Chapter II: The Habitat (4)

=122. The clinometer.= In the simplest form, this instrument is merely a semicircle of paper, with each half graduated from 1–90°. It is mounted on a board and placed base upward, upon a wooden strip, 2 feet long and 2 inches wide, which has a true edge. At the center of the circle is attached a line and plummet for reading the perpendicular. A more convenient form is shown in figure 28, which is both clinometer and compass. This also necessitates the use of a basing strip to eliminate the inequalities of the surface. The dial face is graduated to show inches of rise per yard, as well as the number of degrees, but the latter, as the simpler term, is preferable for ecological work. In making a reading, the basing strip is placed upon a representative area of the slope, and pressed down firmly to equalize slight irregularities. The clinometer is moved slightly along the upper edge, causing the marker to swing freely. After the latter comes to rest, the instrument is carefully turned upon its back, when the angle of the slope in degrees may be read directly. Two or three such readings in different areas will suffice for the entire habitat, unless it be extremely irregular. The clinometer with compass may be obtained from the Keuffel and Esser Company, 111 Madison St., Chicago, Illinois, for $5.

=123. The trechometer.= For measuring the effect of slope upon run-off, a simple instrument called the trechometer (τρέχω, to run off) has been devised. This consists merely of a metal tank, 3 × 4 × 12 inches, holding 144 cubic inches of water, with an opening ¼ × 12 inches at the base in front, closed by a tight-fitting slide. Three metal strips, 2 × 12 inches, are fastened to the front of the tank in such a way as to enclose a square foot of soil into which the strips penetrate an inch. In the front strip is an opening, 1 inch square, provided with a drip from which the run-off is collected in a measuring vessel. In use, the instrument is put in position with the metal rim forced down 1 inch into the soil; the tank is filled, the graduate put in place, and the slide raised. The run-off for a square foot is the amount of water caught by the graduate, and is represented in cubic inches per square foot. For obtaining results which express slope alone, comparisons must be made upon the same soil, from which all cover, dead and living, has been removed. They must be as closely together in time as possible, at least during the same day, as rain or evaporation will cause considerable error. It is obvious that with the same slope or on a level the trechometer may also be used to advantage to determine the absorptive power of soils of different texture. It serves well a similar purpose when used in different habitats to measure the composite action of slope, soil, and cover in dividing the rainfall into run-off and absorbed water.

_Exposure_

=124. Exposure= refers primarily to the direction toward which a slope faces, i. e., its exposition or insolation with respect to sun and wind. It is not altogether separable from slope, however, inasmuch as the angle of the slope has some effect upon the degree of exposure. The chief influence of exposure is exerted through temperature, since slopes longest exposed to the sun’s rays receive the most heat. This is supplemented in an important degree by the fact that a group of rays 1 foot square will occupy this area only on slopes upon which they fall at right angles. In all other cases the rays are spread over a longer area, with a consequent reduction in the amount of heat received. This effect is felt principally in evaporation from the soil, and in soil temperatures. For the leaf, it is largely if not entirely negligible, since the angle of incidence is determined by the position of the leaf, which is the same for each species whether on the level or upon a slope. On this account, exposure has little or no bearing upon light, except that the total amount of light received by the aggregate vegetation of a slope will be greater than for a level area of the same size. The effect of wind varies with the exposure. It is naturally most pronounced in those directions from which the prevailing dry or cold winds blow, and it is greatly emphasized by the fact that the opposite exposure is correspondingly protected. The influence of wind, especially in producing evaporation from the plant and the soil, increases with the slope, since the mutual protection of the plants, or that afforded the soil by the cover, is much reduced. Finally, the distribution of the snow by the wind, a matter of considerable importance for early spring vegetation, is largely determined by exposure.

Exposure is expressed directly in terms of direction, to which is added the angle of the slope. A good field compass, reading to twelve points, is sufficient. It should be checked, of course, by the declination of the needle at the place under observation. A convenient instrument is the one already mentioned, in which compass and clinometer are combined, since these are regularly used at the same time.

