s evident that extreme
heat, though of short duration and rare occurrence, may be an im-
portant factor in preventing reproduction generally and in drawing
a line between the natural habitats of the different species. Where
soil drying accompanies high temperatures, as 1t almost invariably
does in nature, temperatures of only 130° F. may do considerable
injury. With a moist soil there is some injury to the leaves of all
species at. 130° F., but many individuals will escape injury until the
temperature reaches 150° F. or more.

Hat hile lodgepole pine seedlings show great sensitivity at an early

e, their superior protection against direct heat injury to the needles
Be etaloncs in part any weakness which they may show in root-
ing, ete., if drying of the surface soil is involved. This is not sufh-
cient, however, to permit this species to occupy so warm and dry a
zone as that chosen by yellow pine. Spruce is quite sensitive poe
either standpoint. Yellow pine is most capable of supplying itself
with water and is fairly immune to heat injury. Douglas fir is well
equipped to obtain its water, but its leaves are most easily super-
heated.

As between yellow pine and Douglas fir, whose habitats merge in
the central Rocky Mountain region, the ability of the latter to replace
the former depends almost entirely upon the critical temperature
conditions which the germinating seedlings must withstand at the
surface of the soil. It is known that Douglas fir and yellow pine
seedlings have almost equal root vigor, and the distinction between
the two is clearly due to greater tolerance of heat on the part of yel-
low pine. On south slopes, where the surface soil temperatures dur-
ing the dry periods of September and October often reach a maxi-
mum of 140° to 150° F., any survival of fir must be confined to spots
which are shaded at least a part of the day.

The relatively great ability of lodgepole pine as compared with
spruce to withstand high temperatures is undoubtedly the deciding
factor in their relations on open, denuded areas. Lodgepole pine re-
produces splendidly on intensely heated sites within its range, pro-
vided only that its moisture needs are guaranteed by a lack of com-

16 BULLETIN 1263, U. S. DEPARTMENT OF AGRICULTURE

peting vegetation and reasonable rainfall. However, spruce seedlings
often come in under stands of pine, where pine seedlings will not
grow at all, and thus ultimately supplant the pine.

4. The resistance of all seedlings becomes slightly greater with
aging and accompanying hardening of tissues and with increase in
the number of needles, some of which no doubt protect others. Older
trees are also much less liable to heat injury than young seedlings,
because of the stem protection furnished by bark and the greater
distance of the foliage from the ground surface.

5. While in nature high temperatures are seldom entirely dis-
sociated from a more or less arid condition, it is evident that ex-
treme heat must often kill seedlings which would be able to cope
with the existing drought conditions. The scale of heat tolerance
for the four species suggested in conclusion 3 (lodgepole pine, yellow
pine, spruce, and Douglas fir) is the same as the inverted scale of
resistance to transpiration, given on page 127 of Bates’ (2) discus-
sion, as an average for two sets of results obtained under slightly
different conditions. The relative heat tolerance of the species is
somewhat at variance with their relative heat requirements for
optimum growth as this may be judged from the prevailing tem-
peratures of their habitats. All of these factors, as well as light
requirements, are important for the forester to have in mind.

The observations presented above bring out the fact that under
certain conditions, perhaps seldom occurring in nature, maximum
temperatures may be the critical factor preventing natural repro-
duction. Where this is the case, planting may be necessary or the
establishment of a given species may not be possible until the heat-
ing action of direct sunlight on the site has been somewhat modified.

LITERATURE CITED.
(1) Bares, C. G.
Experiments in sandhill planting. Proc. Soc. Am. Foresters, Vol. V,

No. 1, 1910.
(2) :
Transect of a mountain valley. Ecology, Vol. IV, No. 1, Jan., 1923.
(3)
Physiological requirements of Rocky Mountain trees. Journ. Agr. Res.,
U. S. D. A., Vol. XXIV, No. 2, Apr. 14, 1923.
(3A)

Forest types in the Central Rocky Mountains as affected by climate and
soil. U. S. Dept. Agr. Bul. No. 1233. 1924.
(4) CErs~taR; Dr. A.
Einfluss der LichtstAarke und der Temperaturhéhe auf die grésse der
Chlorophyllassimilation einiger Waldbaume. Note in Forestry Quar-
terly, Vol. VI, No. 3, Sept., 1908.
(5) Hartiey, Carl.
Stem lesions caused by excessive heat. Journ. Agr. Res., Vol. XIV,
No. 13, Sept. 2, 1918.
(6) Korstian, C. F.. and N. J. FETHEROLF.
Control of stem girdle of spruce transplants caused by excessive heat.
Phytopathology, vol. 11, No. 12, Dec., 1921.
(7) Muwnns, E. N.

High temperatures and eucalypts. Journ. of Forestry, Vol. XIX, No.1, -

Jan., 1921.
(8) MacDovear, D. T., and E. B. Workrne.
Another high-temperature record for growth and endurance. Science,
Vol. LIV, Aug. 9, 1921.
(9) SHReEvE, E. B.
A thermo-electrical method for the determination of leaf temperatures.
The Plant World, vol. 22, No. 4, Apr., 1919.

WASHINGTON : GOVERNMENT PRINTING CFFICE : 1924

pe ee