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erosion rates. The type and magnitude of erosion will
<br />depend on the amount of soil exposed by management
<br />practices, the effect of management activities on infil-
<br />tration rates, slope steepness, soil type and thickness,
<br />amount and intensity of precipitation, and the type of
<br />mitigation treatments applied after the disturbance (e.g.,
<br />seeding, mulching, or ripping) (Swank et al., 1989).
<br />Studies in Colorado and elsewhere suggest that erosion
<br />rates are acceptably low when there is less than about
<br />30% bare soil (Gary, 1975; Benavides - Solorio, 2003).
<br />With respect to forest harvest, the site disturbance from
<br />tree felling is generally considered to be minor. The ac-
<br />tivities of greater concern are the transport of the logs to
<br />a central site, site preparation activities after harvest, and
<br />the network of roads, skid trails, and landings used to ac-
<br />cess the timber and remove it for processing.
<br />Logs are moved (skidded) from the stump to a land-
<br />ing by tractor, cable, aerial systems or animals. Tractor
<br />skidders may be either crawler or wheeled units. The
<br />amount of site disturbance and compaction, and hence
<br />the amount of surface erosion, will vary greatly with
<br />the type of skidding or yarding system. Crawler tractors
<br />generally cause the greatest amount of site disturbance,
<br />followed closely by wheeled skidders, but on some sites
<br />the use of wheeled skidders can result in more compac-
<br />tion than crawler tractors (Bell et al., 1974; Davis, 1976).
<br />In the case of the Deadhorse Creek study, wheeled skid -
<br />ders caused more disturbance in steeper areas because
<br />they had to drop their blade to control their downhill
<br />speed (C. Troendle, pers. comm., 2001). Cable logging
<br />systems result in less site disturbance because yarding
<br />trails are only established to the yarding tower machin-
<br />ery and the tower is placed on roads. Cable systems can
<br />be ranked in order of decreasing soil disturbance as fol-
<br />lows: single drum jammer, high lead cable, skyline, and
<br />balloon (Stone, 1973; Brown et al., 1976; Davis, 1976).
<br />Helicopters and balloons minimize site disturbance, but
<br />are more costly.
<br />Some form of site preparation may be needed to ensure
<br />adequate regeneration after timber harvest. The purpose
<br />of site preparation is to provide optimal conditions for
<br />seed or seedling survival and growth. The two main
<br />goals of site preparation are to provide a mineral seedbed
<br />and control less desirable, competing vegetation. Site
<br />preparation treatments include fire, herbicide applica-
<br />tion, slash windrow, roller chopping, or other mechani-
<br />cal techniques. Fertilizers can be applied to improve
<br />seedling establishment and growth, but fertilization in
<br />forested areas is not a common practice in Colorado.
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<br />Long -term erosion rates for forest management are
<br />typically much lower than other land uses, such as agri-
<br />culture, because the duration, frequency, and amount of
<br />disturbance are much less. Typical timber harvest activi-
<br />ties may only increase erosion rates by 0.05 to 0.25 tons
<br />ac' yr'. More intensive site preparation treatments such
<br />as slash windrowing, stump shearing, or roller chop-
<br />ping may increase soil erosion rates by several orders
<br />of magnitude to around 5 tons ac-` yr'. Once a site is
<br />revegetated erosion rates rapidly decline to pre- treatment
<br />levels (USFS, 1981; Stednick, 2000).
<br />Data from a series of ponderosa pine watersheds in
<br />northern Arizona showed that annual post - harvest sedi-
<br />ment yields ranged from 0.11 to 1.3 tons per acre in a
<br />watershed that was 31% harvested, and only 0.03 -0.3
<br />tons per acre for a watershed that was 77% harvested. A
<br />third watershed that was clearcut and had 100% of the
<br />ground disturbed produced from 0.01 to 27 tons per acre
<br />per year (Brown et al., 1974).
<br />A variety of best management practices (BMPs) are
<br />commonly applied to minimize the adverse effects of
<br />timber harvest on runoff and erosion. The amount of
<br />soil disturbance due to yarding by tractors or wheeled
<br />skidders can be greatly reduced by operator training and
<br />the careful layout of skid trails (Rothwell, 1971). Post-
<br />harvest runoff and erosion from skid trails can be greatly
<br />reduced by installing water bars, or ripping and seeding
<br />compacted areas.
<br />Riparian management zones or vegetative buffer strips
<br />are commonly applied along streams and around wet-
<br />lands and lakes to minimize the potential adverse effects
<br />of forest management on aquatic resources. A major
<br />purpose of these buffer strips is to minimize the delivery
<br />of the eroded soil to the channel network, as in most
<br />cases only a small fraction of the total erosion within a
<br />watershed reaches streams and other aquatic ecosystems
<br />(Walling, 1983). As long as the flow is not concentrated
<br />into channels, the vegetation and surface roughness
<br />within the filter strip should reduce the velocity of any
<br />overland flow (Campbell, 1984). The decreased velocity
<br />allows sediment to settle out. In most cases the undis-
<br />turbed soils in the buffer strip also will allow some or
<br />all of the overland flow to infiltrate into the soil, and this
<br />further reduces the delivery of sediment to the stream
<br />network. Vegetation filter strips are usually effective un-
<br />less the ground is near saturation, such as during spring
<br />snowmelt, or an extreme precipitation event generates
<br />so much overland flow that it overwhelms the filtering
<br />capacity of the buffer strip. Ephemeral channels can be
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