Two aspects of the harvesting operation, each of which may set in motion a distinct chain of effects, are to be recognized. One is the intensity of the cut, which may vary from light thinning to clear-cutting. The other is the time, methods, and equipment used in the logging operation.

Intensity of cut bears upon soil productivity in much the same manner as fire, except that organic matter on the ground is not consumed. That is, the greater the opening of the stand or the greater the changes in species composition, the more profound are the effects on the soil.

In the extreme instance of clear-cutting, the site thus exposed is subject to greater variations or extremes of light, temperature, humidity, and wind. The existing litter undergoes accelerated breakdown, with little annual replacement. Unless a new cover develops soon, the soil may be bared and exposed to the full drying force of the weather and the full force of rain. This, as after fire, can lead to compaction of soil, reduced rates of infiltration, less water storage, more surface runoff, and possible erosion.

The degree to which this chain of events develops depends on soil texture and structure, slope, the character of the precipitation, and the amount of low vegetation that survives the logging or appears immediately afterward. Again, light soils are least subject to adverse effects. Heavier soils, especially those on steep slopes, are most likely to suffer damage. Prompt development of sprout, shrub, and herbaceous cover will greatly ameliorate the effects of exposure. In this connection, sprouting hardwoods can be beneficial in preserving soil values, regardless of their worth as timber species.

Rarely will a site be, or long remain, completely bare after clear-cutting. Natural revegetation soon starts, or sometimes the area may be replanted. The point of concern is that, after clear-cutting, the site and soil will be exposed to some degree perhaps to a large degree for a time and to that extent are subject to some adverse effects.

The forest manager who understands soil values tries to keep the degree and duration of exposure to a minimum consonant with silvicultural and economic objectives. He will clear-cut only when it seems definitely dictated by technically sound considerations and he will manage his cuttings as to obtain prompt reestablishment of an adequate protective cover on the soil.

Partial cuttings have proportionately less impact than clear-cuttings. If enough stand is reserved to maintain a good mantle of litter, the detrimental effects are not likely to be serious. In dense coniferous stands, as we noted earlier, moderate thinnings may be beneficial in terms of soil values by promoting the activity of soil organisms and the decomposition of litter and by facilitating the establishment of understory vegetation.

Changes in stand composition by cutting practices mean changes in future litter composition. They are not likely to be of much significance unless the cutting left a pure coniferous stand, such as cutting all hardwoods from a pine-hardwood stand. In that event, we have established monoculture or (if more than one coniferous species is represented) a situation similar to monoculture. Our earlier comments on litter decomposition under monoculture apply here. Pure coniferous litter tends to be somewhat less effective than mixed litter for building up a soil, but the differences in litter values need not deter one from growing pure stands of adapted species if such monoculture is desirable for any reason.

The logging operation may seriously damage soil values. A substantial proportion of the area up to 40 percent may be compacted by haul roads, skid trails, and yarding activities. Roads and skid trails, improperly located and improperly built, provide channels for surface runoff and, in steep terrain, erosion. Skidding often causes the greatest damage. The widespread practice of downhill skidding on steep slopes by horse or tractor leads almost always to gullying and movement of soil and debris on to lower flats or into streams.

Techniques have been developed for minimizing logging damage, and studies indicate that use of such techniques does not necessarily involve added expense they usually reduce costs.

One of the essential requirements for low-damage logging is proper layout of roads and trails: Lay them out across (not up and down) slopes, do not exceed specified grades, and be sure to avoid undercutting unstable soil or rock masses. Provide adequate drainage to handle heavy storm waters or melting snow. Avoid stream channels as far as possible. Build with a minimum of earth movement. Maintain roads while in use. Provide further stabilization at the end of the job by installing water bars, mulching, and seeding grasses and herbs.

The amount of soil compaction resulting from logging varies with the kind and weight of equipment employed, the frequency with which it traverses the same ground (for example, on and between skid trails), and the weather while the equipment is operating. Among various types of power vehicles, track-laying tractors generally apply the least pressure on the surface between 3 and 9 pounds to the square inch, depending on their weight, length, and width of track. The compaction may extend to a depth of 12 inches directly beneath the tracks or wheels and to a width of 12 to 18 inches on each side of them. The husky horses often used to skid logs generally apply more pressure per unit area than do most tractors of the wheeled type. The logs themselves as they are dragged over the ground and heavy ground-skidding cables also cause some compacting.

Tractor logging, on a previously undisturbed area of silty clay and clay loam soils, increased the density of the upper soil layers 2.4 percent, reduced macropore space 10 percent, and reduced permeability 35 percent over the area as a whole. On the skid roads, volume weight of the soil increased 15 percent, macropore space dropped 53 percent, and permeability declined 93 percent from the values prevailing in undisturbed soil.