=125. Surface.= The most important consideration with respect to surface is the presence or absence of cover, and the character of the latter. With the exception of snow, cover is, however, a question of vegetation, living and dead, and consequently is to be referred to the discussion of biotic factors. The surface of the soil itself often shows irregularities which must be taken into account. Such are the rocks of boulder and rock fields, the hummocks of meadows and bogs, the mounds of prairie dog towns, the innumerable minute gullies and ridges of bad lands, the raised tufts of sand-hills, etc. The influence of these is not profound, but they do have an appreciable effect upon the run-off, temperature, and wind. In many cases, this is distinctly measurable, but as a rule little more can be done than to indicate that the surface is even or uneven, and to describe the degree and kind of unevenness.

=126. Record of physiographic factors.= Altitude, slope, exposure, and surface are essentially constant factors, and are determined once for all, after a few check readings have been made, except in those relatively rare habitats in which dynamic forces are very active. The form of record used is the following:

═══════╤═════════╤════════╤════════════╤════════╤═════╤════════╤═══════
DATE │FORMATION│STATION │ GROUP │ALTITUDE│SLOPE│EXPOSURE│SURFACE
───────┼─────────┼────────┼────────────┼────────┼─────┼────────┼───────
10/7/02│Gravel │Golf │Eriogonare │2700 m. │ 23°│ N.N.W. │Even
│slide │Links │ │ │ │ │
„ │Brook │Jack │Violare │2550 m. │ 5°│ E.N.E. │ „
│bank │Brook │ │ │ │ │
„ │Half │Hiawatha│Achilleare │2600 m. │ 14°│ E. │Uneven
│gravel │ │ │ │ │ │
„ │Spruce │Milky │Opulasterare│2625 m. │ 12°│ N. │Even
│ │Way │ │ │ │ │
───────┴─────────┴────────┴────────────┴────────┴─────┴────────┴───────

=127. Topography.= As heretofore indicated, questions pertaining to the form and development of the land concern groups of habitats within which each habitat is the unit of investigation after the manner already laid down. A knowledge of topography is essential to the accurate mapping of a region, for which the simple methods of plane table and contour work are employed, while the geology of the surface is of primary importance in the study of successions.

BIOTIC FACTORS

=128. Influence and importance.= Biotic factors are animals and plants. With respect to influence they are usually remote, rarely direct. Nevertheless, they often play a decisive part in the vegetation. Their effect is, as a rule, felt directly by the formation rather than the habitat, but in either case the one reacts upon the other. Such factors are not themselves susceptible of exact measurement, but their influence upon the habitat is usually measurable in terms of the physical factors affected. In the case of biotic factors, it must be distinctly understood that these are not properly factors of the habitat as a physical complex, but that they are rather to be considered as reactions exerted by the effect, or formation, upon the cause or habitat. This is most especially true of plants.

=129. Animals.= The activities of man fall into two classes: (1) those that destroy vegetation, and (2) those that modify it. There are rare instances also where the work of man has changed a new or already denuded habitat. In the cases where the vegetation is destroyed, the habitat itself is sufficiently changed to permit the effect to be measured by physical factor instruments. Otherwise, the influence is felt only by the formation, as when man makes possible the migration of weeds, and it can be measured in terms of invasion by the quadrat alone. It becomes especially evident, then, in the case of man’s activities, that where they produce a denuded habitat they are to be regarded as factors in the habitat; when they merely affect the formation, this is not strictly true. The changes wrought by other animals are essentially the same as those produced by man. They are not so marked nor so important, but their relation to habitat and formation is the same. As a rule, however, they affect the habitat much less than they do the formation.

=130. Plants.= As a dead cover, vegetation is a factor of the habitat proper, but it has relatively little importance, since it occurs regularly during the resting period. Its chief effects are in modifying soil temperature, and in holding snow and rain, and thereby increasing the water-content. By its gradual decay, moreover, it not only adds humus to the soil, but it thereby increases the water-retaining capacity of the latter also. The cover of living vegetation reacts upon the habitat in a much more vital fashion, exerting a powerful effect upon every physical factor of the habitat. The factors thus affected are distinctly measurable though it is often impossible to determine just how much of the factor is directly traceable to the vegetation. This is a simple problem in the case of most aerial factors, especially light, but it is extremely difficult for soil factors, such as water-content and soil texture. In the case of all habitats covered with formations, by far the great majority, it is impossible as well as unnecessary to separate the physical factors of the habitat proper from the reaction upon them which the plant covering exerts. Indeed, the great differentiation of habitats is largely due to the universal principle that in vegetation the effect or formation always reacts upon the cause or habitat in such a way as to modify it. As fundamental causes of succession, the discussion of the various reactions of vegetation is reserved for another place.

