Historic, archived document Do not assume content reflects current scientific knowledge, policies, or practices. S. DEPARTMENT OF AGRICULTURE, - FOREST SERVICE—BULLETIN 92. HENRY S. GRAVES, Forester, [ LIGHT IN RELATION TO TREE oe woes BY Se _ RAPHAEL ZON, es BA CHIEF OF SILVICS, ; AND HENRY S. GRAVES, HOh ESTs WASHINGTON : GOVERNMENT PRINTING OFFICE ; 1911. : ev uA ein eS Issued June 80, 1911. U.S. DEPARTMENT OF AGRICULTURE, FOREST SERVICE—BULLETIN 92, HENRY S. GRAVES, Forester. LIGHT IN RELATION TO TREE GROWTH. BY RAPHAEL ZON, CHIEF OF SILVICS, AND HENRY S. GRAVES, FORESTER. Zen peatnes nie TAT i a Mise ii J « fe yo: c=. fe a QO % sO = U WASHINGTON : GOVERNMENT PRINTING OFFICE 1911, LETTER OF TRANSMITTAL. U. S. DeparTMENT or AGRICULTURE, Forest SERVIC#, March Age f Git: Sir: I have the honor to transmit herewith a manuscript entitled “Light in Relation to Tree Growth,” the major portion of which is the work of Mr. Raphael Zon, Chief of Silvics, and to recommend its publication as Bulletin 92 of the Forest Service. Respectfully, Henry S. Graves, Forester. Hon. James WIson, Secretary of Agriculture. CONTENTS: introduction: spai's sys ee ee reas Aeimdistoreelit es eae ie Direct and diffused light ....-..-. Overhead side vandienectedsiolt=) (2) oe same oe I Ae Light intensities and tree growth.... Minimum intensities............ Optimum intensities. ........-- Rolerance and sintolerancevoistreesey sie ee ee ee een ora Lubimenko’s experiments....... Grafe’s experiments.......-... Factors influencing tolerance. ...... Clima tees Meee Uk Geers Pee oh PeNI RES OOK) Sah apo eee a capa Cian Soil moisture. ees ye Gel oe oe Solbientilityse Gwe Se Ne Age, vigor, and origin of the tree Measure of tolerance. ............-- Scales of tolerance.............. i i i i i i re racucalivaltievolrerherscalesiens. enum tee CMe me Nani Ne See ER Methods of determining tolerance. . - Bmpiricalimethods?..2 2222 2252.- Density of crown...... Selicprumines 2 5.2. cy: Numiberoiioranicheordergies est ban sare nee eM ce Ee nye a ee Natural thinning of stand -. Condition of reproduction. . Relativetmeiohties =. £25 25%. Artificial shading........... Se ee Anatonmmcalfand omy siolovicalvimernodss 2.80) 2255 6. ee Structure of leaves........- Assimilative capacity of leaf Physical methods... 25 ies. Measurement of luminous light intensity....... Measurement of chemical light intensity....-.-. Wse of photographic paper-. 92.2.2... 22... ‘‘Normal shade” and ‘‘normal paper”... --- Wiesnensmmsolatoruwod i fae UNO a Clements; photometer: 22 42 ee Absolute and relative light intensity.....-.. Method of making measurements.......... Measurements of direct and diffused light. . - Se ee ee Whiesnermsrimvestiontloms a. focOu ue eh Ne cea set yw) Clementspaimvestigationsas =. 2 Vinnie ice Ek a se oes iHlorest Senvaice imvestigationss 5222 5-5 ue ese ee ae eee Defects of the method. - eee eee eee eee eee ee te ee tt ee et ee ee ee tt ee ee Spectroscopic measurement of light intensity. .......--..-.--------- Zederbavler- samvesticationss so sss 5. see eet aE ie tei: Wiesner’s investigations Relative value of different methods. SORTA. hava stele alciaiuars © elelsmiale s eeseeeese ee ee tee eee ee ete ee ee ee eee ee ee ee ew eseeseuseneveoeeuscetetoeeseee eee esse ocstsee ees eeeents Fig. ONomorh wd LELUSa AP TONS: . (Overhead, side; and reflected Nehts2.22- 22246-22=. 5 Se ee ee . Variation in light requirement with increase in latitude and altitude. - . Results of experiments with shading seedlings.-.............-------- . Cross section: of ‘a beech leat. 203.2352 22. see aoe ee ee . _Cross'section of a spruce needle. 2222-222. 222 eee ee eee . Wiesner’s photometer Gnsolator)= 220s Oe Ss esa. eee eee > Glements’ photometer... Ysecca5en 5 es eee oo eee eee . Maximum and minimum licht intensities within crowns of trees with different arrancement, of toliasespacce =. ae 5 eae See ee eee . Hourly light intensities within the crown of ailanthus (Adlanthus glan- dulosa); horizontal arrangement of leaves: 2.52.2. 2222e2-2 soso . Hourly light intensities within the crown of black locust (Robinia pseudacacia); vertical arrangement of leaves. .......-.-.-.-------- 4 LIGHT IN RELATION TO TREE GROWTH. INTRODUCTION. Light is indispensable for the life and growth of trees. In com- mon with other green plants a tree, in order to live, must produce organic substance for the building of new tissues. Certain low forms of vegetable life, such as bacteria and fungi, do not require light. They exist by absorbing organic substance from other living bodies; but the higher forms of plants manufacture their own organic mate- rial by extracting carbon from the air. The leaves, through the agency of their chlorophyll, or green coloring matter, absorb from the air carbon dioxide, and give off a nearly equal volume of oxygen. The carbon dioxide is then broken up into its elements and converted into organic substances which are used in building up new tissues. Light is not only indispensable for photosynthesis, but it is essential for the formation of chlorophyll itself. Only in exceptional cases, as in the embryo of fir, pine, and cedar seeds, does chlorophyll form in the dark, and, with the exception of some microbes, the green cell is the only place where organic material is built up from inorganic substances. Light also influences transpiration, and consequently the metab- olism of green plants. It influences largely the structure, the form, and the color of the leaf, and the form of the stem and the crown of the tree. In the forest it largely determines the height growth of trees, the rate at which stands thin out with age, the progress of natural pruning, the character of the living ground cover, the vigor - of young tree growth, the existence of several-storied forests, and many other phenomena upon which the management of forests de- pends. A thorough understanding, therefore, of the effect of hght upon the hfe of individual trees, and especially on trees in the forest, and a knowledge of the methods by which the extent of this effect can be determined are essential for successful cultural operations in the forest. The aim of this bulletin is to bring together the principal facts with regard to the part which light plays in the life of the forest, and the different methods of measuring it. It should prove of ma- 5 6 LIGHT IN RELATION TO TREE GROWTH. terial assistance to students of the subject, and it is hoped that it may also stimulate an interest to further research in determining more accurately the light requirements of our forest trees, especially by actual measurements of light in the forest. KINDS OF LIGHT. DIRECT AND DIFFUSED LIGHT. In discussing light with reference to tree growth a distinction must be made between direct sunlight and diffused light. If the earth were not enveloped by an atmosphere, all hght would be direct sun- light, and its intensity at every point on the earth could be calcu- lated mathematically from the position of the sun. The presence of the atmosphere, however, modifies essentially the distribution of light and heat on the earth’s surface. Only a part of the light which emanates from the sun reaches the earth as direct sunlight. An- other part is reflected from the small particles contained in the air, such as dust and minute drops of water, and forms diffused light. Still another part is absorbed and entirely disappears. The diffused hght illumines the atmosphere and forms the skylight. When the sky is cloudless the total daylight consists of both direct and diffused hght. On cloudy days all the light is diffused light. Trees in the forest make use chiefly of the diffused skylight, and for this reason it plays the most important part in their life. Indeed, many plants have developed special contrivances for protecting them- selves from the direct rays of the sun. There are, however, trees and other plants which, in addition to diffused light, need direct light either during their entire life or during a definite period of their life, as, for instance, during the period of fiowering and leafing. Thus, the opening of the buds in many trees proceeds much faster when the tree is exposed not only to diffused light, but also to the direct rays of the sun. Therefore, in determining the effect of light on vegetative processes it may be essential to know what portion of the entire light affecting the tree is diffused and what is direct. Both direct sunlight and diffused ight decrease with increase in latitude, but not in the same proportion. The intensity of direct sunlight decreases much more rapidly with increase in latitude than does the intensity of diffused or sky light. This is well brought out in Table 1, computed on the basis of measurements taken by Bunsen and Roscoe, which gives the relative chemical intensities of the radia- tion received directly from the sun and from the sky upon a horizon- tal surface during a whole day at the spring equinox. It is of considerable interest as showing what kind of lght.is mostly avail- able for tree growth at various latitudes. KINDS OF LIGHT. 7 TABLE 1.—Light intensity of direct sunlight and of diffused light at various latitudes. Chemical inten- sity— Places. ILI OC es) $$$ LE Ofsun- | Of sky- light. light ° PO] Ge Se ER NSE AS 1 DNV ORE RUE AAR BEE ASN AG Be IE EG 8 ee 90 0 20 20 ING lyvpill Vi Learn see Se ACROSS OVERS a nce Nepean ays oc syrah 75 12 106 118 EV eyilajaivallc lice learn Geet ee a ee UE ELE cea I SR PUES Cee ie Siege ener 64 60 150 210 Sie IPSS oles INI CBC ean ea ceseaancaoauadeas Ee at Mn araly 60 89 164 253 Mian cles Gere tei gi) em eae eat ieee ate ia ea Le ABIES ented 53 145 182 327 Tei Pel er ess Gere Gia eect ie MN Ne VER eg sha a 49 182 191 373 INEN OSS NPN Neer ee ei crc ain i meee ia iie pp ein munya aul eed teat Al 266 206 472 Cano NE ory Gace aie 5 Oe Seer Wace sei ao nN ape abe me yyy les ep Teta 30 364 217 581 1SKoioa Oe a ba DENS GG SOMME Nuno e es Ue cuia a men Hel Menard esas ne ey ay 19 | © 438 223 661 Sam Josey Costar ica eee see ey eee Sy sy aieee see cy egoysl cael ese 10 475 226 701 Qari GOR Cura COT See aN de EVN eee RL ater r vay Le ate 0 489 227 716 1 From Handbook of Climatology, by Dr. Julius Hann, p. 116. Diffused light decreases with increase in altitude; direct sunlight, on the contrary, increases. , OVERHEAD, SIDE, AND REFLECTED LIGHT. Besides direct and diffused hght, ight may be distinguished in accordance with the direction from which it comes and the effect it has upon the different parts of the crown. The light which reaches the crowns of trees from above is called a overhead light. (Fig. 1,a.) This determines . \ the arrangement of 3 Se LD \ | leaves on the shoots, their position in rela- | ye = tion to the sun, and : _ | oe j the arrangement of FG a / the) oranches. Tit 1s ‘ bee ff the strongest light, ee ; whether it consists of @ ie ge either direct or dif- eee fused light alone or | both together. In the case of dominant Oy yy 5 a Say yf, yy trees it equals the to- a | Ta ot) a S at cae NY Ni ») raat ate pend SY een 5 . oN » Sa Sas aN wt UG, one = NWN 5 si SUMAN MC tal daylight. NS MG rise se S Ze oa ae The light which oo Gale Fic. 1.—Overhead light (a); side light (0); reflected side reaches the UO aS of light (c) ; reflected ground light, from a water surface (d@). trees from the side is called side hight. (Fig. 1, 6.) It stimulates the development of buds on the lateral branches and is responsible for the development of the branches facing an opening in the forest. 