The effects of timber harvest and soil disturbance on soil processes and water quality in a South Carolina blackwater swamp.

摘要

Blackwater swamps border rivers that originate in the Coastal Plain and have a very distinctive "tea-colored" hue resulting from high concentrations of dissolved organic acids in the water column. These bottomland systems are an integral component of commercial forests in the Southeast. This project was one of three projects designed to document functional changes in critical wetland processes following timber harvest in a blackwater bottomland ecosystem. This particular study focused on soil and water quality, while the other projects addressed vegetative productivity and herpetofaunal populations (Pavel 1993, Phelps 1993). Two operational harvest treatments were installed between January 1991 and November 1991. The treatments were: (1) timber cutting and removal by rubber-tire skidder (skidder), representing silvicultural soil impact; (2) helicopter forwarding (helicopter) representing no soil impact to the site, and; (3) a well-documented undisturbed site serving as a reference for the harvest treatments. The objective of this project was to investigate the relative effects of aerial- and ground-based timber harvesting systems on selected indices of blackwater bottomland ecosystem functions. Indices of ecosystem function were chosen for their sensitivity to reflect site changes and for their proven utility to provide repeatable results.; Cotton strip decomposition assay indicated that organic matter decomposition increased with the magnitude of site disturbance. The decomposition response was attributed mainly to soil temperature, with periodic soil saturation ameliorating this response.; Increased levels of nutrients were found in shallow ground water samples from the more disturbed treatments due to the accelerated decomposition and lack of soil or vegetative uptake.; Harvest treatment areas accumulated greater amounts of sediment from the surface water column due to the enhanced surface roughness from thick herbaceous regrowth and logging debris.; Soil bulk density values were lower across all treatments after harvest, mainly due to the "liquid state" of the soil at the time of treatment imposition, the fluctuation of the water table in the soil, and the deposition of low density sediments on the soil surface.; The overall analysis of variance for soil nitrogen and phosphorus indicated a significant treatment effect on soil nitrogen levels but not for phosphorus. The higher levels of soil nitrogen in the disturbed treatments is attributable to the organic matter that accumulated under the anaerobic conditions at the 7.5-15.0 cm level in the soil profile.; The water table was elevated in the harvested units as compared to the undisturbed controls, mainly due to the lower evapotranspirational capacity of the herbaceous and woody regrowth, as compared to the tree overstory. Water tables were elevated in the skidder treatment, as compared to the helicopter treatment but differences were not significant. This phenomenon occurred even though herbaceous and understory biomass was greater in the skidder treatment than the helicopter treatment. This implies that the evapotranspirational capacity of the helicopter treatment plots was not greater than that of the skidder treatment plots and/or attributable to significantly higher surface storage capacity and decreased soil porosity in the skidder treatment relative to the helicopter treatment. The higher surface storage capacity is an effect of the ruts created during traffic by the skidder.