METHODS OF HABITAT INVESTIGATION

=131.= The use of the various instruments previously described depends largely upon the preponderance of simple instruments or recording ones. The former necessitate a number of well-trained assistants; the latter require only a part of the time of one investigator. For the most satisfactory results, however, an assistant is all but indispensable. Since simple instruments are most easily obtained because of their cheapness, and are especially adapted to purposes of instruction, the method of using them will be described first, and then that of ecograph batteries.

THE METHOD OF SIMPLE INSTRUMENTS

=132. Choice of stations.= This method is based upon simultaneous readings by means of simple instruments in a series of habitats, or of stations in a single habitat. Such readings are necessary for the variable atmospheric factors, humidity, light, temperature, and wind. Frequent readings suffice for water-content and precipitation, while only two or three determinations, enough to check out the error, are necessary for the constant factors, altitude, slope, exposure, and surface. An account of the exact procedure employed in class study at Lincoln and Minnehaha will best serve to illustrate the use of this method. The series of stations chosen at Lincoln were primarily within a single formation, for the purpose of determining the physical factor variation in different areas. One series was located in the prairie-grass formation (_Koelera-Andropogon-psilium_), and consisted of the following stations: (1) low prairie, (2) crest of ridge I, (3) northeast slope of ridge I, (4) grassy ravine, (5) southwest slope of ridge II, (6) bare crest of ridge II, (7) thicket ravine. The other series was established in the bur-oak-hickory forest (_Quercus-Hicoria-hylium_) at the following stations: (1) thicket, (2) woodland, (3) knoll in forest, (4) depression in forest, (5) level forest floor, (6) nettle thicket, (7) brook bank. At Minnehaha the series was primarily one of different formations: (1) the pine formation (_Pinus-xerohylium_), (2) the gravel slide formation (_Pseudocymopterus-Mentzelia-chalicium_), (3) east slope of spruce forest (_Picea-Pseudotsuga-hylium_), (4) ridge in the spruce forest, (5) north slope of spruce forest, (6) brook bank in forest, (7) the thicket formation (_Quercus-Cercocarpus-lochmodium_), (8) the aspen formation (_Populus-hylium_). When permanent or temporary quadrats are established, they are ordinarily used as regular stations, since this enables one to refer the physical factor readings to a few definite individual plants, as well as to the entire formation. The transects in figure 29 illustrate two of the above series of stations.

=133. Time of readings.= The frequency of simple readings and the times at which they are made must be regulated largely by opportunity and convenience. In addition to making readings once or twice a week throughout the season, the series should be read at least once every day for a representative week or two. It is also very desirable to have a series for each hour of a typical day, or of two days, one of which is clear, the other cloudy. When a single daily reading is made, it should be taken at or as near meridian as possible. The usual series is the one obtained by simultaneous observations at the same level in different stations. An important series is also secured by simultaneous readings at the various levels of the same station, though it is not necessary to take this series frequently.