90431°—Bull. 92—11 wv! 3) ~ et 8 LIGHT IN RELATION TO TREE GROWTH. In the case of trees growing near a wall or steep slope the tree may receive ight which is reflected back upon the tree and is called reflected side light. (Fig. 1, ¢.) In some cases, especially where trees are growing near bodies of water, their crowns are illumined by light which is reflected from the ground or from the water’s surface. (Fig. 1, d.) This is called reflected ground light, and is not so insignificant as it may appear at first thought. Thus, actual measurements have shown that, at a height of 1 meter (84 feet) the intensity of light reflected from a road illumined by the sun may be 1/12 of the overhead light inten- sity; the intensity of hght reflected from the water’s surface may amount to 1/6 of the overhead light intensity, measured at a height of about 5 feet from the surface. The intensity of light varies with the direction from which it comes. Thus, the following results were obtained from measure- ments made at the end of April, at noon, in Vienna. If the hght intensity coming from the north is taken as 1, then the intensity of light from the west is 1.19; from the east, 1.25; from the south, 3.12; while the intensity of the overhead light is 4.50. LIGHT INTENSITIES AND TREE GROWTH. Only in exceptional cases do forest trees make use of the total daylight. Isolated individual trees may do so, but, as a rule, the total daylight is considerably weakened by the configuration of the land and by the shade cast by the foliage of the individual tree itself or of neighboring trees. The bulk of forest trees and the interior parts or crowns of even isolated trees depend, therefore, only on a part of the total daylight. Actual measurements of lght intensity have shown that on a clear, sunny day the light intensity on the edge of a forest is only about half that of the total daylight, while in the shade of the trees, even when they were still without any foliage, the light intensity was one-fourth that of the total daylight. MINIMUM INTENSITIES. The minimum intensity of light in which photosynthesis can take place is not sufficiently determined for all species; it differs in differ- ent species with the sensitiveness of the chloroplasts. Trees not only accumulate energy by building up new organic substance, but they also expend energy from the organic substance which they produce. This expenditure of energy is accompanied by oxidation of carbon and exhaling carbon dioxid, or respiration. As long as the light in- tensity is above the necessary minimum for the given species, the process of assimilating carbon from the air, and thus building up new organic substance, goes on with greater energy than the opposite LIGHT INTENSITIES AND TREE GROWTH. 9 process of breaking up organic substance and giving off carbon into the air in the form of carbon dioxid. As the light intensity de- creases the assimilation decreases correspondingly, and the amount of carbon assimilated from the air approaches the amount given off by respiration. As soon as the energy of assimilation falls so low that the amount of carbon assimilated is less than that needed for the maintenance of respiration, the leaf dies. It is possible to gain an idea of the relation between light intensity and the activity of the green cell from the experiments made by Kreusler. He found that a square centimeter of a leaf of Kuropean hornbeam (Carpinus betulus) on a cloudy day decomposed in one hour 13.7 cubic millimeters of carbon dioxide, or seven times as much as was given off by the leaf in breathing; with a 1,000-candlepower electric light (which has an effect upon assimilation similar to that of sunlight), at a distance of 31 centimeters (12.2 inches), a square centimeter of leaf surface decomposed 28.5 cubic millimeters of car- bon dioxide,? an amount 15 times greater (by weight) than that which was given off; by removing the source of light to a distance of from 1 to 1.5 meters (39 to 59 inches), thus making it from one-ninth to one twenty-fifth as intense as before, the amount of carbon dioxide which was absorbed was barely enough to cover the amount given off by respiration. : i According to Wiesner (1907), for Norway maple (Acer plata- noides) the minimum light intensity under which photosynthesis can still take place 1s 1/55 of the total daylight, for birch 1/50, and for beech somewhat less than for birch. The fact that in light intensity of 1/70 of the total daylight, such as prevails in dense spruce forests, no green vegetation occurs under the shade of the trees indicates that the minimum light intensity at which a green leaf can still function must le between 1/60 and 1/70 of the total daylight intensity. As far as woody species are concerned, Wiesner (1907) found in the shade of European hornbeam (Carpinus betulus), with a light intensity of 1/58, an undergrowth of the following species still in a fairly good condition: Beech (Fagus sylvatica), hornbeam (Carpi- nus betulus), field maple (Acer campestre), common elm (Udmus campestris), and dogwood (Cornus sanguinea). In the same light intensity, however, the undergrowth of elder (Sambucus nigra), Huonymus europeus, and EL’. verrucosus, was dying. The minimum light intensity at which green vegetation disappears from under the shade of trees in the forest varies considerably with the climate. Thus, in the Temperate Zone no green vegetation occurs 1 Equivalent to 1 square inch of leaf surface decomposing 0.0054 cubic inch of carbon dioxide. 2 Hquivalent to 1 square inch of leaf surface decomposing 0.0112 cubic inch of carbon dioxide. 10 LIGHT IN RELATION TO TREE GROWTH. in the shade where the light is only 1/70 of the total daylight. In the Tropics the last vestige of green vegetation disappears from under the -tree cover when the minimum light intensity falls to 1/120 of the total daylight. The minimum intensity of hght which is needed for the formation of chlorophyll is much smaller than the minimum required for photo- synthesis. Thus, while the green leaves of Acer platanoides cease to break up the carbon dioxide of the air and form organic substance in a light whose intensity is 1/55 of the total daylight, the forma- tion of chlorophyll still takes place in a hght which is 1/400 of the total daylight; in some herbaceous plants, such as peppergrass (Lepidium sativum), the formation of chlorophyll still goes on at a light intensity of 1/2,000, while in light whose intensity is 1/50 of the total daylhght the leaves are just as green as in higher light intensities. The decomposition of carbon dioxid is produced chiefly by the rays of the red portion of the spectrum, and the leaves in the interior of the crown are able to assimilate, because the largest part of the light which they receive penetrates not through the green leaves, which absorb the red rays, but through the places between the foliage. Chlorophyll may form in light which lacks the rays of the red portion of the spectrum. For this reason light which goes through the green foliage may still be effective in producing the green color of the leaves inside the crown, but is no longer effective in bringing about the process of assimilation of carbon. MAXIMUM INTENSITIES. In general the intensity of light varies directly with the height of the sun. The maximum light intensity, however, does not coincide with the intensity at noon, which is always less than the maximum. Thus, from measurements made at Vienna, the ratio of the average noon light intensity to the average maximum light intensity for the year was found to be as 1:1.08. This is analogous to the well-known fact that the highest temperature during the day occurs not at noon, but later, and is probably due to the same causes. The maximum light available for tree growth is, of course, the total daylight. This varies, as has already been pointed out, with latitude, altitude, and the configuration of the earth. The highest light intensities found by Wiesner (1905) in the United States were in Yellowstone Park. Thus, at Norris, on Sep- tember 1, 1904, at 1 p. m., with the sun at an altitude of 52° 56’, the chemical intensity of the light was found to be 1.7 in Bunsen-Roscoe units.t At Old Faithful, on September 4, 1904, at noon, angle of sun 1 Bunsen-Roscoe unit of chemical light intensity is the amount of light required to pro- duce a standard color in one second on standardized sensitive paper. TOLERANCE AND INTOLERANCE OF TREE GROWTH. 11 52° 99’, the light intensity was 1.9, and an hour later, with the sun forming an angle of 51° 47’, 2.083. These measurements were taken on cloudless days. In Europe the highest light intensities measured were 1.5 near Vienna, at an elevation of 550 feet above sea level (Wiesner, 1896), between 1.5 and 1.6 at Kremsmunster at an elevation of 1,268 feet (F. Schwab, 1904), and 1.8 in Steiermark, at an eleva- tion of 4,550 feet (Thomas V. Weinzierl, 1902). OPTIMUM INTENSITIES. The optimum light intensity at which different species thrive best has not been fully determined, especially since this optimum varies during the life of the tree, and is subject to variations even in dif- ferent parts of the same vegetative season. In a general way it may be stated that with the majority of forest trees the optimum heght intensity at which the leaves function best and at which the pro- duction of flowers and fruits is most abundant lies nearer to the maximum amount of light available for the use of the tree than to the minimum light under which it can still exist. In some species this optimum coincides with the total daylight or with conditions that surround a tree grown in the open; in other species both the vegeta- tive and reproductive functions of the tree are most vigorous when the amount of ght is less than the total daylight; that is, when the trees are grown in a stand. TOLERANCE AND INTOLERANCE OF TREES. The ability of trees to endure shade is called tolerance of shade, or often merely tolerance. Trees that are capable of enduring shade are tolerant; those requiring full light intolerant. Some species are able to absorb enough light for assimilation even in the shade of a forest canopy. Thus, hemlock and spruce spring up and live for many years under other trees. Other species, such as tamarack, aspen, gray birch, and most of the yellow pines, require full light and can not endure shade from above. All trees, however, thrive in full hght, especially if they have it from the very start, and none requires shade except as a protection from drying or from frost. This does not mean, however, that all trees grown in the open absorb equally all the available light. Even in full light they need and use differ- ing amounts of light for their best growth. This is accomplished by a definite orientation of the foliage in relation to the source of hght, by the development of denser crowns which lessen the amount of hght that can penetrate into the interior, or by a change in the structure of the leaves so as to decrease the assimilative energy. In the open they are able to make these modifications for their best growth to better advantage than in the shade of a dense forest, and 12 LIGHT IN RELATION TO TREE GROWTH. adapt themselves to their needs; therefore, open-grown trees usually appear the more thrifty. The primary cause of this difference in tolerance must be sought in the anatomical structure and inherent qualities of the leaves and chlorophyll. Since, however, the anatomical structure of the leaves and even the character