Chapter II: The Habitat (3)

In the sunlight, it makes no difference at what height a light reading is taken. In forest and thicket as well as in some herbaceous formations, the intensity of the light, if there is any difference, is greatest just beneath the foliage of the facies. In forests especially, the light is increasingly diffuse toward the ground, particularly where layers intervene. In woodland formations, moreover, the exact spot in which a reading is made must be carefully chosen, unless the foliage is so dense that the shade is uniform. A very satisfactory plan is to take readings in two or more spots where the shade appears to be typical, and to make a check reading in a “sunfleck,” a spot where sunlight shows through. In forests and thickets, the sunflecks are fleeting, and the light value is practically that of the shade. In passing into open woodland and thicket, the sunflecks increase in size and permanence, until finally they exceed the shade areas in amount and become typical of the formation.

_Reflected and Absorbed Light_

=88. The fate of incident light.= The light present in a habitat and incident upon a leaf is not all available for photosynthesis. Part is reflected or screened out by the epidermis, and a certain amount passes through the chlorenchym, except in very thick leaves. The light absorbed is by far the greatest in the majority of species. Many plants with dense coatings of hairs reflect or withhold more light than they absorb, and the amount of light reflected by a thick cuticule is likewise great. As light is imponderable, the actual amount absorbed or reflected by the leaf can not be determined. It is possible, however, to express this in terms of the total amount received, by means of readings with solio paper, and the knowledge thus obtained is of great importance in interpreting the modifications of certain types of leaves. For example, a leaf with a densely hairy epidermis may receive light of the full intensity, 1; the amount reflected or screened out by the hairs may be 95 per cent of this, the amount absorbed 5 per cent, and that transmitted, nil. In the majority of cases, however, the absorbed light is considerably more than the amount reflected or transmitted.

=89. Methods of determination.= If results are to be of value, reflected and transmitted light must be determined in the habitat of the plant simultaneously with the total light which a leaf receives. An approximation of the light reflected from a leaf surface is secured by placing the photometer so that the light reflected is thrown upon the solio strip. A much more satisfactory method, however, is to determine it in connection with the amount of light transmitted through the epidermis. This is done by stripping a piece of epidermis from the upper surface of the leaf and placing it over the slit in the photometer for an exposure. An exposure in the full light of the habitat is made simultaneously with another photometer, or immediately afterward upon the same strip. When the epidermis is not too dense, both exposures are permitted to reach the same tint, and the relation between them is precisely that of their lengths of exposure. Ordinarily the two exposures are made absolutely simultaneous by placing the epidermis over half of the opening, leaving the other half to record the full light value, and the results, or _epidermis prints_, are referred to a multiple standard. The difference between the two values thus obtained represents the amount of reflected light together with that screened by the epidermis. The amount of light transmitted through the leaf may be measured in the same way by using the leaf itself in place of the epidermis alone. The time of exposure is necessarily long, however, and it has been found practicable to obtain leaf prints by exposing the leaf in a printing frame, upon solio paper, at the same time that the epidermis print is made. In a few species both the upper and lower epidermis can be removed and the amount of light absorbed determined directly by exposing the strip covered with the chlorenchym. Generally, however, this must be computed by subtracting the sum of the per cents of reflected and transmitted light from 100 per cent, which represents the total light.

=90. Leaf and epidermis prints.= In diphotic leaves the screening effect of the lower epidermis may be ignored. Isophotic sun leaves, i. e., those nearly upright in position or found above light-colored, reflecting soils, are usually strongly illuminated on both sides, and the absorbed light can be obtained only by measuring the screening effect of both epiderms. Shade leaves and submerged leaves often contain chloroplasts in the epidermis, and the above method can not be applied to them. In fact, in habitats where the light is quite diffuse, practically all incident light is absorbed. The rare exceptions are those shade leaves with a distinct bloom. In addition to their use in obtaining the amount of light absorbed, both leaf and epidermis prints are extremely interesting for the direct comparison of light relations in the leaves of species belonging to different habitats. The relative screening value of the upper and lower epidermis, or of the corresponding epiderms of two ecads or two species, is readily ascertained by exposing the two side by side in sunshine, over the slit in the photometer. For leaf prints fresh leaves are desirable, though nearly the same results can be obtained from leaves dried under pressure. The leaves are grouped as desired on the glass of a printing frame, and covered with a sheet of solio. They are then exposed to full sunlight, preferably at meridian, and the prints evaluated by means of the multiple standard. This method is especially useful in the comparison of ecads of one species. These differences due to transmitted light are very graphic, and can easily be preserved by “toning” the print in the usual way.

_Expression of Results_

=91. Light records.= The actual photographic records obtained by photometer and selagraph can at most be kept but a few months, unless they are toned or fixed. “Toning” modifies the color of the exposure materially, and changes its intensity so that it can not be compared with readings not fixed. It would involve a great deal of inconvenience to make all comparisons by means of toned strips and standard, even if it were not for the fact that it is practically impossible to obtain exactly the same shade in lots toned at different times. The field record, if carefully and neatly made, may well take the place of a permanent one. The form is the following:

═══════╤══════════╤═════════╤═══════╤════════╤════════╤══════════╤═══════
Day │ Hour │Formation│Station│Altitude│Exposure│ Group │Height
│ │ │ │ │ │ │
│ │ │ │ │ │ │
───────┼──────────┼─────────┼───────┼────────┼────────┼──────────┼───────
14/9/04│ 12:00 M. │ Spruce │ Milky │2600 m. │N.E. 20°│Opulaster │1 foot
│ │ │ Way │ │ │ │
„ │12:05 P.M.│ Spruce │ Moss │2500 m. │ Level │Streptopus│ „
│ │ │ Glen │ │ │ │
„ │12:15 P.M.│ Brook │Grotto │2500 m. │ E. 3° │ Filix │Surface
│ │ b’nk │ │ │ │ │
───────┴──────────┴─────────┴───────┴────────┴────────┴──────────┴───────

