Forest Harvesting Disturbance and Site Preparation Effects on Soil Processes and Vegetation in a Young Pine Plantation

The favorable growth of young loblolly pines (Pinus taeda L.) in response to controlling non-crop vegetation is well documented. However, the beneficial effects of non-crop vegetation on soil quality, nutrient cycling, and biodiversity have not been thoroughly explored. A study was conducted to determine the effects of harvesting-induced soil disturbance, bedding and chemical vegetation control on soil processes and productivity, and to characterize the effects of silvicultural treatments on non-crop vegetation dynamics. Study plots were established on a wet pine flat on South Carolina's lower Coastal Plain. Treatments included a range of 5 disturbance classes (undisturbed, compression tracked, shallowly rutted, deeply rutted and churned), two site preparation treatments (flat planted and bedded) and a gradient of vegetation control (no vegetation control, operational-level weed control and complete weed control). Soil disturbances had relatively small effects on soil quality. Soil compaction reduced soil aeration, but this condition was fully ameliorated by bedding. Churning did not degrade the soil physical environment in any measureable way, largely because slash and litter were incorporated into the surface soil. Bedding and churning increase soil biological activity, which increased nitrogen mineralization in excess of pine demand. When non-crop vegetation was chemically controlled, mineralization rates increased due to increases in surface soil temperatures. With less vegetation on the site, the amount of nitrogen sequestered was less, furthering the potential for nitrogen loss by leaching or denitrification. Soil quality improved somewhat with increasing levels of non-crop vegetation biomass; however, these beneficial effects were marginal during two years of operational vegetation control. The majority of dominant species on undisturbed treatment areas were woody, and soil disturbance, including bedding, reduced the proportions of these species. Silvicultural treatments had little effect on the prevalence of hydrophytic species on these wetland study sites. From a forest management point of view, for this site type, it appears that much is gained by reducing competition from non-crop vegetation; the benefits of controlling the density of non-crop vegetation for encouraging early pine growth are clear. While non-crop vegetation slightly improved system function by sequestering available nitrogen, increasing diversity and increasing soil quality, these improvements do not appear to be critical to forest function on these inherently high-quality sites.