=134. Details of the method.= After the stations have been selected by a careful preliminary survey of the habitat or series of habitats, their location is indicated by a small flag bearing a number, in case there is no danger of these being disturbed. Otherwise, less conspicuous stakes are used. The ordinary practice is to visit each station of the series, and to take readings of water-content, altitude, slope, and exposure. On the first trip these are all made by the instructor, but after a short time the determination of each factor may be assigned in rotation to each of the students. After these constant factors have been read and recorded, one student is equipped with photometer, thermometer, and psychrometer, and, if desirable, anemometer, and left at the first station. At each succeeding station the same plan is followed, so that at the end of the series the constant factors have all been read, and there is an observer at each station prepared to make readings of the variable ones. The task of acquainting the students with the operation of photometer, psychrometer, etc., can best be done in class or at a previous field period, as it is evident that they must be familiar with the instruments before they can use them accurately in the field series. The details of operation have already been given and need not be repeated here. The task of obtaining readings at the same moment may be met by supplying each observer with a watch, which runs exactly with all the others, or by making observations upon signal. The second means has been found most successful in practice, since the signal fixes the attention at the exact moment. The best plan is for the instructor to occupy a commanding position somewhere near the middle of the series, and to give the signals by shout or whistle at the proper interval. Considerable care and experience are necessary to do the last satisfactorily. Sufficient time must be given for the operation of the instrument and the making of the record. In addition, a period must be permitted to elapse which is long enough for every instrument to reach the proper reading. For example, in a series which contains a gravel slide and a forest, the thermometer which has just been used for an air reading will require four or five times as long an interval to respond to the temperature of the gravel as to that of the cool forest floor. In such series, the instructor should regularly take his place in the station where the response is slowest or greatest. He must record the exact time of each signal, and note any general changes of sky or wind that produce temporary fluctuations at the time of reading. When the readings extend over a whole day, the usual plan is to begin at the last station and take a second series of water-content samples, noting the exact time in order that the rate of water loss may be determined. A check series of physiographic factors may be made at this time also, or this may be left for future visits. While it is unnecessary to take soil samples oftener than once a day, it is important to make at least one series at each visit. Sometimes it becomes desirable to know the rate of water loss in different stations during the day, and in this event, samples are taken at one or two hour intervals for the entire day.

In making simultaneous readings at the different levels of one station, the observers are grouped in one spot in such a way that they do not interfere with the correct reading of each instrument. Readings of this sort are most valuable in the case of temperature, which shows greater differences at the various levels. Important differences of humidity and wind also are readily obtained, and, in layered formations, marked variations in the amount of light. In the open, the ordinary levels for temperature are 6 feet, 3 feet, surface, 5, 10, and 15 inches in the ground, and for wind and humidity, 6 feet, 3 feet, and surface. In forests the same levels are used for comparison with formations in the open, but a more desirable series for light especially is secured by making readings at the height of, or better, just below the various layers. Series of this sort are likewise made on signal. The best time of day is that of a period in which the middle station is read near meridian, since the variation due to time is sufficiently small to permit fairly accurate comparisons between the readings for the different stations.

=135. Records.= The form used for recording the observations made by means of simple instruments is shown below. It is hardly necessary to state that it may be readily modified to suit the needs of different investigators. Ordinarily, each sheet is used for the records of one habitat or series alone, but for convenience sake, the records of two different series are here combined. The figures given are taken from records for the prairie and forest formations at Lincoln.

_Koelera-Andropogon-psilium_

April 25, 1901. Clear. South wind.

════╤═════════════════════════════╤═══════════╤═══════════╤══════════════╤═══════════ │ TEMPERATURE │ LIGHT │ HUMIDITY │WATER-CONTENT │ WIND ────┼─────┬─────┬─────┬─────┬─────┼─────┬─────┼─────┬─────┼──────────────┼─────┬───── TIME│3:20 │3:24 │3:30 │3:35 │3:40 │ │ │3:45 │3:55 │ % │2:40 │3:05 P.M.│ │ │ │ │ │ │ │ │ │ │ │ ────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼────┬────┬────┼─────┼───── STA-│1½ m.│Surf.│ 5 │ 10 │ 15 │1½ m.│Surf.│1½ m.│Surf.│ 5 │ 10 │ 15 │1½ m.│Surf. TION│ │ │ │ │ │ │ │ │ │ │ │ │ │ ────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼────┼────┼────┼─────┼───── 1. │27.8 │29.6 │17.8 │15.8 │10.9 │ │ │ 57 │ 59 │17.7│14.6│17.4│ 740│ 280 2. │26.5 │31.3 │18.3 │16.6 │12.8 │ │ │ 59 │ 59 │17.9│12.2│14.3│ 1100│ 510 3. │26.9 │28.5 │18.2 │14.2 │13.5 │ │ │ 58 │ 59 │16.5│16.9│19 │ 980│ 520 4. │26.2 │30 │16.8 │13.2 │11.6 │ │ │ 63 │ 66 │24.4│21.3│24.8│ 920│ 460 5. │28 │32.4 │18.6 │14.6 │14.2 │ │ │ 59 │ 60 │10.7│17 │17.2│ 1080│ 490 6. │28 │40.8 │23.8 │16 │15 │ │ │ 51 │ 51 │ 5 │8.3 │10.3│ 1010│ 410 7. │26.4 │30.8 │16 │13 │11 │ │ │ 68 │ 70 │27 │24.3│21.4│ 680│ 52 ────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴────┴────┴────┴─────┴─────