═══════╤════╤════════╤════════╤═════╤═════╤═════════╤════════╤════════
Day │No. │ Length │Standard│Light│Base │Reflected│Transm’d│Absorbed
│ │ of │ │value│value│ light │ light │ light
│ │exposure│ │ │ │ │ │
───────┼────┼────────┼────────┼─────┼─────┼─────────┼────────┼────────
14/9/04│2:10│ 160 s. │ 3 s │.019 │ │ │ │
│ │ │ │ │ │ │ │
„ │2:12│ 240 s. │ 3 s. │.012 │ │ │ │
│ │ │ │ │ │ │ │
„ │2:13│ 360 s. │ 3 s. │.008 │ │ │ │
│ │ │ │ │ │ │ │
───────┴────┴────────┴────────┴─────┴─────┴─────────┴────────┴────────

=92. Light sums, means, and curves.= Owing to the fact that the selagraph has not yet been used in the field, no endeavor has been made to determine the light value for every hour of the day in different habitats. Consequently there has been no attempt to compute light sums and means. Photometer readings have sufficed to interpret the effect of light in the structure of the formation, and of the individual, but they have not been sufficiently frequent for use in ascertaining sums and means. The latter are much less valuable than the extremes, especially when the relative duration of these is indicated. Means, however, are readily obtained from the continuous records. Light sums are probably impracticable, as the factor is not one that can be expressed in absolute terms. The various kinds and combinations of light curves are essentially the same as for humidity. The level curve through a series of habitats is the most illuminating, but the day curve of hour variations is of considerable value. The curve of daily duration, based upon full sunlight, is also of especial importance for plants, and stations which receive both sun and shade during the day.

TEMPERATURE

=93.= In consequence of its indirect action, temperature does not have a striking effect upon the form and structure of the plant, as is the case with water and light. Notwithstanding, it is a factor of fundamental importance. This is especially evident in the character and distribution of vegetation. It is also seen in the germination and growth of plants, in the length of season, and in the important influence of temperature upon humidity, and hence upon water-content. Because of its intimate relation with the comfort of mankind, the determination of temperature values has received more attention than that of any other factor, and excellent simple and recording instruments are numerous. For plants, it is also necessary to employ instruments for measuring soil temperatures. The latter unquestionably have much less meaning for the plant than the temperatures of the air, but they have a direct influence upon the imbibition of water, and upon germination.

_Thermometers_

=94. Air thermometers.= The accurate measurement of temperature requires standard thermometers. Reasonably accurate instruments may be standardized by determining their error, but they are extremely unsatisfactory in practice, since they result in a serious waste of time. Accurate thermometers which read to the degree are entirely serviceable as a rule, but instruments which read to a fraction of a degree are often very much to be desired. The writer has found the “cylindrical bulb thermometer, Centigrade scale” of H. J. Green, to be an exceedingly satisfactory instrument. The best numbers for general use are 247 and 251, which read from –15° to 50° C. and are graduated in .2°. They are respectively 9 and 12 inches long, and cost $2.75 and $3.50. These instruments are delicate and require careful handling, but even in class work this has proved to be an advantage rather than otherwise. In making readings of air temperatures with such thermometers, constant precautions must be taken to expose the bulb directly to the wind and to keep it away from the hand and person.

=95. Soil thermometers.= The thermometer described above has been used extensively for soil temperatures. The determination of the latter is conveniently combined with the taking of soil samples, by using the hole for a temperature reading. When carefully covered, these holes can be used from day to day throughout the season without appreciable error, even in gravel soils. Repeated tests of this have been made by simultaneous readings in permanent and newly made holes, and the results have always been the same. It has even been found that the error is usually less than 1 degree when the hole is left uncovered, if it is more than 9 inches deep. A slight source of error lies in the fact that the thermometer must be raised to make the reading. With a little practice, however, the top of the column of mercury may be raised to the surface and read before the change of temperature can react upon it. This is especially important in very moist or wet soils where the bulb becomes coated with a film of moisture. This evaporates when the bulb is brought into the air, and after a moment or two the mercury slowly falls.

Regular soil thermometers are indispensable when readings are desired at depths greater than 12–18 inches. They possess several disadvantages which restrict their use almost wholly to permanent stations. It is scarcely possible to carry them on field trips, and the time required to place them in the soil renders them practically useless for single readings. Moreover, the instruments are expensive, ranging in price from $7 for the two-foot thermometer, to $19 for the eight-foot one. When it is recognized that deep-seated temperatures are extremely constant and that the slight fluctuations affect, as a rule, only the relatively stable shrubs and trees, it is evident that such temperatures are of restricted importance. Still, in any habitat, they must be ascertained before they can well be ignored, though it is unwise to spend much time and energy in their determination. Soil thermometers of the form illustrated may be obtained from H. J. Green, Brooklyn.

