270 FORESTS, RESERVOIRS, AND STREAM FLOW 
These depend upon causes of far greater magnitude, and are influ- 
enced, if at all, only to an insignificant degree by the operations of 
those who occupy the planet. 
The fourth proposition of the forestry argument is that forests 
are necessary to prevent erosion on steep slopes and the consequent 
silting of reservoirs and watercourses below. Here again there is 
the same deficiency of evidence to support the theory that has char- 
acterized the three propositions already considered. The writer has 
been unable to find anything to confirm it. In his observations, em- 
bracing pretty nearly all varieties of timber land in the northern 
two-thirds of the United States, he has still to see a single example 
where the mere cutting off of forest trees has led to an extensive erosion 
of the soil. Almost invariably, and it may be said always except 
in very unusual conditions, a soil that will sustain a heavy forest 
growth will immediately put forth, when the forest is cut down (or 
even burned down), a new growth, generally in part different from 
the first, but forming an equally effective cover to the soil. The only 
approach to an exception to this rule that he has observed is in some 
of the high mountain forests where the soil is extremely thin and 
weak and the action of Nature in producing vegetable growth is slow. 
Fig. 2, Plate XL, shows one of the best examples of this class of 
timber land; but even here the slow growth is very evident and no 
large amount of soil erosion has resulted. In, the forest areas of the East, 
the growth that follows tree-cutting, consisting not only of new trees 
but of briars and small brush of every description, accumulates very 
rapidly and forms a more effective mat against erosion than the 
original forest itself and is equally effective in storing water. Such 
low growths have also a better effect upon snow-melting because they 
give both wind and sun freer play. Certainly the ground in a forest 
under culture, with the débris raked up, is more easily eroded than 
that of a slashing or second-growth area, or even good meadow or 
pasture. A forest soil unprotected by forest débris is almost as erosible 
as a field under culture. 
The increased erosion of the soil, of which so much is heard, does 
not result from forest cutting, but from cultivation, using that term 
in its broad sense to include all of Man’s operations for the occu- 
pancy and utilization of the ground from which the forests have 
been removed. It is the “breaking of the soil” that leads to its 
