4i6 



CONSERVATION 



off or sink into the ground and will 

 come under those heads. It does not 

 appear that forests influence the amount 

 or distribution of snowfall. 



Forests ill relation to ground rua- 

 ter. — Inasmuch as ground water and 

 soil moisture are wholly derived from 

 precipitation, any effect that forests 

 may have in regulating rain extends to 

 them. As compared with bare ground 

 which receives all the rain and snow 

 that fall, the ground beneath trees gets 

 only a part, as they catch some propor- 

 tion in their foliage. The percentage 

 caught varies from twenty to forty per 

 cent, of the precipitation measured 

 in adjoining fields, according to the 

 kind of trees, the density of stand, and 

 other factors. Thus, if the conditions 

 for percolation are the same vmder the 

 trees and in the open the earth under 

 the forest receives less than that under 

 the bare surface. An approximation to 

 this assumption is reached on level 

 plains, provided the soil in the open be 

 in an excellent state of tilth. 



That proportion of the rain and snow 

 which reaches the ground under the 

 trees wets the mulch of mosses, leaves 

 and decaying wood, and such part as 

 is not thus absorbed seeps into the 

 ground below. Only a small part es- 

 capes as run-off, unless the slopes be 

 very steep. The mulch of decaying 

 vegetable matter is extremely absorbent 

 and will hold as much as fifty per cent, 

 of its own volume, or two to three times 

 its weight of water. From extended 

 experiments in Germany with straw, 

 leaves, and humus it appears that the 

 moisture thus caught is evaporated to 

 the air in dry seasons, and it would 

 therefore be uneconomical from the 

 point of view of ground-water storage 

 to increase the surface litter beyond 

 that required to prevent run-off. Un- 

 der some conditions of forest mainte- 

 nance it would probably be economical 

 to keep the surface comparatively clean 

 of litter and in an open state, such that 

 the rain sifting gently through the 

 foliage would be absorbed by the 

 ground in a maximum degree. 



The subsoil beneath the forest near 

 the surface is moister than that beneath 

 the adjoining fields. This is due chief- 

 ly to the covering of moist surface litter 

 and to the relatively small evaporation 

 at the ground surface. Deeper in the 

 subsoil, at depths below a foot, the for- 

 est soil is often drier than that below 

 the adjoining fields, for the tree roots 

 take a large amount of water which is 

 given off through the foliage. Thus, if 

 we contrast the soil condition under 

 the forest with that under the bare field 

 we shall find the ground beneath the 

 forest moister near the surface and 

 drier below. The continuity of water 

 films in the ground is a condition wdiich 

 promotes percolation, and the relatively 

 moist surface layer thus aids the falling 

 drops in reaching the deeper, drier lay- 

 ers when precipitation occurs. In the 

 open, wdien the surface layer becomes 

 measurably dried out and filled with 

 air, a sudden rain may so occupy the 

 pores of the immediate surface as to 

 seal them against the escape of the un- 

 derlying air, and the latter then pre- 

 vents the percolation of water, and the 

 impervious surface becomes one on 

 which run-off is inevitable. 



The total effect of trees in preventing 

 some proportion of the rain from reach- 

 ing the ground and in using some part 

 of the moisture contained in the ground 

 in their physiological processes is in- 

 dicated by the depths to which they de- 

 press the level of ground water as com- 

 pared with that in adjacent fields. Ob- 

 servations on this point have been made 

 in Russia, France, and Germany, and 

 the results are in agreement for Euro- 

 pean conditions. They were made on 

 level ground, and the conclusions do 

 not apply where run-off from the sur- 

 face is copious. It is shown that — ^ 



(a) The water table lies deeper be- 

 low the surface under a forest than out- 

 side of it, if account be taken of the 

 relief; 



(b) The depression of the water ta- 

 ble is more pronounced under old for- 

 ests than under young plantations ; 



(c) The depression is more pro- 



'Henry, E. : Les Forets de Plaine, Les Eaux et Forets, vol. 42, 1903, p. 161. 



