Fig. 27. Mountain barometer: (a) in carrying case; (b) set up for use.
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.
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.
Fig. 28. Combined clinometer and compass.
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.