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5.3 Impacts of Forest Operations

Forest operations alter the environment. Some of these effects are intended; others are undesirable consequences. Most impacts are associated with driving equipment and moving material in the forest. Soil, water, and residual vegetation can be affected. Effects must be considered in terms of their quantity, severity, persistence and location within the landscape. Some impacts are short-lived, while others may affect the long-term productivity of the forest. Impacts that are concentrated may be significant, while the same impacts spread across a stand may not be ecologically important. Chapters AQUA-3, AQUA-4, and HLTH-3 provide more information about the effects of forest management on water and soil.

The principal impact of most forest operations is soil disturbance. Soil disturbance results from road or trail construction, equipment traffic, and the dragging of material. Soil disturbance includes physical dislocation and loosening, compaction, or puddling. Disturbance effects are the cumulative result of all operations in a silvicultural system. Soil disturbance from felling is covered by soil disturbance from skidding, which is subsequently ameliorated by the soil disturbance associated with site preparation.

Conventional clearcut skidder harvesting systems cover about 15 percent of the stand in trails and landings. The most heavily impacted areas are the primary skid trails, gate delimbing areas, and landings. Detailed tracking of total soil disturbance on a Piedmont clearcut showed about 22 percent of the stand affected by more than five passes of machinery (McDonald and others 1998). At least 30 percent of the stand remains undisturbed, even in clearcuts. Reisinger and others (1988) summarized studies from the South and noted that 63 to 99 percent of the stand areas were undisturbed, depending on the system used

More difficult sites tend to have a greater amount of undisturbed area than more easily accessible areas. Stuart and Carr (1991) and Stokes and others (1998) observed that disturbed area decreased with increasing slope. On slopes greater than 35 percent in central Virginia, skidtrail disturbance ranged from 3 to 10 percent of the stand. In contrast, Aust and others (1993) found 34 percent of a wet flat rutted.

Harvest intensity also affects the amount of soil disturbance. Kluender and others (1993) and Carter and others (1997) found that clearcuts and shelterwood cuts had similar amounts of skidtrail disturbance (about 15 percent of the stand). Shelterwoods, however, had more area in undisturbed condition. Single-tree selection had the least amount of soil disturbance, but that prescription calls for more frequent entries with additional impacts over time.

Cut-to-length(CTL) systems carry wood rather than dragging it over the soil. The result is less soil disturbance. Vidrine and others (1999) and Lanford and Stokes (1995) found that 7^th and 5^th row thinning in pine plantations with a harvester/forwarder combination resulted in 11 to 30 percent of the total stand area disturbed by traffic. Both of these studies were on Coastal Plain sites in winter. Seixas and others (1995) compared five CTL configurations in various prescriptions and found the least disturbance occurred with a feller-buncher, manual processing, forwarder system. About 26 percent of the stand area was disturbed. A system with a drive-to-tree harvester and forwarder disturbed 39 percent and a horse logging crew disturbed 42 percent of the stands.

Cable logging reduces soil disturbance because wheeled traffic is eliminated in the stand. Disturbance still occurs from dragging logs, however, Miller and Sirois (1986) compared skidder and cable logging in southwestern Mississippi. About 16 percent of the cable units were disturbed, mostly in cable corridors. Skidders disturbed about twice as much area. Cable logging disturbance tended to be oriented up-and-down slope while skidder disturbance was more irregular.

Forestry tires have gotten larger to provide better flotation and reduced rutting and disturbance. Wider tires typically reduce rut depth but increase track width (McDonald and others 1995). Thus, the primary application of wide tires appears to be on very soft soils where sinking and rutting are concerns. Carruth and Brown (1996), for example, noted that when moisture content exceeds 40 percent on lower Coastal Plain sites, the only systems that can operate are tracked feller-bunchers and wide-tired skidders operating on trees and mats. On drier soils in eastern North Carolina, Seixas and McDonald (1997) observed that the least rutting developed with narrower tires on a forwarder rather than wider tires or tires with tracks. Rummer and Sirois (1984) observed that carrying larger loads on wider tires probably offset any reduction in soil loading.

