Visually determined soil disturbance classes used as indices for forest harvesting disturbance

Visual estimates of soil and site disturbances are used by foresters, soil scientists, logging supervisors, and machinery operators to minimize harvest disturbances to forest sites, to evaluate compliance with forestry Best Management Practices (BMPs), and to determine the need for ameliorative practices such as mechanical site preparation. Although visual estimates are commonly used by field personnel, the actual relationships of visually determined soil disturbance classes to various soil physical properties and site characteristics have not been determined. The purpose of this investigation was to evaluate if visually determined soil disturbance classes are related to quantitative soil and site properties that are known to influence soil productivity and hydrologic function. Several types of quantitative data were evaluated within the soil disturbance classes: static data (bulk density, saturated hydraulic conductivity, total, capillary, noncapillary pore space, and soil roughness) and dynamic data (mechanical resistance, volumetric soil moisture, subsurface water table depth). All data were collected from a long-term forest productivity study located in the Coastal Plain of South Carolina. The study is a randomized complete block design with two harvest disturbance levels (wet-weather harvest vs. dry-weather harvest) and a maximum of five site soil disturbance (SD) classes. Disturbance classes included undisturbed (SD0), compressed but not rutted (SD1), rutted (SD2), deeply rutted (SD3), and churned (SD4). Analyses revealed that three static variables (soil bulk density, saturated hydraulic conductivity, macropore pore space) and two dynamic variables (depth of the subsurface water table and mechanical resistance) were significantly related to disturbance. Although undisturbed and compressed areas generally were affected less than the more severe disturbance classes, the three most severe disturbance classes, churned areas, deeply rutted areas, and rutted areas were not different from one another. Thus, it appears visual disturbances do not necessarily equate to site damage. The overall implications are that visually determined soil disturbance classes have merit as indices of some soil and site changes, but they should not be equated to soil damage categories.