=96. Maximum-minimum thermometers.= These are used for determining the range of temperature within a given period, usually a day. Since they are much cheaper than thermographs, they can replace these in part, although they merely indicate the maximum and minimum temperatures for the day, and do not register the time when each occurs. The maximum is a mercurial thermometer with a constriction in the tube just above the bulb; this allows the mercury to pass out as it expands, but prevents it from running back, thus registering the maximum temperature. The minimum thermometer contains alcohol. The column carries a tiny dumbbell-shaped marker which moves down with it, but will not rise as the liquid expands. This is due to the fact that the fluid expands too slowly to carry the marker upward, while the surface tension causes it to be drawn downward as the fluid contracts. The minimum temperature is indicated by the upper end of the marker. In setting up the thermometers, they are attached by special thumbscrews to a support which holds them in an oblique position. The minimum is placed in a special holder above the maximum which rests on a pin that is used also for screwing the pivot-screw into position. The support is screwed tightly to the cross-piece of a post, or in forest formations it is fastened directly to a board nailed upon a tree trunk. A shelter has not been used in ecological work, although it is the rule in meteorological observations. The minimum thermometer is set for registering by raising the free end, so that the marker runs to the end of the column. The mercury of the maximum is driven back into the bulb by whirling it rapidly on the pivot-screw after the pin has been taken out. This must be done with care in order that the bulb may not be broken. As soon as the instrument comes to rest, it is raised and the pin replaced, great care being taken to lift it no higher than is necessary. When the night maximum is sought, the thermometer should be whirled several times in order to drive the column sufficiently low. Usually, in such cases, a record is made of this point to make sure that the maximum read is the actual one. If the pivot-screw is kept well oiled, less force will be required to drive the mercury back. In practice, the thermometers have been observed at 6:00 A.M. and 6:00 P.M. each day, thus permitting the reading of the maximum-minimum for both day and night. Pairs of maximum-minimum thermometers are to be obtained from H. J. Green, 1191 Bedford Ave., Brooklyn, or Julien P. Friez, Baltimore, Maryland, at a cost of $8.25.

=97. Radiation thermometers.= These are used to determine the radiation in the air, and from the soil, i. e., for solar and terrestrial radiation. The latter alone has been employed in the study of habitats, chiefly for the purpose of ascertaining the difference in the cooling of different soils at night. The terrestrial radiation thermometer is merely a special form of minimum thermometer, so arranged in a support that the bulb can be placed directly above the soil or plant to be studied. It is otherwise operated exactly like the minimum thermometer, and the reading gives the minimum temperature which the air above the plant or soil reaches, _not_ the amount of radiation. As a consequence, these instruments are valuable only where read in connection with a pair of maximum-minimum thermometers in the air, or when read in a series of instruments placed above different soils or plants.

=98. Thermographs.= Two types of standard instruments are in general use for obtaining continuous records of air temperatures. These are the Draper thermograph, made by the Draper Manufacturing Company, 152 Front St., New York city ($25 and $30), and the Richard thermograph sold by Julien P. Friez, Baltimore ($50). After careful trial had demonstrated that they were equally accurate, the matter of cost was considered decisive, and the Draper thermograph has been used exclusively in the writer’s own work. This instrument closely resembles the psychrograph manufactured by the same company. It is made in two sizes, of which the larger one is the more satisfactory on account of the greater distance between the lines of the recording disk. The thermometric part consists of two bimetallic strips, the contraction and expansion of which are communicated to a hand carrying a pen. The latter traces a line on the record sheet which is attached to a metal disk made to revolve by an eight-day clock. In practice the thermograph is set up in the shelter which contains the psychrograph, and in exactly the same manner. The clock is wound, the record put in place, and the pen inked in the same way also. The proper position of the pen is determined by making a careful thermometer reading under the shelter, and then regulating the pen hand by means of the screws at the base of it. A similar test reading is also made each week, when the clock is rewound. A record sheet may be left in position for three weeks, the pen being filled each week with a different ink. The fixed order of using the inks is red, blue, and green as already indicated.

Owing to the fact that they are practically stationary, soil thermographs are of slight value, except at base stations. Here, the facts that they are expensive, that the soil temperatures are of relatively little importance, and that they can be determined as easily, or nearly so, by simple thermometers, make the use of such instruments altogether unnecessary, if not, indeed, undesirable. In a perfectly equipped research station, they undoubtedly have their use, but at ordinary stations, and in the case of private investigators, their value is in no wise commensurate with their cost.

_Readings_

=99. Time.= The hourly and daily fluctuations of the temperature of the air render frequent readings desirable. It is this variation, indeed, which makes single readings, or even series of them, inconclusive, and renders the use of a recording instrument almost imperative in the base station at least. Undoubtedly, a set of simultaneous readings at different heights in one station, or at the same height in different stations, especially if made at the maximum, have much value for comparison, but their full significance is seen only when they are referred to a continuous base record. Such series, moreover, furnish good results for purposes of instruction. In research work, however, it has been found imperative to have thermographs in habitats of widely different character. With these as bases, it is possible to eke them out with considerable satisfaction by means of maximum-minimum thermometers in less different habitats, or in different parts of the same habitat. Naturally these are less satisfactory, and are used only when expense sets a limit to the number of thermographs. In a careful analysis of a single habitat, more can be gained by one base thermograph supplemented by three pairs of maximum-minimum thermometers in dissimilar areas of the habitat than by two thermographs, and the cost is the same.

=100. Place and height.= For general air temperatures, thermograph and thermometer readings are made at a height of 3 feet (1 meter). Soil temperatures are regularly taken at the surface and at a depth of 1 foot. When a complete series of simultaneous readings is made in one station, the levels are 6 feet and 3 feet in the air, the surface of the soil, and 5, 10, and 15 inches in the soil. When sun and shade occur side by side in the same formation, as is true of many thickets and forests, surface readings are regularly made in both. Similarly, valuable results are obtained by making simultaneous readings on the bare soil, on dead cover, and upon a leaf, while the influence of cover is readily ascertained by readings upon it and beneath it. A full series of station readings made at the same time upon north, east, south, and west slopes is of great importance in studying the effects of exposure.