of the chlorophyll may be influenced by en- vironment, the tolerance of trees is not a fixed quality, but is subject to variation. Each species, however, inherently requires a certain amount of light, which can not be changed by any environment. LUBIMENKO’S EXPERIMENTS. The latest experiments by Prof. Lubimenko?! established with sufficient accuracy the difference in the sensitiveness of the chloro- plasts of different tree species. The species with which he experi- mented were Scotch pine (Pinus sylvestris), noble fir (A bées nobilis) , white birch (Betula alba), and linden (Tilia cordata). Of.these, pine and birch are light-needing species and stand close to each other in this respect. The fir and basswood are classed as tole- rant, and are also fairly similar in their demands upon light. More- over, the anatomical structures of the leaves of pine and fir are very similar, and of birch and basswood practically identical. These experiments brought out clearly the following points: (1) The initial hght intensity at which assimilation begins varies with the species, since the fir and basswood began assimilation at 1/5 of the hght intensity at which pine and birch began to assimilate. (2) If it be accepted that the light intensity at which assimilation equals respiration is the minimum for the existence of the leaf, then birch appears to be the most light-demanding species, followed by pine, basswood, and finally fir. (3) With increase in light, the assimilation at first increases in all four species, but in direct sunlight, when the rays strike the surface of the leaves perpendicularly, the pine and birch still continue to show an increase in assimilation, while the fir and basswood show a decrease. This may be seen in Table 2. TABLE 2.—Relative amounts of carbon dioxide absorbed by 1 gram of leaf during one hour of work. Pine. Fir. Birch. Basswood. Direct rays of the sun striking the surface of the leaf at ancacute-angle cocoa Ge sete i tee blo opera tester 49. 74 53. 10 68. 08 68. 71 Direct rays of the sun striking the surface of the leaf perpendicularly. <.132~ see ceine esses See me eee eee eee 56. 83 38. 50 75. 69 43. 21 Since the anatomical structures of the leaves of pine and fir are very similar, and of birch and basswood almost identical, this differ- ence in assimilation must be explained exclusively by the difference 1 Carried on in 1904 at the Laboratoire de Biologie Vegetale, in Fontainebleau, France. TOLERANCE AND INTOLERANCE OF TREE GROWTH. 13 in the sensitiveness of their chloroplasts. These figures show also, in a general way, that a unit (by weight) of leaf substance of broad- leaf species assimilates considerably more than the same unit of leaf substance of coniferous species. This is probably due to the differ- ence in the anatomical structure of the leaves of the broadleaf and coniferous species. The extremely small size of the cells of the leaf parenchyma in broadleaf species means the presence of a large amount of living protoplasm and chloroplasts, and consequently a larger amount of living, acting substance within the same space. On the basis of these experiments, Lubimenko claims to have established the following ‘facts in regard to the photochemical work of different forest trees: (1) The photochemical work of the green leaf is determined by its anatomical structure and the inherent qualities of its chloroplasts. (2) The influence of the anatomical structure of the leaf is felt mainly in light of medium intensity; the influence of the inherent qualities of the chloroplasts, on the other hand, is strongly apparent in light of low and high intensities. , (3) Chloroplasts of different species are sensitive to light in dif- ferent degrees; chloroplasts of shade-enduring species are more sensi- tive than chloroplasts of light-needing species. (4) Species with more sensitive chloroplasts begin to decompose carbon dioxide and reach a maximum of assimilative photochemical energy in ight of a much lower intensity than species with less sen-, sitive chloroplasts. (5) The curve of the photochemical work of a green leaf has a dis-’ tinct optimum, which is reached in different species under differ- ent intensities of light and is determined by the inherent qualities of the chloroplasts. (6) It is very probable that the difference in sensitiveness of chloroplasts of different species is due to a difference in the absorp- tive capacity of the chlorophyll. (7) The conception of “ light-loving ” and “ shade-enduring ” trees has a real foundation as far as the process of assimilation of carbon is concerned. Leaves of shade-enduring species are able to replace the carbon dioxide expended in the mere process of respiration in much weaker light than are leaves of light-loving species. The opti- mum of assimilation in tolerant species lies within the limits of nor- mal sunlight, while the optimum of assimilation in light-needing species lies beyond those limits. GRAFE’S EXPERIMENTS. These conclusions are also corroborated by Grafe’s experiments carried on at the Institute of Plant Physiology, at Vienna (Wiesner, 1907: 266-267), though for another purpose. Green leaves of birch (a light-demanding species) and of beech (shade enduring) were 14 LIGHT IN RELATION TO TREE GROWTH. exposed to hight of various intensities and tested for the presence of starch. At a hght intensity of 1/6 of total daylight the leaves of beech showed distinctly the presence of starch, while birch gave a less distinct reaction. At a light intensity of 1/10 beech still gave a pro- nounced reaction for starch, while in birch only traces of starch could be detected. At a lght intensity of 1/50 birch leaves showed no presence of starch, while the leaves of beech still continued vigorously to form it. These facts tend to show the same point brought out by Lubimenko, that there is a distinct difference in the sensitiveness of the chloroplasts of beech and birch. FACTORS INFLUENCING TOLERANCE. Tolerance varies not only with species, but even within the same species, according to conditions under which the tree is growing. These variations are due largely to changes in the structure of the leaves brought about by changes in transpiration. Among the impor- tant factors influencing transpiration, and therefore tolerance, are climate, altitude, moisture and nourishment in the soil, age of tree, and vigor and origin of the individual. CLIMATE. Plants need less light the higher the temperature and more light the lower the temperature. Consequently the higher the temperature of a given locality, the more shade a tree can stand. This explains, in part, the frequent differences of opinion regarding the tolerance of the same species when the observations are taken in different regions. For example, white pine in Maine is less tolerant than in the Southern Appalachians. In Vienna the minimum lght intensity in which the leaves of Norway maple (Acer platanoides) can exist is 1/55 of the total daylight. In Hamar, Norway, it is 1/37; in Drottningholm, 1/28; and in Tromso, 1/5. This substitution of light for heat in plant growth, and vice versa, was well demonstrated by Wiesner (1907) with regard to annual meadow grass (Poa annua). The minimum light necessary for annual meadow grass at the beginning of March, at Cairo, is equiva- lent to about 53 calories, while at Vienna it is equivalent to 109. The mean temperatures are 59.9° and 35.6° F., respectively. ALTITUDE. The light requirement of a species increases also with increase in altitude, but only to a certain limit, beyond which it remains con- stant or even decreases. For example, observations by Wiesner showed that lodgepole pine, at an elevation of 6,400 feet, required a FACTORS INFLUENCING TREE GROWTH. 15. minimum light intensity of 1/6 of the total daylight; at an elevation | of 8,500 feet it sinks to 1/6.4 and even to 1/6.9°. Thus the behavior of trees in their extension toward higher lati- tudes and higher altitudes is not the same. This is due to the fact that the intensity of both direct and diffused light decreases with the increase in latitude, and the hght limit of a species is reached when the intensity of the total daylight becomes equal to the tree’s mini- mum lght requirement. With increase in altitude, however, diffused light decreases, but direct sunlight increases. With an increase in the intensity of direct sunlight, even though there be a decrease in the diffused light and a lower temperature, the light requirements of a species remain constant or even become less at higher altitudes. A A 4 S ae s 8 ; ‘ N Ni W 8 9 N a : | \ OF 2) A) >= S Ny ; Rb N S| N Nf 8 ; - . 50° 60° 70° Boe B— = = Sere : 7 JIOO0O -6000 3000" /2000 POZE: ALTITUDE AB, total light intensity; BO, lati- AB, light intensity ; BC, altitude; ay, curve tude; az, total light intensity; ba, of the total light intensity; ab, curve of minimum light requirement of a the light requirements of trees. (After species; #, point at which total Wiesner.) . light intensity equals minimum light needed by the species, also limit of latitudinal distribution of the species. (After Wiesner.) Fie. 2.—Variation of light requirements with increase in latitude and altitude. This difference in the light requirements of trees in the horizontal and vertical distribution is well illustrated by the two diagrams shown in figure 2. SOIL MOISTURE. Tolerance of trees is emphatically influenced by moisture condi- tions in the soil, as well as by its quality. Fricke (1904: 315-825) clearly demonstrated by a series of convincing experiments that defi- cient moisture in the soil, brought about by competition of the roots of older trees, may cause the death of young growth under the shelter of mother trees. His experiments were made in a Scotch pine stand on poor, dry, sandy soil, on which, according to all authorities, the light requirements are greatest. In a stand from 70 to 100 years old,’ 90431°—Bull, 92—11——3 16 LIGHT IN RELATION TO TREE GROWTH. with a crown density of 0.7, there were isolated groups of suppressed young pines, among which were no old trees. The young pines were 10 years old and but little over a foot and a half in height. Ditches 10 inches deep were cut around these groups, and in this way all the roots extending from neighboring old trees were cut through to the depth of the ditches. The relative amount of light received by these groups of undergrowth was not affected by the experiment, since not a single tree was cut down or trimmed. The ditches were made in the spring. In the first summer the needles that appeared on the little pmes within the isolated groups had doubled the length of the preceding summer, the terminal shoots became longer, and this thrifty growth continued up to the time the results of the experiments were described (1904), while the same undergrowth outside the areas surrounded by the ditches preserved the same suppressed character. The old trees, whose superficial roots were cut through, apparently did not suffer, and none of them were uprooted by the wind. On the areas inside the ditches a rich flora sprung up during the first summer. Entirely unexpected, there appeared bellflower (Campanula), wild strawberry (/ragaria), hawkweed (Hieracium), sorrel (Rumex), ironweed (Veronica), wil- low herb (£'pilobium), star thistle (Centeurea), geranium (Gera- nium), violet (Viola), five finger (Potentilla), and other herbaceous plants, and in addition there sprung up a number of seedlings of birch and mountain ash. Most interesting and significant of all, however, is the fact that none of these species appeared in the neigh- boring stand. The herbaceous vegetation which was present on the ground before the experiment, such as sweet vernal grass (Anthoxan- thum), hair grass (Azra), bentgrass (Agrostis), and woodrush (Lwa- ula), and which had led a very precarious existence, developed luxu- riantly, so that the areas surrounded by the ditches bore green, suc- culent vegetation, in striking contrast to the grayish-brown ground cover of the rest of the stand. In another pine stand, 100 years old, growing on poor soil, several natural openings on which there were no old trees or young growth of any kind were surrounded by ditches and sown with seeds of Scotch pine, spruce, beech, and red oak without any preparation of the ground. Similar sowings were made on similar natural open- ings not surrounded by ditches. On the area inside the ditches there are now remarkably well-developed seedlings of pine and oak, although with characteristics peculiar to shaded plants, and also seedlings of beech and spruce. At a distance of some 7 or 10 feet outside of the ditches, although a part of the seeds did come up, the seedlings of pine, beech, and spruce were poorly developed from the start and soon died; the seedlings of oak which still persist have scarcely reached 1/5 the height attained by those inside the ditches FACTORS INFLUENCING TREE GROWTH. 