_Quercus-Hicoria-hylium_

April 20, 1901. Clear. Southeast wind.

════╤═════════════════════════════╤═══════════╤═══════════╤══════════════╤═══════════ │ TEMPERATURE │ LIGHT │ HUMIDITY │WATER-CONTENT │ WIND ────┼─────┬─────┬─────┬─────┬─────┼─────┬─────┼─────┬─────┼──────────────┼─────┬───── TIME│10:40│10:46│10:50│10:55│11:00│12:00│12:05│11:10│11:20│ % │11:30│11:45 A.M.│ │ │ │ │ │ │ │ │ │ │ │ ────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼────┬────┬────┼─────┼───── STA-│1½ m.│Surf.│ 5 │ 10 │ 15 │1½ m.│Surf.│1½ m.│Surf.│ 5 │ 10 │ 15 │1½ m.│Surf. TION│ │ │ │ │ │ │ │ │ │ │ │ │ │ ────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼────┼────┼────┼─────┼───── 1. │16 │25 │ 9.6│8.4 │7.8 │ .08 │ .06 │ 73 │ 81 │24.2│19.2│19.5│ 298│ 0 2. │16.2 │30.5 │ 8.5│8.4 │7.8 │ .11 │ .09 │ 73 │ 86 │22 │22.5│19.4│ 375│ 2 3. │16.2 │17.8 │ 7.6│7.8 │8 │ .08 │ .06 │ 73 │ 95 │22.1│20.4│21.6│ 640│ 6 4. │15.6 │26.2 │ 10.6│8.4 │8.2 │ .06 │ .03 │ 81 │ 95 │25.4│23.1│22.4│ 275│ 12 5. │17.6 │25.4 │ 7.6│7.4 │7.2 │ .03 │ .02 │ 90 │ 95 │27.2│19.8│18.8│ 178│ 2 6. │16.2 │20.2 │ 8.4│7 │6.2 │ .02 │ .01 │ 82 │ 90 │27.6│20.8│18.8│ 115│ 4 7. │15.8 │17.2 │ 6.4│6.4 │6.1 │ .05 │ .04 │ 82 │ 90 │23.8│19 │19.3│ 60│ 0 ────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴────┴────┴────┴─────┴─────

METHOD OF ECOGRAPH BATTERIES

=136.= A battery of recording instruments consists of a selagraph, a psychrograph, and a thermograph, to which an anemograph is added when possible. As stated before, the determination of water-content by the geotome method is more satisfactory than by any automatic instrument yet devised. When the base station is located where the sunlight is unobstructed, which should be the case whenever possible, it is unnecessary to include a selagraph in those batteries placed in similarly exposed stations, since the light values will be the same. As a rule, batteries are established within different zones or different habitats, except where a highly diversified habitat is made the subject of special inquiry. Such a restriction arises from the fact that expense, care of operation, etc., place a limit upon the number of batteries, and, in such case, the task of primary importance is to establish the physical character of representative habitats. For these reasons, the first series of thermographs established in 1903 was located with respect to altitude, the instruments being placed at Manitou 2,000 m., Minnehaha, 2,600 m., and Mount Garfield 3,800 m. In 1904, the stations established for the record of temperature and humidity were situated with respect to habitats representing the four formations: gravel slide, half gravel slide, spruce forest, and brook bank.

