There is a widespread belief that it is plant roots that prevent soil erosion by keeping the soil in place. When these are removed, as happens when trees are cut down or smaller plants are uprooted, the soil is ploughed in order to prevent undesired plants from growing, or poisons are sprayed to prevent the growth of other plants besides the cultivated crops, the soil is vulnerable to being dried by the sun, blown away by the wind, and washed away by the rain. However, this common belief is only part of the story of soil erosion.
One of the interesting facts about a forest where there is an abundance of mature trees is that there exist bare patches of earth underneath the trees. This is because the trees’ overlapping canopies completely block the ground, and thus prevent other plants from growing because they cannot collect enough sunlight. And yet, the soil is not eroded by either the wind or rain. If we consider what is happening in these situations, we will find there is another very important living mechanism that prevents the soil underneath the trees from being eroded away.
Mature trees protect the soil – with their trunk, branches, and leaves – from the harmful effects of the wind, rain, and sun. In the case of rain, a mature tree can hold a considerable amount of water on its various parts, meaning that, when it rains on a forest, a considerable amount of the water never reaches the ground. Instead, it falls on the trees’ leaves, branches, and trunk, where it remains until it evaporates back into the air. In the case of a light shower, it will be found that almost none of the rain reaches the ground. This fact can easily be verified by measuring the rainfall at ground level in a forest and the rainfall in a nearby spot that is completely bare of vegetation.
These protective parts of the tree also serve the function of breaking the water’s fall and thereby reducing its velocity, which in turn reduces the force with which it strikes the bare earth. Moreover, the rain falls more gradually over a longer period of time, as anyone who has stood beneath a rain-soaked tree can attest when it is shaken by the wind. A saturated tree will continue releasing water onto the ground even after the rain has stopped falling. Hence, a forest canopy breaks the fall of rain and transforms even a violent storm into a less violent and longer-lasting drizzle of water. To determine the erosive effect of rain, it is not enough to measure only the total quantity of rain that falls on a given area of land. One must also measure its velocity and the quantity of water that actually reaches the ground. The protective effect of trees and their canopies against erosion is most evident in the case of hilly terrain where the soil is bare due to the presence of many mature trees. When these areas are cleared of vegetation, they are especially vulnerable to the harmful effects of erosion.
Provided they are not raked and removed, leaves also protect the soil even after they fall, in autumn, winter, and spring, when they are not hanging from the trees. And even the particular way that some leaves dry and curl up serves to protect the earth as much as possible, instead of simply crumbling into smaller particles, as they do when they are crumpled in one’s hand. For organic matter that is as seemingly fragile as leaves, they are nevertheless remarkably durable in their dried state. They can easily last through the snowfall and ice of a cold winter, repeated drenchings from storms, being blown and tossed about by the wind, and scorched by the sun, until they are gradually eaten by microbes and other small organisms, and thereby enrich the soil with organic matter and nutrients that have been brought up from deeper strata of the ground by the tree’s penetrating roots. And the fact that some kinds of leaves curl up when they dry up also allows the surface of the soil and the multitude of tiny organisms that live there to receive adequate amounts of oxygen, instead of being smothered to death by a compacted layer of dead, putrefying leaves.
The great importance of this protective relationship is shown when a forest is clear-cut and the soil is left completely exposed to the elements. Even though the trees’ roots are still there, the exposed soil is now vulnerable to erosion. It is dried out by the sun, which makes it liable to be blown away by the wind, and when it rains, the force of the rain is not tempered by the trees’ protective canopy, which serves as a large, permeable natural umbrella over the ground. Just as a baby suffers when it is deprived of the protective care of its parents, in particular its mother, the earth suffers when it is deprived of the protective care of the plants that grow on it. It is precisely this sense of desolation that one feels when one looks at a clear-cut area, a feeling that is no different from what one would feel after seeing the devastation wrought by a merciless invading army, which plunders and kills every living creature that it encounters.
Although mature trees prevent the soil from receiving as much rain as it would if they were not there, by shielding the soil from the drying effects of both the wind and sun, these trees ensure that this reduced precipitation is sufficient for whatever is growing there. Except in times of extreme drought, it will be found that the ground in a mature forest is very rarely completely dry. Thus, trees protect the soil in which they grow from the harmful effects of the sun, wind, and rain, a mutually-beneficial relationship that is not at all surprising when we consider that it is the soil on which the tree depends for its nourishment, stability, and survival. Most people are completely oblivious of this relationship – that, in a very real sense, each tree cares for and protects the patch of ground in which it has taken root, which enables an abundance of other organisms also to live there and thrive. But unlike the tree’s life-giving generosity, wherever modern human beings have settled and taken root, we greatly reduce rather than increase the amount of diversity and the total amount of life by our selfish actions and our incessant efforts to reshape all things to accord with our strange and artificial conceptions about the best way to order life and the world we live in.
 Since all plants consist of one part that lies above the soil and another part that lies below or in the soil, this common way of describing a tree is not entirely accurate. However, since we humans are not soil-dwellers, we are usually most aware of the above-ground part of the tree or plant.
 It has been found that a tree’s root system is roughly as extensive as its canopy, which again is not surprising since the canopy protects the soil from drying out, which could lead to the death of the tree’s thinner, more delicate roots. However, this is a generalization, and like most generalizations that are sometimes true, there are probably exceptions to this one.