17 and will hardly live very long. In this stand also there appeared in the openings surrounded by the ditches a luxuriant herbaceous vege- tation strikingly different from that in the surrounding forest. In order to prove also that deficient soil moisture brought about by the competition of the roots was the sole cause of the death of pine reproduction under the shelter of mother trees, Fricke deter- mined the soil moisture in sample plots of which some were pene- trated by living roots, while in others the competition of the living roots was eliminated. In all these experiments the proportion of moisture in the soil was invariably greater in the plats free from living roots—usually two or three times greater, and occasionally four and even six times. The results of these experiments clearly show that the unsatis- factory condition of an undergrowth shaded laterally or from above is not due altogether to insufficient light, but to competition of the roots of the neighboring old trees; likewise that the presence and condition of shrubs, grasses, and mosses in the forest depends very little upon the amount of light, but are chiefly influenced by the degree of desiccation of the soil caused by the roots of the old trees. Thus the poor development of young growth directly under or near seed trees may be explained, not by shading alone, nor by the mechan- ical action of water dripping from the leaves and branches of the old trees, nor by excessive light reflected from the trunks, but by the moisture-sapping competition of the roots of the older trees. In these experiments, however, Fricke, by eliminating competition of the neighboring roots, created artificial conditions which do not exist in nature. Moreover, no photometric measurements of any kind were made during these experiments, and without such measure- ments the question of light requirements of species can not be settled. While the experiments bring out the importance of considering other factors besides hght in the hfe of the forest, they do not prove that the light requirements of all species is the same. On fresh soils, with an abundant supply of moisture, root competi- tion affects the growth of the seedlings only a little or not at all, and for this reason it is assumed that trees are more tolerant on fresh or moist soils than on dry soils. But even on the same kind of soil the effect of trees of different species upon the growth of seedlings is not the same. Trees with a strongly developed superficial-root system naturally desiccate the upper layers of the soil much more than trees with a compact, deep-root system. In filling fail places in plantations, the competition of the roots determines the success or failure of the operation. It happens only too often that the planting of fail places on dry or only moderately fresh soils meets with entire failure, because growth of the new 18 LIGHT IN RELATION TO TREE GROWTH. seedlings is impossible in the dry soil produced by the roots of the older seedlings, which in the porous soils of the planting holes attain extra good development. In dry situations, therefore, fail places must be filled not later than three years after the first planting; otherwise it will be necessary to give up entirely the filling of the blank places, or the older competitors must be removed from small areas and these then replanted. These facts show that the so-called “ light increment,” or increase of growth after logging or thinning, is not due alone to the greater access of light to the remaining trees. By thinning a stand not only are the hght conditions changed, but the competition of the roots is diminished, which leads to an increase of moisture in the soil. The leaf litter is also more readily decomposed and the soil in this way becomes enriched with nutritive substances, all of which results, of course, in an acceleration of growth after thinning. SOIL FERTILITY. Plenty of chemical nourishment in and favorable physical condi- tions of the soil increases tolerance. At the same hght intensity the assimilative energy of the green leaf increases with increase of nour- ishment in the soil. Thus, the assimilative energy of trees grown in a deficient light but on good soil may be the same as that of trees grown in full light but on poor soil; or, in short, trees on good soils can stand more shade than trees on poor soils. This has been clearly demonstrated by Hartig (1897: 142-143), who thought that the weight of the young leaf-bearing shoots may, to some degree, serve as an indication of the amount of foliage in a tree. At the same time, the amount of wood produced in a tree for each pound of small leaf-bearing twigs serves as a criterion of the work of the leaves. Thus, measurements made by him on oaks in the Bavarian Spessart gave the average results shown in Table 3. TABLE 3.—Amouwint of wood produced annually fer each pound of small twigs. Pasiaus Age (years). | Cubic inches of wood. 33 45.9 90 16.8 246 16. 0 400 14.0 These figures show that the leaves of young trees function with greater energy than the leaves of older trees. The same method may be applied to determine the effect of nourishment upon the work of the leaves. It was found that on different soils the annual produc- tion of wood in trees of the five different classes (dominant, codomi- FACTORS INFLUENCING TREE GROWTH. 19 nant, intermediate, oppressed, and suppressed) for each pound of small twigs was as given in Table 4. TABLE 4.—Amount of wood produced annually by different classes of trees for each pound of small twigs. Classes. Good soils. | Poor soils. Cu. in Cu. in ATO) TNT TATA Ge a ee alee neg rH Day a Ey aaah CN ae i Te Me ep eyed Oo 7 15.8 (Qroyskosaatih avs wal atte ee veh Bes er yet le oN 5 IE gu Oe I, a Se WOE ee 73.2 20. 4 AMAT AiG SRT CTE Ga aera ayia Goa ato a Eee dh HUANG ita PU MRR GN gh MLE MolTare AUP ML TS 49.6 16.5 TOTES SE CL Reese sie et aes rte reece NI SS aS WNC Vara NAL CLES ct 32.8 17.3 SUL DRESSC Chie ie Ase lea ecec CL Eh pat ee A So OI Uaioe acai ek I Sc PS Spies seen e These figures clearly show the influence of soil conditions upon the assimilative activity of the leaves, and consequently upon the variation in tolerance of trees according to soil fertility. AGE, VIGOR, AND ORIGIN OF TREE. Trees are more tolerant in early youth than later in life. In fact, it may be said that during the first year or two all trees are tolerant. As they grow their demands for light increase and the distinctive hght requirements of different species become more and more empha- sized. This increase in light requirements becomes especially ap- parent at the time of the most rapid height growth, after which the light requirement remains stationary for a long time and increases again only late in life. White pine is a very good example of a tree which in early youth is tolerant, but in later life is distinctly intol- erant. White pine seedlings will start in dense shade; but can not often live under such conditions for more than 10 to 15 years; and may perish from lack of hght in the first 2 to 5 years, even if abun- dant moisture and nourishment are present. The amount of light needed for flowering and fruiting is greater than for mere growth, so that demands for light vary not only with age, but with season. Thus, early in spring at the time of bursting of buds, the minimum light intensity for larch is 1/2 of the total daylight, later it falls to 1/5; for beech the minimum light intensity at the time of unfolding of buds is 1/4; and later it sinks to 1/60 of the total daylight. These differences in light demands are well known from common experience. Thus, in a coppice forest it is often necessary, in order to awaken the dormant buds of the stumps, to clear away the brush around them and expose them to higher light intensities. It is a well-known fact that trees in the open begin to bear seed earlier than trees in a close stand. Foresters esti- mate the average retardation due to close stand at 20 years. The preparatory cuttings for natural reproduction under the shelter-wood °0 LIGHT IN RELATION TO TREE GROWTH. system are based on this principle of allowing more light to the remaining trees in order to stimulate seed production. In general, the more vigorous the individual, the greater is its tolerance. Any factor which reduces its vitality reduces its tolerance. trees from seed, all other conditions being equal, stand shade much better than sprouts, and in artificial plantations nursery stock is more tolerant than trees which have been started by direct seeding. MEASURE OF TOLERANCE. Since tolerance is affected by so many different factors, it is evident that the tolerance of any particular species is necessarily variable and exceedingly difficult to measure. Until recently, therefore, it has been customary not to attempt to measure tolerance of shade in positive, but only in relative terms. SCALES OF TOLERANCE. The tolerance of different species is usually compared in lists or scales, the most tolerant being usually placed first and the least tolerant last. The position which a species occupies in relation to these extremes is the expression of its tolerance. Broad and rather indefinite terms have been used, such as * very tolerant,” “ tolerant,” “ intermediate,” “ intolerant,” “ very intolerant.” The following are examples of scales of tolerance prepared by European authorities, beginning with the most tolerant: GAYER (1898). BUHLER (Morozov, vol. 2: 1286-1295). Taxus baccata (English yew). Tazrus baccata (English yew). Abies pectinata (silver fir). Abies pectinata (silver fir). Fagus (beech). Fagus (beech). Picea (Norway spruce). Picea (Norway spruce). Carpinus betulus (European horn- Pinus strobus (white pine). beam). Ulmus montana (wych elm). Pinus strobus (white pine). Tilia (linden). Tilia parvifolia=T.: cordata (small- Alnus incana (hoary alder). leaf linden). Quercus (oak). Alnus incana (hoary alder). Acer platanoides (Norway maple). Acer (maple). Pinus montana (Swiss pine). Ulmus montana (wych elm). Pinus sylvestris (Scotch pine). Frazinus excelsior (European ash). Populus (poplar). Quercus pedunculata (oak). Betula (birch). Populus tremuloides (aspen). Lariz europea (larch). Pinus sylvestris (Scotch pine). Betula verrucosa (birch). Betula pubescens (birch). Larix europea (European larch). MEASURE OF TOLERANCE. TuURSKY (1904: 32). Abies pectinata (silver fir). Fagus (beech). Picea (Norway spruce). Carpinus betulus (European horn- beam). Ulmus montana (wych elm). Pinus laricio, var. austriaca (Austrian pine). Alnus glutinosa (black alder). Fraxinus excelsior (Kuropean ash). Quercus (oak). Populus (poplar). Pinus sylwestris (Seotch pine). Betula (birch). Larixs europea (larch). 