416 CONSERVATION 



off or sink into the ground and will The subsoil beneath the forest near 



come under those heads. It does not the surface is moister than that beneath 



appear that forests influence the amount the adjoining fields. This is due chief- 



or distribution of snowfall. ly to the covering of moist surface litter 



Forests in relation to ground tc'a- and to the relatively small evaporation 

 ter. Inasmuch as ground water and at the ground surface. Deeper in the 

 soil moisture are wholly derived from subsoil, at depths below a foot, the for- 

 precipitation, any effect that forests est soil is often drier than that below 

 may have in regulating rain extends to the adjoining fields, for the tree roots 

 them. As compared with bare ground take a large amount of water which is 

 which receives all the rain and snow given off through the foliage. Thus, if 

 that fall, the ground beneath trees gets we contrast the soil condition under 

 only a part, as they catch some propor- the forest with that under the bare field 

 tion in their foliage. The percentage we shall find the ground beneath the 

 caught varies from twenty to forty per forest moister near the surface and 

 cent, of the precipitation measured drier below. The continuity of water 

 in adjoining fields, according to the films in the ground is a condition which 

 kind of trees, the density of stand, and promotes percolation, and the relatively 

 other factors. Thus, if the conditions moist surface layer thus aids the falling 

 for percolation are the same under the drops in reaching the deeper, drier lay- 

 trees and in the open the earth under ers when precipitation occurs. In the 

 the forest receives less than that under open, when the surface layer becomes 

 the bare surface. An approximation to measurably dried out and filled with 

 this assumption is reached on level air, a sudden rain may so occupy the 

 plains, provided the soil in the open be pores of the immediate surface as to 

 in an excellent state of tilth. seal them against the escape of the un- 



That proportion of the rain and snow derlying air, and the latter then pre- 

 which reaches the ground under the vents the percolation of water, and the 

 trees wets the mulch of mosses, leaves impervious surface becomes one on 

 and decaying wood, and such part as which run-off is inevitable. 

 is not thus absorbed seeps into the The total effect of trees in preventing 

 ground below. Only a small part es- some proportion of the rain from reach- 

 capes as run-off, unless the slopes be ing the ground and in using some part 

 very steep. The mulch of decaying of the moisture contained in the ground 

 vegetable matter is extremely absorbent in their physiological processes is in- 

 and will hold as much as fifty per cent, dicated by the depths to which they de- 

 of its own volume, or two to three times press the level of ground water as corn- 

 its weight of water. From extended pared with that in adjacent fields. Ob- 

 experiments in Germany with straw, servations on this point have been made 

 leaves, and humus it appears that the in Russia, France, and Germany, and 

 moisture thus caught is evaporated to the results are in agreement for Euro- 

 the air in dry seasons, and it would pean conditions. They were made on 

 therefore be uneconomical from the level ground, and the conclusions do 

 point of view of ground-water storage not apply where run-off from the sur- 

 to increase the surface litter beyond face is copious. It is shown that 3 

 that required to prevent run-off. Un- (a) The water table lies deeper be- 

 der some conditions of forest mainte- low the surface under a forest than out- 

 nance it would probably be economical side of it, if account be taken of the 

 to keep the surface comparatively clean relief ; 



of litter and in an open state, such that (fr) The depression of the water ta- 



the rain sifting gently through the ble is more pronounced under old for- 



foliage \vould be absorbed by the ests than under young plantations ; 



ground in a maximum degree. (r) The depression is more pro- 



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



