Forest Practices and Productivity

V. L. Harper, Bernard Frank, and W. E. McQuilkin.

Four forest practices are related closely to the productivity of soils: Prescribed and controlled burning; monoculture (growing a single species in pure stands) ; grazing by livestock; and harvesting commercial timber.

Three of them burning, grazing, and harvesting if not done properly, may destroy or alter the organic material, compact the soil, and cause erosion.

FIRE has been a major force in shaping the development and character of timberlands. The regeneration and perpetuation of many species and forest types in the past have depended largely on fire. Examples are the longleaf pine in the South, pitch pine in the East, lodgepole pine and Douglas-fir in the West, aspen, white birch, and various types of scrub oak.

Some of our best species are among those that regenerate after fires, but most wildfires are highly destructive to whatever forest growth is present at the time and to the soils that support the forest.

The fact that some valuable species regenerate most abundantly after fire, however, has contributed to an increasing interest in prescribed, controlled burning as a tool in the management of forests.

Prescribed burning is the burning of a specified area at a specified intensity for a definite purpose. It calls for the selection of the proper year and season and a day and time of day when moisture in the fuel, wind, and related factors are such that the fire will have the specified intensity. So controlled, these fires are unlike wildfires, which we make every effort to prevent.

Controlled burning usually is prescribed for one or more purposes: To reduce the volume of fuel and thereby facilitate the prevention of wildfires and the suppression of any that may occur; to kill or weaken undesirable hardwoods or reduce grass and shrub competition in pine stands; to facilitate the establishment of pine reproduction by removing competing plants and thick litter; to control disease, notably the brown spot disease on longleaf pine; to improve habitat for upland game birds and other wildlife; to remove brush, debris, or other impediments to livestock grazing and otherwise to improve forage production on forest ranges.

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. Altogether, controlled fires ordinarily are not hot enough or so deep-burning as to destroy all of these plant and animal populations, and the temporary reductions in numbers probably are of little consequence.

Fire releases the mineral nutrients that are bound up in the litter and humus. Some may be utilized 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 volatilized 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 phosphorus. The ash minerals and the accelerated decomposition of organic matter combine to stimulate a quick flush of vegetative growth.

A fire hot enough to oxidize humus from the upper layers of mineral soil alters the texture of those layers. Fires of that intensity seldom occur with prescribed burning, however, except possibly in slash piles, but even in them, temperatures in the mineral soil at a depth of 1 inch have rarely been found to exceed 175 to 200 F.

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 season and latitude) are unfavorable to 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. Impermeable concrete frost forms to a greater degree in exposed soil than under litter in the cooler regions. 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. Therefore burning is prescribed most widely in areas of coarse sandy soils. If burning is done on fine-textured soils, it should be done at a low intensity and infrequently, so as to preserve part of the organic matter.

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, unless grass or herbaceous growth come in soon afterward and provide new sources of organic matter. At worst, soil washing or blowing may occur and gullies develop on sloping land. Such effects are to be avoided in prescribed burning. The burner must understand these tendencies, however, so that he may keep the intensity of the fire below the point that causes such damage.

Changes in the plant cover will be reflected in the type and amount of litter deposited after the burn. If the cover has been greatly reduced, as might be done where the objective is to establish pine reproduction after harvest cutting, the annual litter fall for some years may not be enough to form a continuous mulch over the soil. Much of the beneficial effects of a continuous litter mulch in maintaining soil porosity, equalizing temperature, reducing evaporation, and releasing nutrients would be sacrificed until a tree cover develops again.

A less drastic treatment for instance, a burn to kill back sapling and sprout hardwoods in an established pine stand might not reduce seriously the amount of litter deposited annually but would alter its composition. If the hardwood elements in the stand are set back or killed, the subsequent litter fall would be essentially pure pine. Straight pine litter tends to decompose more slowly than mixed pine-hardwood litter, and there is less release of nutrients and less incorporation of humus into the soil. Rapid leaching of the bases from the upper soil layer frequently occurs in northern climates. Particularly under dense stands of the coarse-needled southern pines, the slower decomposition may cause thick accumulations of duff. These processes do not necessarily mean reduced soil productivity, but the stimulating effects of most hardwood litter on bacterial activity (and hence on decomposition of humus) generally favor the inclusion of most types of broad-leaved vegetation in the overstory or understory of established pine stands.

Burning also directly or indirectly affects plant roots, especially when it brings about marked changes in species composition. The vegetation that remains or becomes established on the site after fire will have less competition for soil moisture and nutrients, since the other plants will have been killed.

The changes may be limited to certain depths or may extend throughout the depth of root penetration. For instance, the destruction of shallow-rooted plants affects moisture and nutrient relations in the upper soil levels, but the effects of killing or retarding deep-rooted tree species oaks, yellow-poplars, hickories, elm, and the like may extend to considerable depths.

The decay of roots following the death of plants has numerous beneficial effects. Nutrients are released, and the physical and biological reactions deep within the soil favor improved soil structure, internal drainage, and aeration. We must note some exceptions, however. Sometimes, especially on steep slopes, the death of roots may cause soil instability and prescribed fire there is of questionable value.

Fire is used mainly in the Douglas-fir region, to dispose of logging slash after clear-cutting operations. The purposes are to reduce the hazard of wildfire and to improve seedbed conditions for the new crop of timber.

Studies of the effects of this practice upon the soil emphasize the importance of burning only when weather conditions assure low intensity that is, shortly after a heavy rain when only the uppermost layers of litter have dried. Heavier burns have resulted in losses of up to 75 percent of the organic matter. In some instances, nutrients were so depleted that coniferous reproduction was unable to compete successfully against weed and brush growth that would not have been an important problem on an unburned site.

In properly controlled slash burning,only about half the area is touched by fire at all, and only 1 to 4 percent of the area the spots of heavy slash accumulation burns so hotly as to have notably adverse effects on the soil.

Prescribed burning in the ponderosa pine types of the West and Southwest is somewhat experimental, although some thousands of acres have been so burned. The main purpose has been reduction of fuel grass, litter, brush, and snags to reduce the wildfire hazard.

Other possible objectives are being investigated. Among them are seedbed preparation, improvement of wildlife habitat, and thinning of too dense pine thickets. Burning for these purposes is not now generally recommended, but further research may point the way to at least limited use of fire in that way.