Forest Soil

S.A.Wilde

1958

 

Every tree expresses a rhythm resulting from organic impulses such as sunshine, wind and rain; and the result is something perfectly harmonious and immensely satisfying. At the base of this natural coordination and harmony rests the soil, the medium that supports and nourishes different members of the forest community. The soil influences the patterns of the forest community, rate of growth, reproductive vigour, degree of resistance to diseases, and stability against the wind. An understanding of the forest lies just as much below as above the ground.

Soil formation begins with parent geologic material weathered by wind and water. Crumbled components of this rock collect in cracks and small depressions. And there, by the roots of lichens and mosses, or the decaying roots and leaves of wattles and gums, soil is formed.

The rock offers little in the way of food and moisture, but on its surface pioneer plants may gain a foothold. Such plants must have the ability to dry up and lie dormant through times of dry weather, then waken to absorb the moisture of every rain or heavy dew. Many species of lichen have this power. Their root like fibres secretes an acid which dissolves minerals from the rock. Eating their way into it, they prepare an entrance for moisture that may later freeze, expand and loosen particles of rock — the beginnings of soil. The lichen thus offers to other more delicate plants a seedbed with moisture, a foothold on the rock, and mineral solutions for food.

In this seedbed, mosses, annual “weeds,” and hardy ferns may grow, catch wind-blown dust, and die, adding their substance to it, and building a deeper bed where seedling trees and other plants may find food and moisture. The growing community of plants slowly spreads till the roots and tiny root hairs fill the earth so compact­ly that they may touch every soil particle, tying it firmly into place, making a secure foundation for the further spread of new plants.

But these growing roots are doing far more than just binding together the rock particles and dust. They are taking the first step toward creating soil.

As the roots of herbs, shrubs, trees, and other plants spread through the soil, they fill it with a new living organic substance built from air, sunlight and water.

Solar energy is absorbed by leaves and needles containing chlorophyll, the green colouring matter that is carried in the leaves, and through the process of photosynthesis builds carbohydrates. This building process is the essential first step which prepares the way for all life that exists on earth. In other words, life on earth is sustained by the finite amount of solar energy that is fixed by green plants.

Chlorophyll has not yet yielded to man all the secrets of its composition or of the processes by which it transforms inert building blocks into living material, but we do know certain basic facts. The essential first step consists of building sugar carbohydrates out of sunlight, carbon dioxide from the air, and water. To make one molecule of sugar carbohydrate the chlorophyll unites six molecules of water and six of carbon dioxide. With them it binds the energy from the sunlight, and in the process, six molecules of free oxygen are removed from the water and carbon dioxide, and are released into the air.

The carbohydrates produced by plants are transformed into the vast number of chemical combinations that form the living substance of the plant — into roots, leaves and branches, and into flowers with their male and female parts which together produce seeds.

But this organic substance built from air, sunlight and water has not yet become a part of the soil. It is not until the plant itself dies that the dramatic change takes place in the soil. For now the dead plant’s roots and leaves offer food to the small organisms which are among the most important factors in the whole cycle of life: the bacteria, the moulds, and other living creatures, many of them too small for the eye to see. Their most important function lies in decomposing the remains of the higher plants and animals, changing them into new chemical combinations that can be used again by succeeding plant generations for food.

During decomposition some parts of the dead plants and animals are more resistant to decay than others. These parts stay in the soil for a long time forming a dark, spongy, very absorbent material called humus. Humus will hold many times it own weight in water. Humus stores rainwater and its dissolved minerals, holding them as a reservoir for plants to draw from.

The decomposition or humification process requires moisture in combination with moderate temperatures. That is, the rate of decomposition is determined by the amount of moisture and warmth in the soil. Very cold winter temperatures (as found in many parts of Northern hemisphere) even though accompanied by moisture, drastically reduce the decomposing activities of the organisms. And likewise, if the soil is dry in the summer their activity is slow. Moist, cool-to-warm soil as found under wet sclerophyll forests,  and low to mid elevation dry sclerophyll forests during fall to spring, provide the optimum climate for the decomposition and recycling of organic matter.