21 WARMING (1909: 18). Abies pectinata (silver fir). Fagus (beech). Carpinus betulus beam). Tilia (basswood). Picea (Norway spruce). Pinus montana (Swiss pine). Acer pseudoplatanus (sycamore maple). Ulmus effusa (spreading elm). Quercus (oak). Fraxinus excelsior (Kuropean ash). Pinus strobus (white pine). Pinus sylwestris (Scotch pine). Alnus incana (hoary alder). Populus tremula (aspen-poplar). Betula (birch). Larig europea (Huropean larch). (European horn- The following are scales of tolerance prepared for some of our American trees: WESTERN SPECIES. VERY TOLERANT. Tasus brevifolia (western yew). Picea engelmanni (Engelmann spruce). Abies lasiocarpa (alpine fir). Abies concolor (white fir). Thuja plicata (western red cedar). Tsuga heterophylla (western hem- lock). Tsuga mertensiana (mountain hem- lock). Picea sitchensis (Sitka spruce). TOLERANT. Chamecyparis lawsoniana (Port Or- ford cedar). Libocedrus decurrens (incense cedar). Picea parryana (blue spruce). Sequoia sempervirens (redwood). INTOLERANT. Abies magnifica (red fir). Pinus ponderosa (western pine). Pinus jeffreyi (Jeffrey pine). Pinus lambertiana (sugar pine). Pinus aristata (bristle-cone pine). yellow VERY INTOLERANT. Larixv tyallii (alpine larch). Larix occidentalis (western larch). Pinus albicaulis (white-bark pine). Pinus attenuata (knobecone pine). Pinus balfouriana (foxtail pine). Pinus coulteri (Coulter pine). Pinus flexilis (limber pine). Pinus monophylla (single-leaf pine). Pinus edulis (pifion). Pinus sabiniana (digger pine). Pinus contorta (lodgepole pine). 22 LIGHT IN RELATION TO TREE GROWTH. INTERMEDIATE. Pseudotsuga taxifolia (Douglas fir). Pseudotsuga spruce). Macrocarpa (bigcone Abies amabilis (amabilis fir). Abies grandis (grand fir). Abies nobilis (noble fir). Chamecyparis cypress). nootkatensis (yellow Sequoia washingtoniana (bigtree). Pinus monticola (western white pine). EASTERN VERY TOLERANT. Chamecyparis white cedar). Abies balsamea (balsam fir). Abies fraseri (Fraser fir). Tsuga canadensis (hemlock). thyoides (southern TOLERANT. Picea canadensis (white spruce). Picea rubens (red spruce). Thuja occidentalis (northern white cedar). Fagus atropunicea (beech). Acer saccharum (sugar maple). Acer saccharinum (silver maple). Acer rubrum (red mapie). Nyssa sylvatica (black gum). Ulmus americana (white elm). Ulmus racemosa (cork elm). Tilia americana (basswood). Platanus occidentalis (sycamore). SPECIES. INTOLERANT. Pinus divaricata (jack pine). Pinus resinosa (red pine). Oxrydendrum arboreum (sourwood). Liriodendron tulipifera (tulip poplar). Q. rubra (red oak). Q. velutina (yellow oak and others). Betula papyrifera (paper birch). Liquidambar styracifiua (red gum). Hicoria alba (mockernut). Hicoria ovata (shagbark hickory). Hicoria pecan (pecan). VERY INTOLERANT. Pinus palustris (longleaf pine). Pinus echinata (shortleaf pine). Taxodium distichum (bald cypress). Lariz laricina (tamarack). Robinia pseudacacia (black locust). Salix (willows). Populus deltoides (cottonwood). Populus grandidentata (largetooth as- pen). INTERMEDIATS. Pinus taeda (loblolly pine). Pinus virginiana (scrub pine). Pinus rigida (pitch pine). Pinus strobus (white pine). Castanea dentata (chestnut). Quercus alba (white oak). Betula lenta (river birch). Betula tutea (yellow birch). Juglans nigra (black walnut). Magnolia tripetala (umbrella-tree). Magnolia acuminata (cucumber-tree). MEASURE OF TOLERANCE. Ue MOST IMPORTANT SPECIES IN CENTRAL NEW YORK. [Beginning with the most tolerant. ] Ulmus americana (white elm). Quercus coccinea (searlet oak). Acer rubrum (red maple). Quercus velutina (yellow oak). Pinus strobus (white pine). Castanea dentata (chestnut). Quercus alba (white oak). Hicoria (hickories). Quercus platanoides (Swamp white Quercus prinus (chestnut oak). oak). Fraxinus americana (white ash). Quercus rubra (red oak). Juniperus virginiana (red cedar). In actual practice there is seldom any doubt as to the light re- quirements of the extreme members of the tolerance scale. The doubt comes with species which, under certain conditions of climate and soil, may be classed either as tolerant or intolerant. Instead of dividing species into “very tolerant,” “tolerant,” ‘“ intermediate,” ‘ intoler- ant,” and “ very intolerant,” it would therefore be more simple and practical to divide them into only three groups— tolerant,” “ par- tially tolerant,” and “intolerant.” This classification does not, how- ever, preclude the possibility of species with extreme requirements becoming, under certain conditions of climate and soil, partially tolerant. Thus, beech and fir, which are ordinarily very tolerant, on very poor soil and in a very cold climate may become only par- tially tolerant; and, on the other hand, pine and larch, which are ordinarily very intolerant, in most favorable soil and climate may become partially tolerant. To the group of tolerant trees belong the following genera: Taxus, Fagus, Abies, Picea, Tsuga, Pseudotsuga, Thuja, Avsculus, ever- green oaks and other evergreen broadleaf trees, and others. To the group of partially tolerant trees belong Carpinus, Tilia, Acer, Fraxinus, Ulmus, Alnus, the five-needled pines, Chamecypa- ris, Libocedrus, Sequotia, white oaks, and others. To the group of intolerant trees belong the black oaks; the two and three needled pines, such as Pinus contorta, P. jeffreyi, and others; Larix, Salix, Populus, Betula, Taxodium, Magnolia, Robinia, Lirio- dendron, and others. Of the secondary trees and arborescent shrubs, the evergreen broad- leaf shrubs, such as Buwus, are classed as tolerant; Corylus, Cornus, Ligustrum, Euonymus, Lonicera, and others are classed as partially tolerant; Prunus, Spartium, Calluna, Crategus, Viburnum, and others as intolerant. Of forest weeds, Polythrichum, Hypnum, Aspidium, Vaccimum, Hedera, and others are tolerant; Anemone and Pteris, partially tol- erant; Circium, Silene, Fragaria, and most of the grass and clover species, intolerant. ‘90431 °—Bull. 92—11——4. 24 LIGHT IN RELATION TO TREE GROWTH. PRACTICAL VALUE OF THE SCALES. These scales have their practical use in silviculture, particularly in ~ giving a broad comparison of the different species. If one is seek- ing a species for underplanting, he naturally concerns himself with the most tolerant. If he is developing a plan for reproduction cut- tings, he knows that the least tolerant will require a clear cutting method, and that he can not reestablish new growth under the old cover. Thus, empirical knowledge of hght requirements establishes the fact that young growth of a tolerant species under the shade of a fully stocked stand of a tolerant species begins to die off, as a rule, after from 10 to 20 years of suppression; intolerant trees under a tolerant species perish after about 5 years; a tolerant species under an intolerant species may continue to live indefinitely, because the amount of light that penetrates through the thin crowns of the light- demanding species is sufficient for the growth and development of the tolerant species. Reproduction of an intolerant species under the shade of an intolerant species is capable of enduring for from 10 to 20 years, or almost as long as reproduction of tolerant trees under the shade of tolerant species. Since the least amount of hght is ad- mitted to the ground in a fully stocked stand in its pole stage, only a tolerant species under the shade of an intolerant species can live through this period without injury. Tolerance scales based on experience are of distinct value to the practitioner and to the student of forestry first learning the char- acteristics of the different species. For accurate and scientific in- vestigation, however, they are defective, for they show only that one species is more or less tolerant than another. They do not show how much. What is needed is a mathematical expression for each species based on some definite scale. Thus, for example, that the tolerance of hemlock is 89, of yellow birch 61, of white oak 32, or whatever the actual values might be proved to be. METHODS OF DETERMINING TOLERANCE. The tolerance of trees may be determined by observation of their behavior in the forest in light of different intensities; by studying the anatomical structure and the functions of the different organs, especially the assimilative organs of trees grown in light and shade; and finally by measuring instrumentally the hght intensities them- selves. These methods may be classed respectively as empirical, anatomical or physiological, and physical. EMPIRICAL METHODS. The density of crowns of individual trees; the rate of natural pruning of different species in stands of the same density, or the same species in stands of different densities; the number of succes- sive branch orders found on trees of different species; the rate of METHODS OF DETERMINING TOLERANCE. 25 natural thinning of stands; the rapidity of growth; the ability of young seedlings to come up under the shade of older trees; and the ratio between the diameter and height of a tree, may all be used as a basis for determining the relative tolerance of different species. DENSITY OF CROWN. Density of crown is, without doubt, a fairly good criterion for determining the tolerance of a tree. The denser the crown the less light is received by the leaves hidden within the crown, and conse- quently the more tolerant is the species, and vice versa, the more open the crown the more intolerant the species. The degree of den- sity or looseness of the crown can not, however, be accurately meas- ured and must always be ocularly estimated. This introduces a sub- jective element into the observation, and while it is possible to de- termine with a fair degree of accuracy the extreme members in the scale of tolerance, the intermediate species must always be arranged more or less arbitrarily. SELF-PRUNING. The rapidity with which the trunk clears itself of lower branches is a splendid indication of the degree of tolerance. The dying of live branches on the lower portion of the trunk and crown in dense stands is, without any question, due to lack of light, which is not sufficient for the assimilative processes of the leaves. The more in- tolerant the species and the denser the stand, the more rapid is the pruning. lLight-needing species clear themselves of branches even in isolated positions, although of course m a less perfect way than in dense stands. Shade-enduring species, however, lose the lower branches only in dense stands, and in the open the crowns reach almost to the base of the trunk. This criterion of tolerance, however, has the same objection as the previous one; it leaves too much to the subjective impression of the observer. Furthermore, this process of self-pruning is often confused with the mere falling off of the dead branches, which is due not to light, but to mechanical causes. | NUMBER OF BRANCH ORDERS. Mathematically, a tree or branch should contain as many