_Expression of Results_

=101. Temperature records.= Neither field nor permanent form is required for thermographic records, other than the record sheet itself, which contains all the necessary information in a fairly convenient form. Although the temperature of a particular hour and day can not be read at a mere glance, it can be obtained so easily that it is a waste of time to make a tabular copy of each record sheet. For thermometer readings, either single or in series, the following form is used:

═══════╤═════════╤═════════╤════════╤════════╤════════╤═══════════
Day │ Hour │Formation│Station │Altitude│Exposure│ Community
───────┼─────────┼─────────┼────────┼────────┼────────┼───────────
„ │ „ │ „ │ „ │ „ │ „ │ „
───────┼─────────┼─────────┼────────┼────────┼────────┼───────────
17/8/04│6:30 A.M.│ Spruce │ Jack │2550 m. │N.E. 5° │Mertensiare
│ │ │ Brook │ │ │
„ │ „ │ Half │Hiawatha│2550 m. │N.E. 7° │ Asterare
│ │ gravel │ │ │ │
„ │6:30 P.M.│ Spruce │ Jack │2550 m. │N.E. 5° │Mertensiare
│ │ │ Brook │ │ │
„ │ „ │ Half │Hiawatha│2550 m. │N.E. 7° │ Asterare
│ │ gravel │ │ │ │
───────┴─────────┴─────────┴────────┴────────┴────────┴───────────

═══════╤═════════╤═══════════════════╤═══════════╤══════╤════
Day │ Hour │POSITION OF READING│Thermograph│ Sky │Wind
───────┼─────────┼──────┬─────┬──────┼───────────┼──────┼────
„ │ „ │3 feet│Surf.│12 in.│ „ │ „ │ „
───────┼─────────┼──────┼─────┼──────┼───────────┼──────┼────
17/8/04│6:30 A.M.│ 9° │ 9° │ 9.8° │ 10° │Clear │ 0
│ │ │ │ │ │ │
„ │ „ │11.2° │11.2°│14.8° │ 10° │Clear │ 0
│ │ │ │ │ │ │
„ │6:30 P.M.│11.4° │11.4°│ 9.8° │ 11° │Cloudy│ 0
│ │ │ │ │ │ │
„ │ „ │ 12° │13.8°│16.4° │ 11° │Cloudy│ 0
│ │ │ │ │ │ │
───────┴─────────┴──────┴─────┴──────┴───────────┴──────┴────

=102. Temperature sums and means.= The amount of heat, i. e., the number of calories received within a given time by a definite area of plant surface, can be determined by means of a calorimeter. From this the temperature sum of a particular period may be obtained by simple addition. In the present condition of our knowledge, it is impossible to establish any exact connection between such results and the functional or growth effect that can be traced directly to heat. As a consequence, temperature sums do not at present contribute anything of value to an understanding of the relation between cause and effect. The mean daily temperature is readily obtained by averaging twenty-four hour-temperatures recorded by the thermograph. The method employed by Meyen[6], of deriving the mean directly from the maximum and minimum for the day, is not accurate; from a large number of computations, the error is always more than two degrees. On the other hand, the mean obtained by averaging the maximum and minimum for the day and night has been found to deviate less than 1 degree from the mean proper. This fact greatly increases the value of maximum-minimum instruments if they are read daily at 6:00 A.M. and 6:00 P.M.

=103. Temperature curves.= The kinds and combinations of temperature curves are almost without number. The simple curves of most interest are those for a series of stations or habitats, based upon the level of three feet, or the surface, or the daily mean. The curves for each station representing the different heights and depths and the season curve of the daily means for a habitat are also of much importance. One of the most illuminating combinations is that which groups together the various level curves for a series of habitats. Other valuable combinations are obtained by grouping the curves of daily means of different habitats for the season, or the various station curves.

=104. Plant temperatures.= The direct effects of temperature as seen in nutrition and growth can be ascertained only by determining the temperature of plant tissues. The temperatures of the air and of the soil surface have an important effect upon humidity, and water-content, and through them upon the plant, but heat can influence assimilation, for example, only in so far as it is absorbed by the assimilating tissue. The temperatures of the leaf, as the most active nutritive organ of the plant, are especially important. While it is a well-known fact that internal temperatures follow those of the air and soil closely, though with varying rapidity of response, this holds less for leaves than for stems and roots. Owing to the very obvious difficulties, practically nothing has yet been done in this important field. A few preliminary results have been obtained at Minnehaha, which serve to show the need for such readings. Gravel slide rosettes in an air temperature of 24° C. and a surface temperature of 40° C. gave the following surface readings: _Parmelia_, 40°, _Eriogonum_, 38.6°, _Arctostaphylus_, 35°, _Thlaspi_, 31.8°, and _Senecio_, 31°. The leaf of _Eriogonum flavum_, which is smooth above and densely hairy below, indicated a temperature of 31.8° when rolled closely about the thermometer bulb with the smooth surface out, and 28° when the hairy surface was outside. The surface readings of the same leaf were .5°–1° higher when made upon the upper smooth surface. This immediately suggests that the lower surface may be modified to protect the leaf from the great heat of the gravel, which often reaches 50° C. (122° F.).