The batteries are located and set up according to the directions already given. A 2–meter quadrat with the battery as the middle is staked and mapped. Within this, all readings of water-content, soil temperature, and physiographic factors are made. Altitude, slope, exposure, and cover are recorded when each battery is located, and a soil sample is taken for mechanical analysis. When the position of the batteries permits it, water-content readings should be made frequently, once or twice a week at least. In addition, a complete series of samples should be taken daily for a period sufficient to indicate the ordinary extremes of water-content.

The ecograph battery of each habitat constitutes a standard to which the results obtained by simple instruments may be referred with accuracy. It not only does this, but it also serves as a basis for interpreting the readings of simple instruments in distant habitats of the same character. In this way a few batteries judiciously placed make possible the exact physical investigation of a large number of habitats, covering a considerable area. The only limit, indeed, upon this method is that placed by time. The proportionate use of batteries and of simple instruments must be largely determined by the conditions which confront the investigator. It is obvious that, where expense is not a decisive factor, the gain in time and in completeness of results is enormously in favor of the battery. There is an additional value in the automatic and continuous record which can not be overlooked. When the use of instruments in the study of habitat and formation becomes universal, the importance of the ecograph will be immeasurably enhanced. It will be possible to secure duplicate records of batteries located in the most remote and diverse regions, from the equator to the poles, and comparative phytogeography upon a scientific basis will for the first time be possible. This opens an alluring vista of the future when ecologists the world over will cooperate in such a way that the results obtained by ecograph batteries anywhere on the globe will permit of exact comparison.

THE EXPRESSION OF PHYSICAL FACTOR RESULTS

=137. The form of results.= It is almost inevitable that the general adoption of precise methods of measuring the habitat will result in a common form for expressing the physical character of the latter. An actual diagnosis of each habitat is not a difficult matter, after the factors are carefully measured, and will unquestionably lead to very desirable definiteness and precision. The accurate investigation of the physical factors of a number of habitats for one growing season furnishes the necessary material for a diagnosis based upon the mean for the growing season. Similar results for two or three seasons will yield a diagnosis as accurate and as final as that of a formation, or, indeed, as that of many species. The author’s investigations have not yet gone far enough to warrant proposing a final form for this, but the following diagnosis is offered as a suggestion:

_Elymus-Muhlenbergia-chalicium._ Habitat: holard 9 per cent, chresard 8 per cent, relative humidity 40 per cent, light 0.6, soil colluvial gravel (gravel 70 per cent, sand 27 per cent, silt 3 per cent), air temperature 65°, surface 82°, soil 59°, wind 10 miles, rainfall 8 inches, altitude 2,800 m., slope 23°, exposure south, surface even, cover open, no active biotic agencies.

The detailed comparison of habitats is made most readily by the graphic method of curves, which constitute the most desirable form of expression in connection with the original record upon which they are based. Factor means are particularly desirable for diagnostic purposes, and they furnish valuable curves also. Factor sums are impracticable at present, and it seems doubtful that they will ever be of much value. It is by no means impossible, however, that a more detailed and exact knowledge of the physiology of adaptation, coupled with methods of precision in the habitat, will render them necessary.

_Factor Records_

=138.= Experience has shown that the practice of making hasty and often formless records in the field is unwise and is apt to be inaccurate as well. The time saved in the field is more than counterbalanced by that consumed in copying the results into the permanent form. The danger of error in field notes rapidly taken is very grave, and the chance of confusion and the waste of time in deciphering them are great. Moreover, the task of checking a copy with the original, which is absolutely necessary for accuracy, involves a further expenditure of time and energy. For these reasons the field record should be made in permanent form. Definite record sheets are used, and the invariable rule is made that all readings are to be noted in ink at the time and spot where they are taken. On a long journey, or in the face of many observations, the tendency to take notes or to record observations rapidly is very great, but this will correct itself after a few attempts to use such notes. The record forms for various factors have been indicated in the proper place, as well as the one for simultaneous readings. Ecograph sheets are carefully filed, and constitute permanent records. With a little practice they may be read almost as easily as tables, and any attempt to put them into tabular form is a mere waste of time. For purposes of study and of publication, it often becomes necessary to bring together all the results obtained for a particular habitat, both by simple instruments and by ecographs. The form of record used for this is essentially that already indicated for simultaneous readings on page 92, since general features and constant factors can not well be included in the table. Record sheets of this type have been printed at a cost of $5 per thousand, and the various factor records can be obtained at about the same rate. The size of sheet used is 9½ × 7¾ inches. The record book is the usual notebook cover, which has been found neither too large nor too small. It is protected from dirt and rain by a covering of oilcloth which overlaps the edges. Record books should be carefully labeled, and each one should contain a single year’s records.