Removal of the forest canopy or disruption of the litter on the forest floor reduces the natural insulation on the soil; during the winter causes the loss of warmth stored in the soil, and during the summer causes increased solar radiation resulting in hotter and drier soil. This results in reduction of decomposing (nutrient recycling) activities by soil organisms.

A host of small insects and mammals help in the decomposition process. Earthworms eat dead organic matter, mixing it with the soil particles that pass through their bodies, digesting the whole, and casting it up on the surface — a revitalized and richer soil. The number of earthworms in the soil depends largely on soil chemistry and on the amount of plant material found in the earth.

Ants, beetles, wasps, spiders, and many other small creatures spend part of their lives in the soil. Some of these come to eat the plants, and many meat eaters come to eat the plant eaters. All this activity combines to carry on the work of ploughing, mixing and fertilizing the soil as the creatures add their remains to the land.

Fungi are another living member of forest soil involved in the recycling process. Fungi are plants but have no chlorophyll and therefore lack the power to make sugar from sunlight, air, and water. Thus they have to get their organic food in other ways. In this respect they are like animals. There are two sources of food supply open to them. Some fungi make use of dead plant and animal matter of one kind or another. Fungi which feed in this way are called saprophytes. Some fungi live on organic matter which they obtain from the tissues of a living host. Fungi, which do this, are called parasites.

Some parasitic fungi live in partnership with forest plants. For example, mycorrhiza fungi grow in what appears to be symbiotic alliance with forest trees. The fungi grow on the root hairs of the tree in the proximity of the interface between the “organic” and mineral layers of the soil. The fungi collect and supply moisture and phosphorus to the tree, and protect the tree roots from attack by pathogens.

Trees may survive without the mycorrhiza fungi but they are not as vigorous or healthy as trees with the fungi. The trees in turn provide the fungi with plant sugar, and the damp, shady microclimate required by the fungi for survival and reproduction.

By now it must be apparent that individual organisms do not live alone. Organisms living in any given area, whether large or small, are associated together in what are known as biotic communities. Forest plants and animals grow in communities, associations and alliances.

The community concept is one of the most important principles in ecological practice. It is important in ecological theory because it emphasizes that diverse organisms usually live together in an orderly manner, not just haphazardly strewn over the earth as independent beings. The community concept is important in the practice of ecology because “as the community goes, so goes the organism.

In summary, forest soil may be described as a portion of the earth’s surface which serves as a medium for the sustenance of forest vegetation; it consists of mineral and organic matter permeated by varying amounts of water and air and inhabited by organisms; it exhibits peculiar characteristics acquired through the influence of three organic components uncommon to other soils — forest litter, tree roots, and specific organisms whose existence depends upon the presence of forest vegetation.

Decaying humus, interwoven roots, and underlying weathered rock form a stable fabric which retains water and minerals, protects the mountain slope from rapid erosion, and provides plants with the substances needed for growth. Soil is a living tissue that pulses with billions of microscopic units of life, the fragile skin of the earth. Under the best conditions, it may take five hundred years to produce one inch of topsoil. Yet the slightest disturbance — a small landslide — may wipe out in a few hours the work of thousands of years.

A forest is, basically, a living community of plants, animals, and the inanimate parts of the environment in which trees are the dominant organisms. Here plants and animals live and die in a cycle of competitive harmony that includes water, light and the soil.

In nature the normal way in which trees flourish is by their association in a forest. The mosses, lichens, herbs, shrubs, trees, and animals of the forest assist each other in preserving the conditions for survival.

The members of the forest community collect and store energy and water. The soil is preserved and shaded and the microbes necessary for its fertility are neither scorched, nor frozen, nor washed away.

The forest is the triumph of the organization of mutually dependent species.

In the words of Gautama Buddha, “The forest is a peculiar organism of unlimited kindness and benevolence that makes no demands for its sustenance and extends generously the products of its life activity; it provides protection to all beings … offering shade even to the axeman who destroys it.”

