The Southeastern Lower Coastal Plain wet pine flats include thousands of acres of jurisdictional wetlands that are economically, socially, and environmentally important. These highly productive forests have been intensively managed as pine plantations for the past few decades. More recently, harvesting and site preparation practices have become a concern among natural resource managers because intensive forestry practices may alter soil physical properties and site hydrology. These alterations could decrease seedling survival, growth, and future site productivity. However, the effects of soil disturbance on long-term site productivity and the effects of amelioration techniques on site hydrology are uncertain. The overall objectives of this study were (1) to characterize disturbed forest soil morphology and physical properties, (2) to assess their impact on the processes that control site hydrology and site productivity, (3) to determine effects of harvesting and site preparation on site hydrology, specifically on the overall hydrological balance and on spatial and temporal patterns of surface water storage. The study site is located in an intensively managed loblolly pine (Pinus taeda L.) plantation in the lower coastal plain of South Carolina. This study was established in winter 1991, and dry- and wet-weather harvesting treatments were installed in summer 1993 and winter 1994, respectively. Bedding and mole channel/bedding treatments were installed in both dry- and wet-harvested plots in fall 1995. Soil profiles were described for a recently disturbed, deeply-rutted area, and 2-year-old deeply-rutted and churned areas, bedded and undisturbed areas. Intact soil core samples and composite loose soil samples were collected from each morphological section for soil physical characterizations. Automated weather station and wells were used to collect continuous climatic and surface water level data since 1996. Surface water levels were monitored monthly on a 20 x 20 m grid of 1-m wells since 1992. Total groundwater heads were determined from differential piezometer measurements at high and low elevation places in each treatment plot. Soil profile descriptions and soil physical property measurements indicated that significant amounts of organic debris were incorporated into the surface horizons, and subsurface soil horizons showed significant soil structural changes and increased redoximorphic features caused by soil disturbance. The disturbed soil layers in recently created traffic ruts consisted of exposed and severely disturbed subsurface soils, but this layer was naturally ameliorated 2 years after the disturbance. Bedding site preparation had little amelioration effects on the physical properties of surface soil horizons because the surface horizons already had some incorporation of organic debris. Overall, the main consequence of bedding in a disturbed wet site was to increase the aerated soil volume. The bedding appeared to have little effect on disturbed subsurface horizons. Groundwater head in the study site was constantly higher than -25 cm during the study period, which caused groundwater inflow when the surface water level was low. Frequent fluctuation of the surface water level and constant water supply from the groundwater probably explain the high productivity of the study site. Results of the annual water balance showed that surface soil water storage changes were very small, and annual precipitation and potential evapotranspiration were approximately equal. Silvicultural practices and minor topography on the study site had significant effects on the water balance because they influenced surface water level. Surface water hydraulic gradient evaluation and multivariate cluster analysis indicated that micro-site hydrology and water flow patterns were significantly altered by wet-weather harvesting and bedding site preparation, but overall site hydrology was not altered. Evaluation of predicted surface water level indicated that micro-topography and precipitation patterns had significant influences on surface water levels during the site establishment period. These results revealed that the hydrologic components of wetland delineation are complex in the wet pine flatwoods. / Ph. D.

Mid-rotation fertilization and vegetation control are some of the most common silvicultural treatments in loblolly pine (Pinus taeda L.) plantations in the southeastern United States. Competing vegetation is commonly thought to sequester fertilizer nitrogen (N) and reduce the potential growth response to a mid-rotation fertilization treatment. This experiment aims to identify what proportion of applied N fertilizer is retained in the crop tree pine foliage, and the degree to which understory vegetation is competing for this resource. Our mid-rotation loblolly pine plantation received an application of 15N fertilization (urea 365 kg/ha, at 46% N by weight, i.e. 168 kg/ha of N) and a portion of plots received an understory vegetation control (basal spray application of triclopyr; 13.6% active ingredient) treatment either before fertilization or not at all. One-year post-fertilization, 15N contents within pine foliage, leaf fall/leaf litter, forest floor, and soil were measured, as was competing vegetation presence. There was significant variation in applied nitrogen acquisition among the different ecosystem components measured, with 0-15 cm soils retaining a majority at 32-37% added 15N. Differences in fertilizer N acquisition in pine foliage between plots with and without understory vegetation control was marginally significant (p = 0.06) with pine foliage in plots without understory vegetation capturing greater 15N (4.3% greater). Red maple (Acer rubrum) and oak species (Quercus spp.) were the most common competitors but neither had a uniquely pronounced effect on pine nitrogen sequestration. My data indicate that increasing competition reduces fertilizer N foliar concentrations in crop pine trees but at a modest rate and equally across species groups. An unrefined threshold determining when fertilizer N capture in crop pine trees was affected was found at 3.1 m2/ha of competing vegetation basal area. This site will continue to be monitored over time to assess fertilizer N retention in loblolly pine each year after fertilization and evaluate the fertilizer N capture within competing vegetation. / Master of Science / Some of the most prevalent management practices for mid-rotation (age 15, i.e., roughly halfway through a crop cycle) loblolly pine (Pinus taeda L.) plantations in the southeastern United States are fertilization and vegetation control. Nitrogen (N) is consistently one of the most limiting factors to productivity. The addition of N via fertilization is therefore a common forestry practice. However, when a stand is fertilized, the added resource is partitioned and cycled throughout the ecosystem. It is presumed that the amount of fertilizer N obtained by crop trees in a plantation is dependent on the level of competing vegetation (i.e., weed-trees and shrubs) present on site. Controlling competing vegetation prior to fertilization may therefore be warranted under certain conditions. To date, the amount of competing vegetation where it begins to impact fertilizer uptake by the crop tree is unknown. This study aims to elucidate this competing vegetation threshold to better inform mid-rotation management of loblolly pine plantations. This study examined applied fertilizer N capture in ecosystem components with varying levels of understory vegetation, and found more fertilizer N in pine foliage when understory vegetation was completely removed prior to fertilization. No single understory hardwood weed species had a uniquely strong influence on crop tree productivity uptake. Plots that ranked in the upper third in competing vegetation presence did have significantly less foliar fertilizer N in the pine crop trees. Additional replication of this study would be necessary to determine a universal threshold of competing vegetation which would trigger the removal of competing vegetation prior to fertilization.