NATIVE WOODY PLANTS OF THE UNITED STATES 3 



Many workers are of the opinion that a plant with a large root 

 system is necessarily the best one to hold soil in place. This idea is 

 so universally held that it may not be amiss to review briefly just 

 what takes place when a soil is eroded, especially by water. In doing 

 this we may come to understand more clearly how erosion can be pre- 

 vented by restoring vegetation. 



In exceptional and rare instances, washing may take place below 

 the surface, but under ordinary conditions the washing away of soil 

 occurs at the surface. It is the movement of soil particles downhill in 

 water or into the air as dust that we wish to prevent. Since such 

 movements start in the very uppermost layer of the soil, our efforts 

 must of necessity be directed to holding topsoil in position. Kpots of 

 woody plants do not, in themselves, offer much help in the top inch or 

 so of soil; they penetrate deeper and may be said to be holding the 

 soil in place below but not necessarily at the surface. 



Observations in the field show that soil easily washes away from 

 roots and that the better the surface of the soil is covered, the less 

 the soil washes. Experimental evidence has confirmed such observa- 

 tions. Kramer and Weaver (346) 2 conducted a series of tests on 

 many kinds of plants, mostly grains and other herbs. In their experi- 

 ments they discovered that it was not the soil-binding effect of roots 

 that produced the best protection, but the plant cover, which pre- 

 vented most of the water from coming in direct contact with the soil. 

 When the plant cover was intact its effectiveness in controlling erosion 

 exceeded that of underground parts alone many times, and com- 

 paratively little relation was found between the quantity of under- 

 ground parts and resistance to erosion. They found also that cover 

 need not be living to be effective ; any kind of cover protects the soil. 

 In the course of their experiments they noticed accidentally that a 

 single elm leaf protected the soil below it until a column over 3 inches 

 high, capped by the leaf, remained after the surrounding soil had 

 been washed away. Undercutting eventually toppled the column. 

 These experiments would appear to justify the use of a mulch in 

 planting on eroded lands. 



It is clear that since erosion is greatest at the surface the roots 

 of woody plants have comparatively little to do with its prevention. 

 As^a matter of fact, when water washes the covering soil off and 

 begins cascading over roots, they may only aggravate soil washing. 



The surface layer of the, soil can best be held in place by some sort 

 of protective cover. This may consist of close-growing plants, litter, 

 or a combination of the two. An effective protection for soil is 

 afforded by a mat of grasses, which are among the best of all plants 

 for erosion control. An equally good cover consists of an established 

 forest and the duff produced by it. A third type might be made up of 

 close-growing, thicket-forming shrubs and the litter produced by them 

 or of a mat of entangled vines. Run-off and removal of soil are 

 reduced to a minimum under such covers as these. It is almost un- 

 necessary to mention that grass sod can be formed more quickly than 

 can a thicket of shrubbery and that the establishment of a forest 

 and forest litter takes longer than either of these. 



2 Italic numbers in parentheses refer to Bibliography, p. 293. 