Each small living thing adds its tiny bit to the building of the living earth until, in the average of one acre of good topsoil, with 4% organic matter, there is stored about 80,000 pounds of organic matter from plants and animals, containing energy from the sunlight equal to that in 20 to 25 tons of anthracite coal.

The energy transmitted daily from the sun to the earth is calculated in the millions of horsepower. A portion of this vast quantity of energy is trapped by the photosynthesising foliage of forest vegetation and is eventually incorporated into the soil as humus. “The ability of forest soils to maintain or even increase their supply of organic matter is probably the most essential feature distinguishing them from cultivated soils”(Wilde, 1958).

Energy stored in the soil is constantly being used for food by the teeming life it supports and, as we have seen, it must be constantly renewed by the plants in order to maintain this life. For good soil is actually a living thing, and its health is a matter of life and death to the plants and animals that live on its surface. We ourselves are as dependent on its health as the smallest of its creatures.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

IMPACTS OF FIRE ON SOIL

USFS USDA YEARBOOK OF SOILS

1957

Fire affects soils biologically, chemically, and physically.

Some of  the forms of plant and animal life — bacteria, fungi, insects, millipedes, earthworms — which are beneficial in breaking down litter and incorporating organic material into the soil, are consumed in burned litter.  Others in layers immediately beneath may be killed by heat.  Changes also occur in the populations and species of organisms, as evidenced by the increase in nitrification, a product of the activities of certain kinds of bacteria.

Fire releases the mineral nutrients that are bound up in the litter and humus.  Some may be utilised by plants and some may be lost by washing or leaching.  In any event, the quick release by fire means fewer nutrients for gradual release later under the normal processes of litter decomposition.  Furthermore, nitrogen is volatilised and is lost entirely.

Soil acidity is temporarily reduced by the release of calcium, potassium, and other elements that form an alkaline ash.  The reduced acidity, in the case of strongly acid soils, may temporarily increase the availability of such mineral nutrients as phosphorous.  The ash minerals and the accelerated decomposition of organic matter combine to stimulate a quick flush of vegetative growth.

A fire hot enough to oxidise humus from the upper layers of mineral soil alters the texture of those layers.

The reduction in organic matter on the surface touches off dire secondary, or indirect, effects.  The effects may be minor if a generally unbroken mat of partly decomposed litter (or of organic matter incorporated in the uppermost layer of soil) remains after the burn.

But if the insulating blanket of organic matter is removed entirely, the upper layers of soil undergo greater and more frequent fluctuations in temperature, which probably (depending on aspect, season and latitude) are unfavourable to soil flora and tree growth.  They also undergo more frequent and more severe drying, which definitely harms growth.  The feeding roots of forest trees typically are concentrated near the soil surface, where the mineral nutrients released from decomposing litter are most abundant.  The nutrients can be absorbed only when the soil contains available water.  Destruction of the surface litter therefore destroys this reservoir of nutrients.  It also curtails the availability of the remaining nutrients by exposing the soil to excessive drying.

The lack of moisture is aggravated by the action of rainfall on bare soil.  Puddling may occur at the surface and seal the pores, slow infiltration, and lead to greater runoff.  The water that enters the soil carries fine particles, which reduce pore spaces, slow down infiltration and percolation even more, and inhibit aeration.

Without surface litter, the activities of soil-burrowing insects, worms, and similar animal forms are reduced.  Soil compaction results.  The increased runoff means less storage of water in the soil and smaller supplies of water for the forest vegetation and for the maintenance of streamflow.  In cooler regions, impermeable concrete frost forms to a greater degree in exposed soil than under litter.  That means still more runoff and less infiltration and storage of water.

Those effects generally develop most markedly in soils of finer texture and least in sands, loamy sands, and coarse sands.

To the extent that the vegetative cover is killed or set back and the soil is exposed to the elements, the soil surface and the site as a whole will be made drier by the action of more sunlight, higher temperatures, more wind movement, and lower humidity.  Organic matter remaining after the burn is subject to accelerated oxidation and dissipation.  Drastic opening of the vegetative cover  thus compounds the detrimental effects of burning the organic mantle.