PRECIPITATION

=105. General relations.= As the factor which exerts the most important control upon water-content and humidity, rainfall must be carefully considered by the ecologist. It is such an obvious factor, and is usually spoken of in such general terms that the need of following it accurately is not evident at once. When it is recognized that the fluctuations of water-content are directly traceable to it, it becomes clear that its determination is as important as that of any indirect factor. This does not mean, however, that the amount of yearly rainfall is to be taken from the records of the nearest weather station, and the factor dismissed. Like other instruments, the rain gauge must be kept at the base station of the area under study, and when this is extensive or diverse, additional instruments should be put into commission. While the different parts of the same general climatic region may receive practically the same amount of precipitation during the year, it is not necessarily true that the rainfall of any particular storm is equally distributed, especially in the mountains. Nothing less than an exact knowledge of the amount of rain that falls in the different areas will make it possible to tell how much of the water-content found at any particular time in these represents merely the chance differences of precipitation.

The forms of precipitation are rain, dew, hail, snow, and frost. Of these, hail is too infrequent to be taken into account, while frost usually occurs only at the extremes of the growing season, and in its effect is rather to be reckoned with temperature. Snow rarely falls except during the period of rest, and, while it plays an important part as cover, it is merely one of several factors that determine the water-content of the soil at the beginning of spring. The influence of dew is not clearly understood. It is almost always too slight in amount and too fleeting to affect the water-content of the soil. It seems probable that it may serve by its own evaporation to decrease in some degree the water loss from the soil, and from bedewed plants. If, however, the dew is largely formed by the water of the soil and of the plant, as is thought by some, then it is negligible as a reinforcement of water-content. From the above, it is evident that rainfall alone exerts a profound effect upon the habitat, and it is with its measurement that the ecologist is chiefly concerned.

=106. The rain gauge=, as the illustration shows, is a cylindrical vessel with a funnel-shaped receiver at the top, which is 8 inches in diameter. The receiver fits closely upon a narrower brass vessel or measuring tube in which the rain collects. The ratio of surface between receiver and tube is 10 to 1. For readings covering a general area, the rain-gauge is placed in the open, away from buildings or other obstructions, and is sunken in the ground sufficiently to keep it upright. In localities where winds are strong, it is usually braced at the sides also or supported by a wooden frame. In measuring the amount of rain in the measuring tube, the depth is divided by ten in order to ascertain the actual rainfall. The depth is measured by inserting the measuring-rod through the hole in the funnel until it touches the bottom. It is left for a second or so, quickly withdrawn, and the limit of the wetted portion noted. In the case of standard rods, the actual rainfall is read directly in hundredths, so that the division by ten is unnecessary. After each reading, the measuring tube is carefully drained, replaced, and the receiver put in position. No regular time for making readings is necessary. During a rainy period, it is customary to make a measurement each day, but it has been found more satisfactory for ecological purposes to measure each shower, and to record its duration. These two facts furnish a ready clue to the relative amount of run-off in each fall of rain. The measurement of snowfall is often made merely by determining its depth. For comparison with rainfall, the rain gauge with receiver and tube withdrawn is used. The snow which falls is melted, poured into the measuring tube, and measured in the ordinary way. The U. S. Weather Bureau standard rain gauge, with measuring stick, may be obtained of H. J. Green, or of J. P. Friez for $5.25.

=107. Precipitation records.= From the periodic character of precipitation, rainfall sums, means, and curves have little importance in the careful study of the habitat. The rainfall curve for the growing season is an aid in explaining the curve of water-content, and the mean rainfall of a region gives some idea of its vegetation, though even here the matter of its distribution is of primary importance. The rain and snow charts published by the U. S. Weather Bureau furnish data of some importance for the general study of vegetation, but it is evident that they can play little part in a system which is founded upon the habitat. Precipitation records, for reasons of brevity and convenience, are united with wind records, and the form will be found under the discussion of this factor.

WIND

=108. Value of readings.= On account of its direct effect upon humidity, and its consequent influence upon water-content, the part which wind plays in a habitat can not be ignored in a thorough investigation. It is an important element in exposure, and accordingly has a marked mechanical effect upon the vegetation of exposed habitats, alpine slopes, seacoasts, plains, etc. Owing to its inconstancy and its extreme variation in velocity, single wind readings are absolutely without value. When read in series, anemometers give some information upon the comparative air movement in different habitats, but the chance of error is great, except when the breeze is steady. Anemographs alone give real satisfaction. Accurate results, however, are not obtainable without a series of two or more in different habitats, and it is still an open question whether the results obtained justify the expense. For a completely equipped base station, anemometer, anemograph, and wind vane are desirable instruments, but the study of the habitat has by no means reached the stage of precision in which their general use is necessary.

=109. The anemometer= in its simplest form is adapted only to readings made under direct observation, as a sudden change in the direction of the wind reverses the movement of the indicator needle. This simple wind gauge, shown in figure 23, has been used for instructional purposes, and to a slight extent, also, in ascertaining the effect of cover. In constant winds, successive single readings are found to have value, but, ordinarily, the observations must be simultaneous. Careful tests of this simple instrument show that it is essentially accurate. It may be obtained from the C.H. Stoelting Company, 31 W. Randolph St., Chicago, for $25. The standard anemometer (Fig. 24) is practically a recording instrument up to 1,000 miles, but as the dials run on without any indication of the total number of revolutions, it must be visited and read each day. This renders its use difficult for habitats which are some distance apart. When exact determinations of wind values become necessary, the most successful method is to establish a series of three standard anemometers. One of these should be placed upon the most exposed part of a typically open habitat, the second in the most protected part of the same habitat, while the third is located in the midst of a representative forest formation. If the two habitats are close together, the daily visits can be made without serious inconvenience. The reading of the registering dials requires detailed explanation, and for this the reader is referred to the printed directions which accompany the instrument. In setting up the anemometer it must be borne in mind that the ecologist desires the wind velocity for a particular habitat. In consequence, the precautions which the meteorologist takes to place the instrument at a certain height and well away from surrounding obstructions do not hold here. Standard anemometers are furnished by H. J. Green, and J. P. Friez for $25 each.