succes- sive orders of branches as there are years in the life of the tree or branch, minus one. Thus, a 100-year-old oak should have 99 suc- cessive orders of branches; as a matter of fact, there are found, as a rule, not more than five or six different orders of branches. In a 50-year-old sycamore there should be 49 branch orders; in reality there are found usually not more than 7. If a branch of a birch formed every year two lateral buds and each bud developed every year into a shoot, a 10-year-old branch would have 19,683 twigs, representing 9 yearly ramifications or orders of branches. As a mat- °6 LIGHT IN RELATION TO TREE GROWTH. ter of fact, on a 10-year branch of birch grown in the open Wiesner (1907) Brad only 238, and on a branch grown in the shade only 182 lateral branches; and instead of 9 successive orders of branches there were only 5. The reason for many of the buds failing to develop into shoots and the gradual dying of existing shoots must be sought chiefly in the minimum light intensities under which buds and foliage of a,given species can develop and exist. This is especially true of the buds, since, as has already been pointed out, the unfolding of buds requires higher lght intensities than the mere process of growth. It stands to reason that species capable of withstanding low light intensities will succeed in unfolding a proportionately larger number of buds every year and will preserve during their mature life a larger number of original ramifications than will light- needing species. Wiesner (1907) prepared the following lists of trees and shrubs, arranged according to the number of branch orders found in them: COMMON FOREST TREES. Maximum Maximum Species. ee Species. Dea orders. orders. Warix (archi. cee een aoe ee 3=45| ROUCRCUS! (Oaks) ssoeege see eee eee 6 Gineko(cingiko) Wisse een eee 45) Robiniai (locust) seat cess eee 7 Gleditsia (honey locust)......-.....-.-.- eal | amu Si (Clim) eee tess tae eee es eae a Ropulus(@poplan)s ae eee Gy ie dib-aanbks (PSN) se eek SES eB okodoaee 7 Piceal(SPruce) assess eee cere ieee 5, ||-Carpinus (hornbeam) S25 so-aaeeee eee 8 Pinus laricio (Austrian pine)..........- Lie Ind Brekeq bh Loteter0) OW) ee ee ee ar eee 8 Betula (birch) *=2ss2e5-seese paclea nisi 54 PE ARUSIGVEW) Seams Saas aaeee eee eae 8 SHRUBS. Cornus (dogwood) S252 osas-eeee eee ae 4 || Philadelphus (mock orange) ........-- 6 Sambucusi¢elden)- se e= pene ee 6 || Ligustrum ( ) ected 7 Viburmunt (aiburnum) 2 355.22 sess. 6 Sytinga ce: we Says Se ciee ae ee eee eee 7 In these lists the species having the smallest number of branch orders must be classed as light- heen and those with the largest number of branch orders as eines: shade-enduring. The only ap- parent exception is spruce, which, by this method, would be grouped with the light-needing species. The small number of branch orders found in spruce may not, however, be due entirely to light, since many of the smaller branches of old spruce trees are bitten off by animals, especially squirrels. The interference by animals, insects, and other agencies with the successive branch formations in a tree constitutes a weak point in this method of determining lhght requirements of species; moreover, the method is, of course, altogether too complicated for practical use. NATURAL THINNING OF STAND. The rapidity with which stands thin themselves affords another indication of the hght requirements of trees. As soon as the crowns METHODS OF DETERMINING TOLERANCE. PA in a young stand touch each other there begins a struggle for exist- ence among the individual trees, which finally results in the differen- tiation of the stand into dominant and suppressed trees with all the intermediate stages. The trees which lag behind their neighbors receive less light, their growth is retarded, and they become more and more suppressed, until finally they die and thus gradually reduce the number of individuals in the stand. The rate at which this process takes place depends largely upon the degree of tolerance of the species composing the stand. Natural thinning, however, is a very complex phenomenon which is conditioned by a number of factors. Although light plays an essential part in the process, it has thus far proved im- possible to separate fully its influence from that of the other factors concerned. CONDITION OF REPRODUCTION. The condition of the young growth under an older stand as an indi- cation of tolerance has the same objections, since the extent to which a species can stand suppression is far from being determined exclu- sively by its tolerance. This method, however, although entirely empirical in character, has accomplished a great deal of good, and is one of the most prevalent ways of determining the tolerance of trees in this country. RELATIVE HEIGHT. The relation between the height and the diameter of a tree varies with the amount of hght which it receives. Thus a tree in the open grows more in thickness, while a tree in a dense stand grows more in height. The ratio between the height of a tree and its diameter, ob- tained by dividing the total height by the diameter at breastheight (expressed in the same unit of length), has been called its relative height, and it has been clearly demonstrated by actual measurements that 1t depends upon the amount of light received. On the basis of such measurements, Medevev (1884), a Russian forester, obtained the following maximum and minimum figures of relative heights for isolated and suppressed trees: Tsolated Suppressed trees. trees. NEAT steers sea cee SUR SR RMR a Sen ARR EN ULE Ry AC ANEMIA ROU bat AOA ASL 4.9 126.0 PSION ELG LS ee as a eID CAE COL CM LT Ne ei RR Oo EO EA ce 39.8 130. 0 1BXS1eYO) Oey AARNet ete aie BON ME RS ON UU AU Aba a ae SUN NSN Rr Uea eds 8 157.5 Between these extreme figures are included the relative heights of trees in all the intermediate degrees of illumination. The more shade-enduring a species the lower is the light intensity at which it can grow, and consequently the greater the relative height at. which it becomes completely suppressed and dies. It is possible, therefore, by determining the relative height of a given species to determine a8 LIGHT IN RELATION TO TREE GROWTH. also its ability to endure low light intensities; that is, its tolerance. The best way to obtain such figures would be to determine the rela- tive heights of trees which have grown without any influence upon one another, but under a uniform shading of some other stand. Since, however, such conditions were hard to find, Medevev, who was the author of this method, made use of the numerous results of his forest measurement work and of the data obtained in measuring sample areas, especially in the forests of Transcaucasus, where an unusual number of observations were made on spruce (Picea orien- talis) and on Scotch pine. He found, on the basis of such measurements, that the relative height of spruce is from 1.762 to 1.338, or on the average 1.500 times greater than that of pine; in other words, that the tolerance of spruce is one and a half times greater than that of pine. In the same way the following figures were obtained for the light relations of other species: SpEUCEH OPRIGEG) ALOU BING CPTI a oa eee 1. 500 Pine (Pins). to Ditch, (Betula yes oe bo ae Eee Boer cea oe eevee =) de doo Beech #CE Gaus) tOiSpEUCe sy CRTC Ce) se Se ee ree a 1. 029 Hornbeam=(Carpivus) to peech Ch agus) = oS ee eee eee . 918 Yew: Pavusy) sto beech CHGgus) 22 ee ae eS A ee 2. 816 Wir CA0tes) 1 60 SPRUCE CRC CCCs) ease aera eee Eee es mae oS) TS EE Ete a 1.125 Basswood,-Ciihea) to beech) Chagus)\ a= 0a ee ee eee . 849 Oak. (Quercus) to hornbeam (Cerpinus) —__-___-_---_-_-= ices . 855 Aspen. GPopulus) beam). beam). Fagus (beech). Fagus (beech). ACCORDING TO Hyer (1898). Rapidity of growth. (Most rapid growing Tolerance. (Most tolerant first.) first.) Larig europea (Kuropean larch). Lariz europea (European larch). Pinus sylvestris (Scotch pine). Pinus sylvestris (Scotch pine). Picea (spruce). Picea (spruce). Abies (fir). Abies (fir). Populus tremula (aspen-poplar). Populus tremula (aspen-poplar). Betula (birch). Betula (birch). Alnus incana (hoary alder). Alnus incana (hoary alder). Acer (maple). Acer (maple). Uimus effusa (spreading elm). Ulmus effusa (Spreading elm). Acer pseudoplatanus (sycamore maple). Acer pseudoplatanus (sycamore maple). Quercus (oak). Tilia (linden). Carpinus . betulus (Huropean horn- Quercus (oak). beam). Carpinus betulus (HKuropean horn-. Tilia (linden). beam). Fagus (beech). Fagus (beech). 34 LIGHT IN RELATION TO TREE GROWTH. Miiller (1877) attempted to determine the comparative light re- quirements of different species by arranging leaves of the same species in layers, one above the other, until no light could penetrate through them. This he accomplished by placing the leaves over a small aper- ture in a perfectly dark room and by causing the sun rays to pene- trate into this dark room through the aperture perpendicularly to the surface of the leaves. Light was entirely prevented from enter- ing the dark room by a thickness of 10 leaves of ash, 11 of elm and basswood, 13 of alder and oak, 15 of aspen and maple, 17 of blue beech, and 20 of beech. These figures are indicative of the shade which each species can endure, and may therefore serve as a criterion of their tolerance. PHYSICAL METHODS.’ A more direct way of determining the light requirements of forest trees is to measure with instruments the hght itself, just as weight is measured by means of scales, temperature by means of thermometers, air pressure by means of barometers, and air humidity by means of psychrometers. Sunhght is not homogenous, but is composed of rays of many dif- ferent wave lengths. The white sun ray in passing through a prism reveals seven different colors, known as the solar spectrum, and these in turn are made up of an infinite number of color gradations. The different rays composing the solar spectrum produce different effects upon matter. Thus, the rays which produce by their action upon the retina the sensation of vision are luminous rays and are found chiefly in the yellow portion of the spectrum, decreasing toward the red and the violet. The rays producing the heating effect are calorific rays and are centered in the red portion. They are very low in the ultraviolet and high in the infra-red. The rays which affect photo- graphic paper or other light-sensitive substances are actinic or chemical rays. They are usually greatest in the violet or ultraviolet portion of the spectrum. In measuring light, therefore, it is possible to measure either the luminous effect or the heating effect or the chemical effect. Plant life is differently affected by different rays of the solar spectrum. In order to determine accurately, therefore, the effect of light upon tree life, not merely the luminous effect or the chemical effect, but the effect of each separate ray of the spectrum should be measured. Unfortunately, up to the present time no accurate and practical instrument has been invented by which the effect of the different rays of the spectrum can be measured. The methods of 1 This discussion is based largely on a detailed description of the physical methods prepared by G. A. Pearson, in charge of the Coconino Forest Experiment Station, Forest Service. METHODS OF DETERMINING TOLERANCE. 