_Factor Curves_

=139. Plotting.= The paper employed is divided into centimeter squares which are subdivided into 2–millimeter units. For ordinary curves the size of sheet is 9½ × 7¾ inches, which makes it possible for curve sheets to be filed in the record book. Tablets containing 60 of these sheets can be obtained for 20 cents each from the Central School Supply House, Chicago. For curves longer than 9 inches special sizes of sheets must be used. On account of their inconvenience large sheets are avoided whenever possible. This can usually be accomplished by increasing the numerical value of the intervals. The inks employed in plotting are the waterproof inks of Chas. Higgins & Co., Brooklyn, New York. These are made in ten or more colors, black, violet, indigo, blue, green, yellow, orange, brown, brick red, carmine, and scarlet, and cost 25 cents per bottle. In addition to being waterproof, they make it possible to combine curves in all conceivable ways without destroying their identity. Furthermore, it is a great advantage to use the same color invariably for the same kind of curve: thus, it has been the practice to indicate the 3–foot, surface, 5, 10, and 15–inch temperature curves by violet, green, yellow, blue, and carmine respectively. A fine-pointed pen, such as the Spencerian No. 1, is most satisfactory for inking; drawing pens, such as Gillott’s Crowquill, are too finely pointed for ordinary use.

In plotting a curve, it is first necessary to determine the value of the interval, and the extreme range of the curve or combination. For example, in the case of temperature, it is most convenient to assign a value of 1° Centigrade to each centimeter, since the thermometers used read to one-fifth of a degree, which corresponds exactly to the 2–millimeter units of each square. The length of the sheet permits a range of 22 degrees Centigrade, and the actual limits must be determined for the particular results to be employed. For the same region, it is very desirable that the unit interval and the range be the same, in order that all curve sheets may admit of direct comparison. Indeed, it is greatly to be hoped that in the future ecologists will agree to a uniform system of curve-plotting, cartography, etc., as the geographers are beginning to do in the construction of maps. The major intervals are written, or, better, typewritten, at both sides of the sheet, and the time or space intervals are indicated at the top. Each curve sheet is properly labeled, and essential data indicated. The readings are taken from the field record, and their proper positions indicated by a dot. These are connected first by a pencil line, the curves being made abrupt rather than flowing; and the line, after having been carefully checked, is traced in ink.

=140. Kinds of curves.= Curves are named both with reference to the factor concerned and the position or sequence of the readings. The factors which lend themselves most readily to this method of representation are the variable ones, water-content, humidity, light, temperature, and wind, and corresponding curves are distinguished. Altitude and slope may likewise be shown by means of curves, but the use of cross section or contour lines serves the same purpose and is more natural. With regard to time and position, curves are distinguished as level, station, and point curves. A level curve is one based upon readings made at the same level through a series of stations or of habitats, e. g., the level curve of surface temperature. The station curve represents the various levels or points at which readings are made in a single station. The point curve has for a basis the hourly or daily variation of a factor at a particular point or level in a station. All of these may be simple curves, when established upon a single reading for a series, or mean curves when they are based upon the mean of a number of readings. Curves which show the extremes of a factor, i. e., the maximum and minimum, are also extremely valuable, though a combination of the two for comparison is preferable.

=141. Combinations of curves= are invaluable for bringing similar curves together, and permitting ready comparison of them. For this, and also because they save space, they are regularly employed to the almost complete exclusion of single curves. Combinations are made simply by tracing the curves to be compared upon the same sheet, it being understood that dissimilar curves, e. g., level and station, can not be combined. Colored inks are an absolute necessity in combining; the primary principle underlying their use is that curves that approach closely or cross each other must be traced in inks that contrast sharply. As elsewhere stated, it has been made the invariable rule to use the same color for the same level or point. This applies especially to temperature, but holds also for humidity, light, wind, and water-content, so that the color always indicates the level. For the same reason, it is applied to a combination of point curves for one station, though it is inapplicable to a series of point curves when these lie in the same level. Light readings above 6 feet and water-content readings below 15 inches necessitate the use of additional colors.

