NATURAL NATIVE FORESTS CONTROL SOIL EROSION

Forest and Bird, Issue 59, 1 February 1941, Page 12

NATURAL NATIVE FORESTS CONTROL SOIL EROSION

In a good forest the tree tops usually are close enough to touch and form a closed canopy; and frequently small trees, shrubs, and other forms of lesser vegetation make up a thin or thick undergrowth. Anything less is not first-class forest and is not likely to provide such good protection for the land beneath.

The leaves, the twigs, branches, and stems of a forest expose innumerable little surfaces, aggregating, under good conditions, an area several times greater than that of the ground beneath. This loosely thatched roof, often 100 feet or more in thickness, is the first line of protection against soil erosion and ' excessive runoff. Driving rains beat upon this roof; the raindrops spatter, and the water slips gently down the stems or drips intermittently to the ground. As much as half an inch of rainfall may be completely intercepted by this intervening thatch. Part of this intercepted water is lost by evaporation and so never reaches the ground. The main forest bulwarks against erosion and runoff, however, are still lower down. Covering the forest floor is a blanket of woods litter -—a mass of leaves, twigs, and fragments of bark, in various stages of disintegration. It is not always a smooth blanket, for beneath it are an endless series of little depressions. These catch part of the water that penetrates the thatched ' roof overhead and restrain much of it from running away. The blanket performs a double functionthat of absorbing part of the water and like a sieve directing the remainder downward to filter slowly to the soil beneath. This all-important blanket of vegetative

material exerts a powerful influence on the soil in several ways, making it more permeable to water. The surface of the soil is kept moist and absorbent, even in winter when exposed soil is deeply frozen. The litter and humus form the principal habitat for a vast population of organisms important to soil building and. conditioning. From the standpoint of soil protection, this covering of the forest floor is the most effective element of the forest complex. Here is the gateway to water storage in soil and underground channels.. Its function is to filter water, keep it- clear, and so keep it moving downward into the soil./ Muddy water, such as gathers on bare surfaces, clogs the channels, slows infiltration, and changes beneficial percolation to harmful runoff.

In any system of . sound land use, it is essential that excessively steep slopes, such as usually prevail about the headwaters of numerous streams,, be kept in some dense cover, perferably forests or grass. Trees make the most effective cover for vast areas ■of mountain and hill country, where it is not easy to establish and ‘ maintain an adequate cover of grass. The contrary is true, of course, with respect to the use of grass in those areas not climatically suited to rapid tree growth. Combinations of trees and grass, as well as combinations of trees and shrubs, have important places for special conditions in the control of runoff and erosion.

(From “ Soil Conservation,” by Dr. H. H. Bennett, Chief of Soil Conservation Department, U.S.A., Publisheres, McGraw Hill Book Coy., New York.)

Mew Zealanders

YOU ARE DEPENDENT UPON the PRODUCTIVITY OF THE SOIL. FIRE DESTROYS SOIL FERTILITY.

fohe JJeW fffeafand mountain forests are tfie finest in the temperate zones as protection forests, on account of their undergrowth and the thief Water-holding brgophgte carpet, or cushions, of the ffoor. — vide L. Coclcayne, Pk.D., F.R.S