35 measuring light in the forest so far have been based to some extent on the measurement of its luminous intensity, but principally on the measurement of its chemical intensity. MEASUREMENT OF LUMINOUS LIGHT INTENSITY. In physics the usual method of determining the intensity of a luminous source is to compare it with a light of known intensity. For this purpose there have been adopted a number of hghts which are used as the standard of comparison; among these may be men- tioned the “standard candle,” the Hefner-Alteneck lamp, and the Vernon Harcourt lamp. The use of such standards in field work, however, has been found very inconvenient and impracticable, and a number of attempts have been made to devise an instrument by which the luminous intensity of light could be measured without the use of the standard light. None of these attempts has, however, resulted in a generally adopted instrument. The best-known instru- ments for this purpose are the smoked-glass photometer and the polarization photometer, for which the inventor Wagner (1907), claims great simplicity and accuracy. Nobody, however, except the inventor himself has used these instruments for field work, and no definite results of any permanent value have been obtained with them. The principle upon which these two instruments are based is that of measuring the amount of hght which is intercepted, in one case by a graduated, slding, smoked glass of uneven thickness (wedge shaped), and in the other case by two Nichol’s prisms whose planes of vibration form various angles with each other. If the amount of light intercepted by the thinner and thicker portions of the wedge- shaped smoked glass, or by the two prisms at various angles of their planes of vibration, is known, then the intensity of light in the open and in the forest may be determined. Thus, if the observer looks through the tube of the smoked glass photometer in the open and obtains complete absorption of light by the smoked glass at a point at which its light absorption is equal to 25 units, while in the forest complete darkness is obtained at a point at which its absorption of light is equal to 20 units, then the light intensity in the forest is to that in the open as 20 is to 25, or, expressed in per cent, the light in the forest 1s 80 per cent of that in the open. In the case of the polarization photometer, the amount of light _ absorption in the open and in the forest is measured not by a smoked glass of uneven thickness but by the angle between the planes of vibration of the two Nichol’s prisms. These two photometers are very simple, but they are not free from error, since with both the accuracy of the measurements depends 36 LIGHT IN RELATION TO TREE GROWTH. chiefly on the ability of the observer to determine the point at which total darkness begins. The chance for subjective error, owing to the variabilty of the human eye, is therefore very great. MEASUREMENT OF CHEMICAL LIGHT INTENSITY. Of all the physical methods of measuring light, the measurements of its chemical intensity have proved most satisfactory, and have contributed in a large measure to a better understanding of climate (Bunsen and Roscoe, 1862) and of the effect of light upon plant life. USE OF PHOTOGRAPHIC PAPER. The Austrian botanist, Prof. Julius Wiesner, was really the first who employed in a practical and thoroughly scientific manner the action of light upon silver chloride paper (photographic paper) for determining the lght requirements of plants. Before Wiesner, Theodore Hartig attempted to determine quantitatively the light requirements of forest trees by means of photographic paper, but he did not succeed in developing a thoroughly satisfactory instrument. Use of photographic paper for measuring light intensities was first made by Bunsen and Roscoe for climatological investigations, but Wiesner was the first to adapt this method to studies of plant life. The measurement of the chemical intensity of light is based on the well-known law formulated by Bunsen and Roscoe that products of light intensity and time of exposure correspond to darkenings of silver chloride paper of like sensitiveness. In other words, if two exposures produce the same tints on the same kind of paper in dif- ferent lengths of time, the light intensities are inversely propor- tional to the corresponding periods of exposure. Thus, if a certain shade is reached in light 7 by an exposure of ¢ seconds, and the same shade is reached in light 7, by an exposure of ¢, seconds, then 7: /,= t,:t, or Jt=I/t,. By varying, therefore, the time of exposure, it is possible to obtain identical shades at very different light intensities. “NORMAL SHADE” AND ‘‘ NORMAL PAPER.”’ In order to have a uniform and constant unit of measure of the light intensity a standard or “normal shade” and a standard or “normal paper” has been adopted. The “normal shade” adopted by Bunsen and Roscoe is prepared as follows: One thousand parts of chemically pure white zinc oxide are mixed with one part pure Jampblack heated to incandescence in the absence of air. The mixture is pulverized to a fine grayish powder. This powder is then spread, with the aid of a gelatinous solution, in an even coat on a piece of white paper or pasteboard, and produces a constant, uni- form, and somewhat grayish color. This color is the “normal shade ” to which al! other shades are compared. METHODS OF DETERMINING TOLERANCE. 37 The “ normal paper ” is prepared in a very simple way. Ordinary paper used for photographic purposes is soaked in a 3 per cent solu- tion of common salt, and then hung up to dry. After the paper has become thoroughly dry it is immersed for two minutes, in the ab- sence of chemically active rays, in a 12 per cent solution of silver nitrate. After this the paper is dried in a dark place. The light intensity which produces in one second on such “ normal paper ” a shade equal to the “normal shade” is the Bunsen-Roscoe unit of measure of the chemical light intensity. Thus, if the color of the “normal shade” is sonchel. Tg ag Ie Peg a a n seconds, the corresponding light teenies are 1/1, 1/2, 1/3, a ee WIESNER’S INSOLATOR. The measurement of the chemical light intensity is carried on by means of a very simple device called an insolator (see fig. 6) and a stop watch. The insolator devised by Wiesner consists of a piece of soft wood 38 to 4 inches long, 24 to 3 inches wide, and about one-fourth of an inch thick, covered, with the exception of a narrow slit at one end (S), with black opaque paper. The strip of “normal paper ” (N. P.) is pushed in beneath the black paper, and the standard (“normal shade”) is placed in the slit beside the “ normal paper.” When a reading is to be made the insolator is held in a horizontal position. The photographic “normal paper” is drawn out until a section of it is brought into the open slit, thus exposing it to light. At the same moment that the “ normal paper ” is exposed to the light the time is taken. As soon as the “normal paper ” assumes a shade identical with the “ normal shade ” the watch is stopped and the time of exposure is recorded. The light intensity is determined by divid- ing the light value of the standard by the time which has elapsed from the beginning to the end of the exposure. Thus, if at a given light intensity eight seconds are required to produce the “ normal shade” on the “normal paper,” then the intensity 7=1:8=0.125, Bunsen-Roscoe units. As long as low light intensities are measured, the “ normal shade ” is Sufficient ; as soon, however, as high light intensities are to be meas- ured, in which the “ normal shade ” is assumed in fractions of a sec- ond, and therefore the time of exposure can not be accurately re- corded, standards of different light values greater than that of the “normal shade” become necessary. Such standards are readily ob- tained simply by a comparison with the “ normal shade.” Thus, if it is desired to obtain a standard 10 times greater than the “ normal shade,” a strip of “normal paper” is exposed in the insolator 10 times the number of seconds required to reach the tint of the “ normal shade.” Since such standards to be of any value must remain un- 38 LIGHT IN RELATION TO TREE GROWTH. changed when exposed to the light, the shades obtained by different exposures of the “ normal paper ” are reproduced in permanent water colors. The different standards are then placed in the insolator beside the “ normal paper” for facilitating comparison. (See fig. 6, No S210 In field work, instead of the “normal paper,” ordinary photo- graphic paper, especially Rodamin-B paper, invented by A. An- dresen, of Berlin, may be used. The chief drawback to the use of the “normal paper ” is its poor keeping quality. It can be kept at most only 24 hours, and therefore a fresh supply must be made every day. Any commercial silver chloride or silver bromide photographic papers may be used, provided they are of a uniform known sensi- tiveness with reference to the , in | ‘normal paper.” CLEMENTS’ PHOTOMETER. Prof. F. E. Clements (1905: 38-63), in his studies of the infiuence of ight upon vege- tation, has used a photometer of his own invention which in many ways is simpler than Wiesner’s insolator. Like Wiesner’s photometer, it is also based on the blackening of silver salts in the light. The construction of the in- ers: strument is shown in fig. 7. pe bg ee It is made in two cases, outer = QQ A ])Nywwv SN vilw ae pate aes - Me 6 wiesnetavincolatons Sit in athe tight metal case. A strip of black paper; N. P., “normal paper”; N. S. sensitive paper is fastened 1 pormal singe” (t'unit); 8: 8 29, upon the shoulder of B, which fits into the outer case A. The two parts are held together by means of a central screw C. In the outer case is a hole one-fourth of an inch square, which is opened and closed by means of a slide S working between two flanges (/). By turning the inner case B the strip of paper is made to revolve past the opening. The outer face of B is graduated into 25 equal parts, which are numbered consecutively in such a manner that the number opposite the opening will always indicate the number of the exposure. On the edge of the shoulder bearing the strip of paper are 25 holes, which are engaged by a spring catch as the case B is turned, thus allowing only one twenty-fifth of a revolution to be METHODS OF DETERMINING TOLERANCE. 39 made at a time. The photometer is made of aluminum; it weighs but a few ounces and may be conveniently carried in the pocket. The sensitive paper is a commercial printing paper sold on the market as “solio.” The photographic paper, the most convenient size of which to use in the photometer is the 8 by 10 inch sheet, 1s cut into strips one-quarter of an inch wide. The strip is placed in position on the shoulder of the photometer so that the two ends meet Fic. 7.