Combinations may be made of the curves of a single factor for purposes of comparison, or they may consist of curves of different factors in order to aid in interpreting or indicating their relation to each other. Curves of the same factor may be combined to form various series. The level series consists of all the level curves for the stations under observation, e. g., the six levels for temperature, three levels for wind, etc. Similarly, the station series is a combination of all the station curves, and a corresponding arrangement may be made for point curves with reference either to station or to level. An extremely valuable combination of curves is that of the holard and chresard for a series of stations. The most important combinations of the curves of different factors are naturally those based upon factors intimately related to each other or to the plant. The grouping of water-content and humidity curves is of great value, especially when the transpiration curve is added. Light and temperature curves make an interesting combination, while a humidity, temperature, and wind series is of much aid in tracing the connection between these factors. Finally, it is altogether feasible to arrange the curves of water-content, humidity, light, temperature, and wind upon the same sheet in such fashion as to give a graphic representation of the whole physical nature of a single habitat or a series. In all combinations of curves representing different factors, it must be borne in mind that the position of a curve does not represent a definite value with reference to the others, since some are based upon per cents, others upon degrees, etc. The comparison must be based upon the character of the curves, but even then it is an important aid. An instructive grouping has been employed where series of readings on the same day, or on two successive days in forest and in prairie have yielded the usual level series of curves. The series for the two habitats are arranged on the same page, one at the right and the other at the left, and permit direct comparison of corresponding level or factor curves, both with respect to position and character.

=142. The amplitude of all the curves= described above is determined by the unit values of the factors concerned, while the length is dependent upon the number of stations, points, or times. The value assigned the latter upon the plotting paper is purely arbitrary, but it is most convenient to fix this at the centimeter square. The unit value for temperature is 1° Centigrade per square, each subdivision of the latter representing 0.2, and the range being 22 degrees. For water-content curves, each square represents a value of 2 per cent, the smaller square being 0.4 per cent, and the range 2–48 per cent. The unit value for humidity is taken as 5 per cent, making each small square 1 per cent, and giving room on the sheet for the entire range from 1–100 per cent. Owing to the anemometer used, curves of wind velocity have been based upon the number of feet per minute. One hundred feet is taken as the unit value, and the range is from 0–2200 feet. The unit value for the curve of light intensity is .005. Each small square is .001, which permits a range from .001 to .01 on one sheet. Consequently, when it is desired to plot the curve of a series of habitats with a range in intensity greater than this it is necessary to use a double sheet. This is the usual device when the range of curves is too great, except where the excess is slight. In this case the curve is left open at the top, and the value which the crest attains is indicated. All curves in combination are labeled at the beginning or left to indicate the level, station, or point, and at the end or right to show the time, or day, if this is not the basis of the curve or series.

The discussion that precedes deals exclusively with curves representing factors determined in the field. It applies with equal force to results obtained by instruments in control houses. In these, however, all factors except those directly experimented with, usually water-content and light, are practically equalized, and the curves based upon them are used chiefly to show how nearly equal they have become. The important curves are those of the water-content series, both holard and chresard, and of the shade tents. Where several houses are differentiated with respect to temperature or humidity, curve series of both these factors are necessary.

_Factor Means and Sums_

=143.= It has been shown elsewhere that the daily mean of temperature can be closely approximated from the maximum and minimum of both day and night. Maximum-minimum instruments for the other factors are lacking, however, and for light, humidity, and wind these values can only be obtained from the ecograph which makes it possible to get the exact mean from the sum of all the hour readings. When it comes to the seasonal mean, the ecograph is even more necessary, exception being made for water-content, in which case a number of readings on various days through the season will suffice. The value of factor means for diagnosis and for curves has already been sufficiently commented upon, and the feasibility of factor sums already indicated.