The anemograph is an anemometer electrically connected with an automatic register. It is the only instrument adapted to continuous weekly records in different habitats, but the price, $75 ($25 for the anemometer and $50 for the register) is practically prohibitive, at least until a complete series of ecographs for other factors has been obtained.

=110. Records.= The following form is used as a combined record for precipitation and wind:

═══════╤══════════╤═════════╤════════╤════════╤════════╤═════════ Day │ Time │Formation│Station │Altitude│Exposure│Community ───────┼──────────┼─────────┼────────┼────────┼────────┼───────── „ │ „ │ „ │ „ │ „ │ „ │ „ ───────┼──────────┼─────────┼────────┼────────┼────────┼───────── 29/8/04│ 6:30 P.M.│ Half │Hiawatha│2550 m. │N.E. 17°│Asterare │ │ gravel │ │ │ │ 31/8/04│ 5:45 P.M.│ „ │ „ │ „ │ „ │ „ 2/9/04│ 4:00 P.M.│ „ │ „ │ „ │ „ │ „ 3/9/04│10:00 P.M.│ „ │ „ │ „ │ „ │ „ ───────┴──────────┴─────────┴────────┴────────┴────────┴─────────

═══════╤══════════╤═══════════════╤════╤═════════════════════════╤════ Day │ Time │ RAINFALL │Base│ WIND │Base ───────┼──────────┼──────┬────────┼────┼────────┬──────┬─────────┼──── „ │ „ │Inches│Duration│ „ │Velocity│Heig’t│Direction│ „ ───────┼──────────┼──────┼────────┼────┼────────┼──────┼─────────┼──── 29/8/04│ 6:30 P.M.│ 1 │8 hours │ │ 5 │3 ft. │ N. W. │ │ │ │ │ │ │ │ │ 31/8/04│ 5:45 P.M.│Trace │10 min. │ │ 12 │ „ │ „ │ 2/9/04│ 4:00 P.M.│ .2 │2 hours │ │ 7 │ „ │ W. │ 3/9/04│10:00 P.M.│Trace │ │ │ 18 │ „ │ „ │ ───────┴──────────┴──────┴────────┴────┴────────┴──────┴─────────┴────

SOIL

=111. Soil as a factor.= In determining the value of the soil as a factor in a particular habitat, it must be clearly recognized that its importance lies solely in the control which it exerts upon water-content and nutrient-content. The former is directly connected with the texture or fineness of the soil, the latter with its chemical nature. Accordingly, the structure of the soil and its chemical composition are the fundamental points of attack. These are not at all of equal value, however. Water is both a food, and a solvent for the nutrient salts of the soil. Furthermore, the per cent of soluble salts, as determined in mechanical analyses, is practically the same for all ordinary soils. Indeed, the variations for the same soil types are as great as for entirely different types. For these reasons, soluble salt-content may be ignored except where it is readily seen to be excessive, as in alkaline soils; and determinations of chemical composition are necessary only in those soils which contain salts or acids to an injurious degree, e. g., alkaline soils, peat bogs, humus swamps, etc. The structure of the soil, on the other hand, in the usual absence of excessive amounts of solutes, absolutely controls the fate of the water that enters the ground, in addition to its influence upon the run-off. It determines the amount of gravitation water lost by percolation, as well as the water that can be raised by capillarity. The resultant of these, the total soil water or holard, is hence an effect of structure, while the size and compactness of the particles are conclusive factors in controlling the chresard. It must be recognized, however, that these are all factors which enable us to interpret the amount of holard or chresard found in a particular soil. They have no direct important effect upon the plant, but influence it only in so far as they affect the water present.

=112. The value of soil surveys.= The full appreciation of the preeminent value of water-content, particularly of the chresard, greatly simplifies the ecological study of soils. The ecologist is primarily concerned with soil water only in its relation to the plant, and while a fair knowledge of soil structure is essential to a proper understanding of this, he has little concern with the detailed study of the problems of soil physics. For the sake of a proper balance of values, he must avoid the tendency noted elsewhere of ignoring the claims of the plant, and of studying the soil simply as the seat of certain physical phenomena. Accordingly, it is felt that mechanical and chemical analyses, determinations of soluble salt-content, etc., have much less value than has been commonly supposed. The usual methods of soil survey, which pay little or no attention to water-content, and none at all to available water, are practically valueless for ecological research. This statement does not indicate a failure to appreciate the importance of the usual soil methods for many agricultural problems, such as the use of fertilizers, conservation of moisture, etc., though even here to focus the work upon water-content would give much more fundamental and serviceable results. For these reasons, slight attention will be paid to methods of mechanical and chemical analysis. In their stead is given a brief statement of the origin, structure, and character of soils with especial reference to water-content.

