BIOTIC FACTORS IN RELATION TO INDIVIDUALS 



231 



March. During parts of the last two months 

 in 1924, air movement 2 meters from the 

 forest floor on the windward side of a large 

 tree measured approximately 1 mile per day 

 (twenty-four hours). Another similar re- 

 cording anemometer, placed in the forest 

 canopy 75 feet overhead, recorded an 

 average movement of 10 miles per day for 

 the same period. Overhead, over the tops 

 of the highest trees, the wind was blowing 

 some 240 miles per day. The rain forest in 

 this instance reduced air flow to approxi- 

 mately 0.4 per cent of the unobstructed 

 rate (Alice, 1926); the air movement in the 

 so-called insect climate, within a few milli- 

 meters of the ground, must have been re- 

 duced still further. 



Systematic observations of wind veloc- 

 ities in a low cover of vegetation are scarce. 

 Geiger (1927) summarizes evidence that 

 the gradient of wind velocity above culti- 

 vated fields is the same as that above bare 

 ground and can be expressed by the equa- 

 tion: 



y = Vi • h" 



In this expression, v gives the wind velocity 

 at h meters above the ground in meters per 

 second; Ui signifies the wind velocity at an 

 elevation of 1 meter, and a is a coefficient 

 that varies with changing conditions. In 

 data collected on level ground near Pots- 

 dam, a had a value of approximately 0.3. 



The surface of vegetation acts as does 

 the surface of the ground when we define 

 the former as the level at which resistance 

 to the wind reduces its velocity so as to 

 approach zero. In dense grass, or other 

 types of plant cover, there is an almost com- 

 plete calm at the ground level, like that of 

 the lowest level of the tropical rain forest. 

 In a wind-blown field of wheat only the 

 heads are directly moved by the wind; the 

 stalks swing mechanically after them. This 

 effect has been measured among heather 

 where, on a windy, sunny day at a height 

 of 2 centimeters, the air movement was less 

 than 0.008 meter per sec; among the tops 

 of the heather at 40 cm., it was 1.7 meters 

 per sec, and above the heather at 180 cm., 

 the air was moving at a rate of 5.1 meters 

 per sec. The reduction to 0.15 per cent of 

 the upper velocity is of nearly the same 

 order of magnitude as that measured on a 

 much larger vertical gradient in the tropical 

 rain forest. The velocity of winds is reduced 

 through a considerable height above the 



vegetation cover. Rooted plants act on the 

 same principle in reducing the rate of flow 

 of water currents. 



The calm produced within the plant layei 

 is a feature of great importance in the mic- 

 roclimate, not only as regards animals in 

 the given habitat, but also in relation to 

 the activities of plants themselves. The dif- 

 ference between the plant-produced micro- 

 climate and the general cHmate becomes 

 more important as the latter becomes less 

 favorable. Temperature and humidity are 

 affected as well as air movement. The vege- 

 tation of arctic and of alpine regions is able 

 to utilize solar radiation to produce a micro- 

 chmate suitable for low-growing plants and 

 for many small animals, particularly insects, 

 as a result of the calm maintained even in 

 such vegetation. Documentation and fur- 

 ther discussion of these points are furnished 

 by Geiger (1927). 



Light 



The modification of Hght by the plant 

 matrix is obvious in the contrast between 

 open terrain and forest floor, and is directly 

 correlated with the temperature relations of 

 the same situations. Light values in various 

 biotically modified situations are as follows: 



In the Panama rain forest, a corrected 

 series of readings by Alice (1926) indicates 

 that if a mean light intensity of 18.4 foot 

 candles in shade on the forest floor is taken 

 as representing an index figure of J, the 

 index for the forest half way between floor 

 and canopy is 5, and for the shade in the 

 upper forest canopy 25, at times when the 

 index for full sunlight is over 500. Similar 

 effects of forest cover are illustrated by 

 Figure 61, and by changes in light intensity 

 resulting from forest succession in the 

 Chicago region (Orlando Park, 1931). 



Similar shading is a universal result of 

 plant cover. The grasses and needle-leafed 

 conifers are not nearly so efficient as shade 

 producers as are broad-leaved plants. Meas 

 urements by Angstrom (1925) show that 

 in a good stand of mixed timothy and or- 

 chard grass approximately a meter high, the 

 intensity of incident light is scarcely affected 

 in the upper half of the erect grass stems. 

 Below that point the intensity falls rapidly 

 until only a quarter of the whole penetrates 

 to 10 cm., above the ground, and only a 

 fifth part reaches the base of the plants. 

 With broad-leaved plants in a similarly 

 dense stand, apart from sun flecks, only one- 