Operational configurations that carry, rather than drag, materials generally produce less soil disturbance. Feller-bunchers generate less disturbance than manual felling because trees are carried from the stump to the bunching location. Forwarders generate less disturbance during extraction than skidders because the load is off the ground. Swing machines have arms and rotating upper structures; they cause less disturbance than drive-to-tree designs. Swing machines can often reach into the stand to perform work without driving over every area.

Operating methods can also reduce soil disturbance. Designating skid trails can manage and minimize the amount of area impacted. Shovel logging is a method of logging that limits heavy traffic to a road of felled trees. When the trees are picked up, the underlying soil is minimally affected. Similarly, CTL operations can process trees in front of the machines, building a trail mat of limbs and tops. Seixas and others (1995) found that soil compaction was reduced under the heavier layers of the slash mat.

Cumulative soil disturbance can also be reduced by follow-up treatments to ameliorate adverse effects. Best management practices typically call for vegetative stabilization of exposed soil that may be a sedimentation risk. Compacted areas may be subsoiled, ripped, or disked during site preparation to improve physical properties. On well-drained sites, survival and growth of loblolly pine have been positively affected by subsoiling treatments. On wet sites, bedding can create drier planting sites where harvesting has resulted in raised water tables (Aust and others 1998).

5.4 Accessibility to Various Ownership Groups

Forest operations accessibility depends primarily on economic viability. Economic viability, in turn, depends on whether the perceived value of the treatment exceeds the costs of implementation. A thinning, for example, may not have a short-run positive cash flow, but the increased value of the residual stand is expected to yield a profit over the rotation. Similarly, a landowner may realize no tangible return from creating a wildlife opening, but the intangible benefit of viewing wildlife may be deemed greater than the incurred costs. A prescription to achieve a given management objective establishes a set of operating conditions, such as extraction distance, volume per acre handled, seasonal restrictions, and slope, which will determine the operating costs for a particular forest operations technology. The prescription also determines the time frame over which expenses must be amortized and the values of the anticipated outcomes.

In the context of differences among various ownership groups, economic viability of forest management is primarily affected by the selection of management regimes and variations in tract size. Thompson and Johnson (1996) profiled NIPF landowners in Virginia and identified three sub-groups: farmer-owned, other corporate, and other private individual. Bliss and others (1997) surveyed NIPF owners in the Tennessee Valley and examined differences among income, ownership size, and management activity. Differences in accessibility of forest operations technology to any of these forest ownership groups depends on whether they fundamentally differ in their management objectives or in the size and composition of their forest holdings. See Chapter SOCIO-4 for additional information on the management objectives of various ownership groups.

Tract size may be the most important factor affecting economic viability of management activities. Row (1978) notes that the diseconomies of small tract size may reduce the willingness of landowners to invest in forest management. In the Virginia survey (Thompson and Johnson 1996) about 11 percent of the NIPF holdings were in tracts less than 10 acres and 40 percent were in tracts less than 50 acres. The smallest average tract size was in the natural pine management type.

Generally, an economy of scale is realized by spreading fixed costs of ownership and management over more units of output (Cubbage 1983). In forest management, many costs must be recovered through value generation at some point in the management regime. For example, fire protection, boundary maintenance, and administration and planning are costs that vary little with tract size. In harvesting, costs for moving and planning accrue without respect to tract size. Greene and others (1997) analyzed the effect of tract size on harvesting costs. Their assumptions were based on a survey of recent timber sale volumes and tract sizes in Georgia. Figure 5 illustrates estimated production costs for three alternative systems using cost equations derived from the simulation analysis. Note that above 20 acres, all of the systems have relatively flat cost curves. Conversely, below 10 acres all of the systems demonstrate significantly increasing costs.

Many studies have examined the effect of removal intensity on harvesting costs (Kluender and others 1998, Rummer 1998, Brummel 1993). Generally, there is little reduction in system productivity for prescriptions that leave a moderate residual stand such as a seed tree or shelterwood. However, when harvesting in small blocks, as with group selection or single-tree selection, productivity declines and costs increase. In selection harvests, other factors such as the effect of selection criteria on average tree size may be more important than tract size in determining economic operability.