=113. The origin of soils.= Rocks form soils in consequence of weathering, under the influence of physical and biotic factors. Weathering consists of two processes, disintegration, by which the rock is broken into component particles of various sizes, and decomposition, in which the rock or its fragments are resolved into minute particles in consequence of the chemical disaggregation of its minerals, or of some other chemical change. These processes are usually concomitant, although, as a rule, one is more evident than the other. The relation between them is dependent upon the character of the rock and the forces which act upon it. Hard rocks, i. e., igneous and metamorphic ones, as a rule disintegrate more rapidly than they decompose; sedimentary rocks, on the other hand, tend to decompose more rapidly than they disintegrate. In many cases the two processes go hand in hand. This difference is the basis for the distinction, first proposed by Thurmann, between those rocks which weather with difficulty and those which weather readily. The former were called dysgeogenous, the latter eugeogenous. Thurmann restricted the application of the first term to those rocks which produce little soil, but it seems more logical to apply dysgeogenous to those in which disintegration is markedly in excess of decomposition, and eugeogenous to those rocks that break down rather readily into fine soils. With respect to the general character of the soil formed, rocks are _pelogenous_, clay-producing, _psammogenous_, sand-forming, or _pelopsammogenous_, producing mixed clay and sand. The first two are divided into _perpelic_, _hemipelic_, _oligopelic_, _perpsammic_, etc., with reference to the readiness with which they are weathered, but this distinction is not a very practicable one. The grouping of soils into silicious, calcareous, argillaceous, etc., with reference to the chemical nature of the original rock, is of no value to the ecologist, apart from the general clue to the physical properties which it furnishes.

=114. The structure of soils.= The water capacity of a soil is a direct result of the fineness of the particles. Since the water is held as a thin surface film by each particle or group of them, it follows that the amount of water increases with the water-holding surface. The latter increases as the particles become finer and more numerous, and thus produce a greater aggregate surface. The upward and downward movements of water in the soil are likewise in immediate connection with the size of particles. The upward or capillary movement increases as the particles become finer, thus making the irregular capillary spaces between them smaller, and magnifying the pull exerted. On the contrary, the downward movement of gravitation water, i. e., percolation, is retarded by a decrease in the size of the soil grains and hastened by an increase. Hence, the two properties, capillarity and porosity, are direct expressions of the structure of the soil, i. e., of its texture or fineness. Capillarity, however, increases the water-content of the upper layers permeated by the roots of the plant, while porosity decreases it. On the basis of these properties alone, soils would fall into two groups, capillary soils and porous soils, the former fine-grained and of high water-content, the latter coarse-grained and with relatively little water. A third factor, however, of great importance must be taken into account. This is the pull exerted upon each water film by the soil particle itself. This pull apparently increases in strength as the film grows thinner, and explains why it finally becomes impossible for the root-hairs to draw moisture from the soil. This property, like capillarity, is most pronounced in fine-grained soils, such as clays, and is least evident in the coarser sands and gravels. It seems to furnish the direct explanation of non-available water, and, in consequence, to indicate that the chresard is an immediate result of soil texture.

=115. Mechanical analysis.= From the above it is evident that, with the same rainfall, coarse soils will be relatively dry, and fine soils correspondingly moist. However, this difference in holard is somewhat counterbalanced by the fact that the chresard is much greater in the former than in the latter. The basis of these relations can be obtained only from a study of the texture of the soil. The usual method of doing this is by mechanical analysis. This is far from satisfactory, since the use of the sieves often brings about the disaggregation of groups of particles which act as units in the soil. Furthermore, the analysis affords no exact evidence of the compactness of the soil in nature, and tests of capillarity and porosity made with soil samples out of position are open to serious error. Nevertheless, mechanical analyses furnish results of some value by making it possible to compare soils upon the basis of texture. For ecological purposes, minute analyses are undesirable; their value in any work is doubtful. A separation of soil into gravel, sand, and silt-clay is sufficient, since the relative proportion of these will explain the holard and chresard of the soil concerned. The latter are also affected in rich soils, especially of forests, by the organic matter present. If this is in a finely divided condition, the amount is determined by calcining. When a definite layer of leafmold is present, as in forests and thickets, its water-value is found separately, since its power of retaining water is altogether out of proportion to its weight.

=116. Kinds of soils.= It is very doubtful whether it is worth while to attempt to distinguish soils upon the basis of mechanical analysis. Unquestionably, the most satisfactory method is to distinguish them with respect to holard and chresard, and to regard texture as of secondary importance. A series of soil classes which comprise various soil types has been proposed by the U. S. Bureau[7] of Soils as follows: (1) stony loam, (2) gravel, (3) gravelly loam, (4) dunesand, (5) sand, (6) fine sand, (7) sandy loam, (8) fine sandy loam, (9) loam, (10) shale loam, (11) silt loam, (12) clay loam, (13) clay, (14) adobe. These are based entirely upon mechanical analyses, and in some cases are too closely related to be useful. The line between them can nowhere be sharply drawn. Indeed, the variation within one class is so great that soils have frequently been referred to the wrong group. Thus, Cassadaga sand (gravel 22 per cent, sand 43 per cent, silt 21 per cent, clay 10 per cent) is more closely related to Oxnard sandy loam (26–37–18–12) and to Afton fine sandy loam (28–43–18–8) than to Coral sand (61–29–3–4), Galveston sand (6–91–1–1), or Salt Lake sand (84–15–1–0). Elsinore sandy loam (8–38–35–10) is much nearer to Hanford fine sandy loam (9–36–33–14) than to Billings sandy loam (1–60–22–11) or to Utuado sandy loam (48–23–19–8). The soil types are much more confused, and for ecological purposes at least are entirely valueless. Lake Charles fine sandy loam has the composition, 1–34–52–9; Vernon fine sandy loam, 1–37–54–7, while many other so-called types show nearly the same degree of identity.

