FORESTS AND SOILS

JOHN T. AUTEN, T. B. PLAIR.

Successful reforestation, particularly with the hardwoods, has to take into consideration selection of the proper species and the balance between trees and soil. Perhaps the soil has eroded or all trees have been removed from it: Then it is not simple to choose trees that grow well on bare land; also, the balance that existed in the virgin forests was destroyed when the land was cleared. Basic soil and atmospheric changes often make such areas incapable of supporting the original species.

Soil loss from erosion following fire, overgrazing, clearing, and cultivation is a basic loss. It reduces productivity of cleared land; it also lowers the site quality in existing forests. Any appreciable change in soil necessitates a shift in species composition in order to obtain those best suited to the site. Site deterioration means species of lower value in the stand and a loss to the owner.

Accordingly, the problems of restoring and conserving our trees and forests will be simplified by a knowledge of forest soils and of the relation between forests and soils.

A soil is a natural mineral body with distinct features that identify it, even in widely separated areas. It has definite structure with horizons or layers, one over the other. The topsoil, from which the fine soil has been washed by percolating waters, is the A horizon. Just under it is horizon B, the heavy horizon or subsoil, which receives the fine soil washed out of A. The C horizon is the parent soil material below B.

A fertile soil contains a myriad of living organisms, plant and animal, adapted to the soil conditions. It has pore space, which contains water and air. To some degree, like a living body, it absorbs oxygen and releases carbon dioxide. A soil has characteristic parts in harmony with its environment. Its productivity depends on all of its parts.

The formation of a soil, a slow process, doubtless began on the first crust of the earth, when heating and cooling and wetting and drying cracked the surface rocks, made little patches of loose rubble, and caused little pockets of mineral crystals to settle in crannies and depressions, and allowed lichens, mosses, and other simple plants to grow in the thin soil and on porous rocks. Rain dissolved the softer parts of the rocks and made soluble minerals available to plant roots.

As the soil mantle grew deeper, the soil grains became finer. Water percolated through the soil mass, carried fine particles from the surface layer downward, and deposited them at lower depths to form subsoil. Plant debris fell on the surface and the micro-organic life appeared. Larger and more complex plants appeared until finally trees and forests, as we know them today., emerged with their characteristic soils.

Each forest soil developed its orderly arrangement of horizons, its porous and absorptive structure, and its balanced and active population of bacteria, molds, fungi, worms, insects, and animals. Roots of the trees anchored the soil in place; the leaves provided a protective cover of litter and added fertility yearly.

Soils differ broadly among climatic provinces. Basic differences occur because rainfall, temperature, and rocks are different. Any part of the earth having a characteristic climate and parent-rock material has its special kinds of soil: The gray, desert sagebrush soils of southern Wyoming; the subhumid, chestnut-colored prairie soils of western Nebraska; the black, tall-grass prairie soils of Iowa and Illinois; the gray-brown, hardwood-covered soils of Indiana and Illinois; the gray, leached, pine-covered podzol soils of Maine; and the rich-brown, humid, forest soils of the Northwest.

Even within a climatic province, many differences occur among soils, chiefly because of differences in vegetation, degree of slope, and the nature of the parent rock. Vegetation alters the surface of soils, but the primary local difference is permeability of the soil to water. Permeability is altered according to changes in coarseness of the soil and is controlled largely by the nature of parent rocks and by the subsoil density associated with topography. A basic factor in soil formation is the relation of topography to subsoil.

Rain falling anywhere on bare soil puddles it with muddy water. Such muddy water contains colloidal soil exceedingly small soil particles, some of them almost molecular in size. If the soil surface is sloping, much of the muddy water runs off. If the surface is flat, much of the water seeps into the lower soil, where the colloidal particles are deposited, forming a part of the B horizon. This horizon forms in the lower soil at depths usually ranging from 8 up to 36 inches, depending on height of the water table during the formative period. The thickness of this zone depends on the rate of internal drainage and fluctuation of the water table during the rain), season. Density of the 13 horizon is affected by the flatness of the terrain. In general, the flatter the terrain the denser the B horizon. This horizon sometimes called the subsoil. or where very dense, a clay- pan is the key to drainage.

Soils in any one area are affected by differences in parent-rock material. Coarse sands from sandstone do not puddle as much as clay from shales; hence subsoil formation is less pronounced in sandstone- than in shale-derived soils. Differences in the rock composition do not alter the basic soil-forming process, but do affect the rate of soil formation.

SOILS AFFECT THE TREES principally through soil air and soil moisture. Seasonal available soil moisture commonly determines what species grow in any forest and their rate of growth. Soils affect forests much as soils affect any other crop. Dry soils in the hardwood belt are likely to have dry-site oaks, like scrub oak, blackjack, and scarlet oak. Moist soils support such species as yellow-poplar, beech, maple, black walnut, and red and white oaks. Wet soils are more favorable for sycamore, cottonwood, redgum, pin oak, and willow.

Four general soil conditions influence forests through their effects on available soil moisture : Surface porosity, subsoil density, aspect, and depth. The first affects rate of water absorption; the second, free movement of water in the soil; the third, surface evaporation rate; and the fourth, the volume of water available. Surface porosity is an ever-present and indispensable attribute of the forest soils. A forest soil always develops porosity under a protective litter cover. This porous surface facilitates absorption.

Water movement is governed by subsoil density: The denser the subsoil, the slower the movement of water through it. Furthermore, the shallower the A horizon above a tight subsoil, the less rainfall the soil absorbs because of the smaller volume of porous surface soil.

Graphic relation between equal-aged yellow-poplar and subsoil in the same planting.

Aspect and exposure influence available soil moisture by affecting the rate of evaporation. South- and west-facing slopes normally have less soil moisture than north and east. In hilly or mountainous country, the quantity of moisture available to a tree varies with its position on the slope. Trees on lower slopes normally have more available moisture than trees on otherwise similar upper slopes. Deep soils that have adequate water-holding capacities keep trees growing at maximum rates if other factors are not limiting whereas shallow soils not having adequate water-holding capacity do not.

Some tree species grow well under many soil conditions, others do not. Black locust, for instance, can grow on deep or shallow, moist or dry soils. True, it does not grow equally well on all situations, but it does persist. Other species, such as yellow-poplar, occur on only a few deep moist soils and usually do not become established on shallow dry soils or on tight claypan soils. Segregation of species within their ranges is therefore often due to differences in soils.

Forest types or associations of tree species depend somewhat upon relative tolerance of the several associated species to shade. Some stand more shade than others. For instance, ponderosa pine, cottonwood, and black locust arc less tolerant of shade than yellow-poplar, white oak, beech, and hemlock. Some species appear to be more tolerant under some soil conditions than under others. Two factors, then, chiefly determine forest types in any climatic province: First, the inherent capacity of a species to withstand the shade and, second, the soil conditions.