TIMBER HARVESTING EFFECTS ON SPATIAL VARIABILITY OF SOUTHEASTERN U.S. PIEDMONT SOIL PROPERTIES

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Abstract
Site-specific forestry requires detailed characterization of the spatial distribution of forest soil properties and the magnitude of harvesting impacts in order to prescribe appropriate management schemes. Furthermore, evaluation of the effects of timber harvesting on soil properties conducted on a landscape scale improves the interpretive value of soil survey data. Questions exist regarding the extent and spatial distribution of the effects of timber harvesting on eroded soils of the Alabama Piedmont. We evaluated the impacts of clear-cut harvesting on the temporal and spatial variability of bulk density (ρb), soil strength, and water content (θg) at three sites in the Alabama Piedmont where timber was predominantly mature plantation stands of loblolly pine (Pinus taeda L.). Pre-harvest spatial variability of texture, surface horizon thickness, and soil organic carbon (SOC) within single soil mapping delineations was also evaluated. Soils were moderately to severely eroded and classified in fine, kaolinitic, thermic Typic and Rhodic Kanhapludult and Kandiudult families. Although significant increases (P < 0.05) in ρb were observed after timber harvesting for some of the trafficking class-depth interval combinations at all sites, the largest increases were observed at the moderately eroded site. Harvesting timber increased soil strength by 25.1% on the moderately eroded site, with increases occurring to a 40-cm depth in skid trails. Results suggested the degree of harvesting impacts were erosion phase dependent, with greater impacts on moderately versus severely eroded soils. Geostatistical analyses indicated that pre-harvest % clay and surface thickness were more highly spatially correlated than pre-harvest SOC, which may be related to erosion processes. Analyses also suggested harvesting slightly increased the overall spatial variablity of ρb, soil strength, and θg. These results suggest that the establishment of site-specific forest tillage zones to ameliorate compaction may be impractical to implement because of the increases in spatial variability of these properties.