=117. The chemical nature of soils.= The effect of alkaline and acid substances in the soil upon water-content and the activities of the plant is far from being well understood. It is generally recognized that salts and acids tend to inhibit the absorptive power of the root-hairs. In the case of saline soils, this inhibitive effect seems to be established, but the action of acids in bogs and swamps is still an open question. It is probable that the influence of organic acid has been overestimated, and that the curious anomaly of a structural xerophyte in a swamp is to be explained by the stability of the ancestral type and by the law of extremes. Apart from the effect which excessive amounts of acids and salts may have in reducing the chresard, the chemical character of the soil is powerless to produce structural modification in the plant. Since Thurmann’s researches there has been no real support of the contention that the chemical properties of the soil, not its physical nature, are the decisive factors in the distribution and adaptation of plants. It is not sufficient that the vegetation of a silicious soil differs from that of a calcareous one. A soil can modify the plants upon it only though its water-content, or the solutes it contains. Hence, the chemical composition of the original rock is immaterial, except in so far as it modifies these two factors. Humus, moreover, while an important factor in growth, has no formative influence beyond that which it exerts through water-content.

PHYSIOGRAPHY

=118. Factors.= The physiographic factors of a definite habitat are altitude, exposure, slope, and surface. In addition, topography is a general though less tangible factor of regions, while the dynamic forces of weathering, erosion and sedimentation play a fundamental role in the change of habitats. It is evident, however, that these, except where they affect the destruction of vegetation directly, can operate upon the plant only through more direct factors, such as water, light, and temperature. While they are themselves not susceptible of measurement, they can often be expressed in terms of determinable factors, i. e., slope, exposure, and surface. Fundamentally, they constitute the forces which change one habitat into another, and, in consequence, are really to be considered as the factors which produce succession. The static features of physiography, altitude, etc., lend themselves readily to determination by means of precise instruments. These factors, though by no means negligible, are remote, and consequently their mere measurement is insufficient to indicate the nature or extent of their influence upon the plant. It is necessary to determine also the manner and degree in which they affect other factors, a task yet to be done. Readings of altitude, slope, and exposure are so easily made that the student must carefully avoid the tendency to let them stand at their own value, which is slight. Instead, they should be made the starting point for ascertaining the differences which they produce in water-content, humidity wind, and temperature.

_Altitude_

=119. Analysis into factors.= Of all physiographic features, altitude is the most difficult to resolve into simple factors. Because of general geographic relations, it has a certain connection with rainfall, but this is vague and inconstant. Obviously, in its influence upon the plant, altitude is really pressure, and in consequence its effect is exerted upon the climatic and not the edaphic factors of the habitat. Theoretically, the decrease of air pressure in the increased altitude directly affects humidity, light, and temperature. Actually, while there is unquestionably a decrease in the absorption of the light and heat rays owing to the fact that they traverse less atmosphere, which is at the same time less dense, this seems to be negligible. Photometric readings at elevations of 6,000 and 14,000 feet have so far failed to show more than slight differences, which are altogether too small to be efficient. The effect upon humidity is greater, but the degree is uncertain. Continuous psychrographic records at different elevations for a full season, at least, will be necessary to determine this, since the psychrometric readings so far made, while referred to a base psychrograph, are too scattered to be conclusive. Finally, the length of the season, itself a composite, is directly dependent upon the altitude. This relation, though obscure, rests chiefly upon the rarefaction of the air which prevents the accumulation of heat in both the soil and the air.

=120. The barometer.= To secure convenience and accuracy in the determination of altitude, it is necessary to use both a mercurial and an aneroid barometer. The latter is by far the most serviceable for field work, but it requires frequent standardizing by means of the former. The mercurial form is much more accurate and should be read daily in the base station. It is practically impossible to carry it in the field, except in the so-called mountain form, which is of great service in establishing the altitudes of a series of stations. In use the aneroid barometer may be checked daily by the mercurial standard, or it may be set at the altitude of the base station, thus giving a direct reading. After the normal pressure at the base has once been ascertained, however, the most satisfactory method is to set the aneroid each day by the standard, at the same time noting the pressure deviation in feet of elevation (see p. 46). The absolute elevation of the various stations of a series may be determined either by adding or subtracting this deviation from the actual reading at the station, or by noting the change from the base station, and then adding or subtracting this from the normal of the latter. When it is impossible to check the aneroid by means of a mercurial barometer, the average of a series of readings made at different days at one station, especially if taken during settled weather, will practically eliminate the daily fluctuations, and yield a result essentially accurate. Even in this event, the accuracy of the aneroid should be checked as often as possible, since the mechanism may go wrong at any time. The barograph, while a valuable instrument for base stations, is not at all necessary. These instruments can be obtained from all makers of meteorological apparatus, such as H. J. Green, and J. P. Friez. Aneroid barometers reading to 16,000 feet cost about $20; the price of the Richards aneroid barograph is $45. Ordinary observatory barometers cost $30–$40; the standard instrument sells at $75–$100. The mountain barometer, which is altogether the most serviceable for the ecologist, ranges from $30–$55, depending upon accessories, etc.

_Slope_

=121. Concept.= This term is used in the ordinary sense to indicate the relation of the surface of a habitat to the horizon. Although it is a complex of factors, or rather influences several factors, these are readily determinable. The primary effect of slope is seen in the control of run-off and drainage, and consequently of water-content, although these are likewise affected by soil texture and by surface. Slope, moreover, as a concomitant of exposure, has an important bearing upon light and heat by virtue of determining the angle of incidence, and also upon wind, and, through it, upon the distribution of snow. At present, while it can be expressed definitely in degrees, it has not yet been connected quantitatively with more direct factors. This is, however, not a difficult task, and it is probable that we shall soon come to express slope principally in amount of run-off, and of incident heat.