—Clements’ photometer. a, Perspective view of the photometer in use; b, rear view of the inner case; c, perspective view, showing the inside construction. at H’, care heing exercised that the strip fits into the groove all around. The strip is then drawn tight and the ends are wedged into the slit with a piece of cork. In this way Clements dispensed with the “normal paper” of Wiesner. He has also done away with the “normal shade,” substi- tuting for it what he calls a “ multiple standard.” This “multiple standard ” consists of a strip of paper exposed for 1, 2, 3, 4, and 5 seconds successively, thus producing a series of shades. For the unit 40 LIGHT IN RELATION TO TREE GROWTH. of measure is taken the shade made by an exposure of one second at meridian at a given day and place when the sun is not obscured by clouds. Since the standard is not “ fixed,” it should therefore always be kept in a dark, cool place. The exposures are made by quickly draw- ing back the slide, thus exposing a section of the sensitive paper. The paper is usually exposed until a medium shade is obtained, be- cause very light or very dark shades are difficult to match with the standard. After each exposure the disk B is turned to the next number. When the entire strip has been exposed, it is removed, labeled, and placed in an envelope or light-proof box, where it is kept until it is compared with the standard. At the end of the day’s field work the exposed strips are compared with the standards, in the absence of chemically active rays. The standard is laid upon the table and fastened at each end with pins to prevent curling. The exposure to be compared is passed along the standard until the shade is found with which it matches. The relative light intensity is then found from the time ratio between the particular shade in the standard and the exposure with which it matches. Thus, if the tint has been obtained by an exposure of 60 seconds in the forest, while the shade of the standard which it matches has been obtained by an exposure of only 3 seconds in the open, then the light intensity in the forest is 3/60, or 1/20, or 0.05 of that of meridional sunlight in the open. The difficulty with Clements’ method, even more than Wiesner’s, lies in matching the tints obtained in the forest with those of the standard; yet with some practice fairly accurate results are obtained by means of Clem- ents’ photometer. ABSOLUTE AND RELATIVE LIGHT INTENSITY. Since plants seldom make use of the total daylight but only of a portion of it, it is of greater practical value to know how much of the total daylight a given species actually requires for its growth than simply to know the light intensity in the crown expressed in absolute figures without relation to the light intensity in the open. The ratio between the light intensity which a given plant actually enjoys and the hght intensity in the open is the “ relative” lght intensity. The light intensity in the open, expressed in Bunsen- Roscoe units, is the “ absolute ” ight intensity. Thus, if the hght in the open has a chemical intensity equal to 0.80, Bunsen-Roscoe units, while at the same time the light intensity in a tree crown is only 0.20, then the relative hght intensity, or the amount of light the tree actually receives, is 20/80, or one-fourth of the total daylight. METHODS OF DETERMINING TOLERANCE. 4] METHOD OF MAKING MEASUREMENTS. Measurements of light should be made on perfectly clear days, since cloudy, smoky, and hazy skies reduce the light intensity and are apt to lead to errors. For the same reason no measurements should be made when the sun is at the horizon or low altitude, for the composition of the solar spectrum at that time differs consider- ably from that when the sun is at a high altitude. Absolute light intensity is also influenced by the altitude above sea level. Its in- fiuence, however, is noticeably felt only when the difference in eleva- tion is 3,000 feet or more. The measurement of the relative light intensity enjoyed by forest trees presents considerable difficulties because the different parts of the crown are differently illumined. It is not enough, therefore, in measuring the relative light intensity to measure only the light on the periphery of the crown, but rather the minimum light intensity at which green leaves still continue to live or assimilate carbon from the air. This minimum light intensity is of the greatest importance in forestry, since it indicates the limit of light beyond which a given species can not exist, and consequently serves as a basis for classify- ing species as tolerant and intolerant. The maximum light intensity under which forest trees grow best does not differ as much with dif- ferent species as does the minimum light intensity which 1s charac- teristic of each species and which must be taken into account in car- rying on cultural operations in the forest. It is evident that in attempting to measure the maximum and minimum light intensities which are enjoyed by different species we are necessarily dealing with a variable quantity. Not only does the Neht vary with the altitude of the sun, but the light intensity within the crown varies with the density of the crown and the arrangement of the foliage. There are two possible ways of measuring the amount of light which is received by a given tree. One way is to obtain a summa- tion of the light intensities for the vegetative period. Another way, much simpler and yet probably as serviceable for the comparison of different species, is to make a series of measurements during the day at points within the crown where the foliage just continues to exist. An average of these measurements gives the average minimum light intensity for that species. ‘The measurements should be taken every hour, so as to include the periods of greatest and least illumination of the interior of the crown. These periods vary considerably with different species. Thus, thinly foliaged trees have a light intensity in the interior of the crown in direct proportion to the intensity of the daylight, and the relative minimum light intensity remains about constant. through- 49, LIGHT IN RELATION TO TREE GROWTH. out the day. In densely foliaged trees in which the foliage has a. more or less horizontal position, as a rule the ight intensity in the interior of the crown at noon, or when the sun is at its highest alti- tude, is greatly reduced, since the position of the leaves prevents to a large extent the entrance of the vertical rays. In such trees the greatest light intensity within the interior of the crown occurs in the forenoon and in the afternoon when the sun is in the east and west— that is, when the light penetrates into the interior of the crown, not from above, but from the side. The light intensity at noon, there- fore, lies between two maxima, which occur in the forenoon and the afternoon. In thinly foliaged trees of species, such as locust, whose WU eg. SAWN YZ Nt ee aw U Fic. 8.—Maximum and minimum light intensities within crowns of trees with different arrangement of foliage. a, Tree with horizontal arrangement of foliage, in which noon light intensity lies between two maxima; 6b, tree with vertical foliage in which the maximum illumination of the crown occurs at noon. leaves avoid the midday light by assuming a profile position, a maxi- mum intensity may be attained at noon. In trees of the same spe- cies which have a dense foliage, however, only the uppermost leaves assume a profile position at noon, and the relative light intensity within the interior of the crown, therefore, remains more or less constant during the day. (See fig. 8.) This difference in the occur- rence of maximum and minimum light intensities within the tree crowns during the day is clearly brought out in the diagrams (figs. 9 and 10), which represent actual hourly measurements of light inten- sities in the interior of the crown of ailanthus (Adlanthus glandulosa) and black locust (Robinia pseudacacia). METHODS OF DETERMINING TOLERANCE. 438 MEASUREMENTS OF DIRECT AND DIFFUSED LIGHT, Plants depend chiefly for their growth and development on dif- fused light; yet for certain functions, such as leafing, flowering, and fruiting, direct sunlight is essential, and it is therefore desirable to determine the re- quirements of trees for direct and dif- fused light. By means of Wiesner’s insolator the inten- sity of direct and diffused hght may be measured sepa- rately in the man- ner suggested by Roscoe. The ob- rf) + aan nia iseiEAEa server faces the sun, Au” holding the insola- mye. 9.—Hourly light intensities within the crown of ailan- tor in 2, horizontal thus (Ailanthus glandulosa). Horizontal arrangement of leaves. ee position before him. The time required to obtain the shade of a given standard is recorded. He then turns 180° so that his back is toward the sun, and holding the insolater in the shadow of his body again observes the time required for the “normal paper” to reach the tint of the same standard. The reading taken facing the sun gives the intensity of the total hght— that is, the sum of the direct and dif- fused light. The sa one facing away Eu wie ing WON? ie A we SGN 7 8 from the sun gives copy pet sit only the diffused Fic. 10.—Hourly light intensities within the crown of black hight, since here the eee (Robinia pseudacacia). Vertical arrangement of direct light ig cut off by the body of the observer. The intensity of the direct light alone is then found by subtracting the value of the diffused light from that of the total light. SNTIEN SITY OF LIGHT 4 AM, 44 LIGHT IN RELATION TO TREE GROWTH. If, for instance, 8 seconds are required to obtain the “ normal shade ” in full sunlight and 27 seconds are required when the insolator is held in the shadow of the body, then the intensity of the total light (direct plus diffused hght) 7t=1=0.125; the intensity of the dif- fused light 7d=;;—0.037, and the intensity of the direct light 7D= [t—Id=0.125—0.037=0.088. Since the body of the observer ab- sorbs a portion of the diffused light, this method gives correct values for the total light, but does not give correct values for the diffused light. In order to minimize the error for diffused light it is sug- gested that the insolator be held as far from the body as possible and in such a position as to receive only the shadow cast by the head. WIESNER’S INVESTIGATIONS. By means of the chemical method Wiesner (1907) determined the average minimum light intensities of a comparatively large number of forest trees of the temperate and tropical climates, as follows: Average | Average Species. ; light in Species. light in- tensities. tensities. Boxus sempervirens (bOX).-..---.------ 11/108 || Populus alba (white poplar)............ 1/15 Fagus silvatica (beech)......-..-.------ 1/60 || Populus nigra (black poplar). = 227.25. 1/1i Bsc hippocastanum (horse chest- | Pinus laricio (Austrian pine).........-_- 1/11 Seatac yh RCO AA ge es 1/37 || Betula verrucosa (birch) ..-.----.-...--- 1/9 Garpinus betulus (hornbeam).......... 1/56 || Liriodencron tulipifera (tulip tree)..-_. 17.5 Acer platanoides (Norway maple)...--- 1/55 || Populus deltoides (cottonwood)......_. 1/6 Acer campestre (field maple)........-.. 1/48 || Fraxinus excelsior (ash).............-.- 1/5.8 Acer negundo (box elder)....----.---.- 1/28 | arix. decidua, (larch) eseee na eee 1/5 Quercus. pedunculata (oak).............| 1/26 || Corylus avellana (hazel)...............- 1/3 Ailanthus glandulosa (tree of heaven). . 1/22 Prunus spinosa (cherry).........------- 1/3 Thuja occidentalis (arborvite) -......-- 1/20 1 Measurements of light intensities below 1/80 are unsafe. This scale of tolerance corresponds very closely to the empirical scales that have been worked out by foresters, with the exception perhaps of ash, whose minimum light requirement is given as greater than that of birch, which is scarcely correct. Of special interest are the minimum light requirements found by Wiesner (1905:77-150) for some of our native trees and shrubs and European species commonly grown here | Minimum Species. Region. Elevation. light intensity. | | | Fa | contorta (incl. P. murr)..| Yellowstone Park, Wyo.....-......-.----- 6, 400 1/6 See Se ees Ser e= GOs Sos ste Es Be Ey 8,500 | 1/ 6.4-1/6.9 Pinus “fiexilis ee RI Ae tS Se SEO GOL 4 eee. oR aS eae ee ogee ees 6,000 | 1/8 -1/9 Picea Parryana. cece