Effects of sedimentation on productivity, nutrient cycling, and community composition in riparian forests associated with ephemeral streams at Ft. Benning, GA, USA

Abstract

The societal value of riparian forests in improving water quality by trapping sediments and pollutants is well understood. However, it is not clear how sedimentation impacts the overall integrity of riparian systems. In an effort to understand the consequences of sedimentation in riparian forests, a 5-year study was performed at Ft. Benning, GA. The specific objectives were to quantify above- and belowground forest productivity along a gradient of sedimentation rates and disturbance classes, determine how biogeochemical processes and nutrient circulation in riparian forests are affected by sediment deposition, and examine how differing rates of sedimentation drive changes in the understory, midstory, and overstory canopy layers in riparian forests. Significant declines in litterfall, woody biomass production, fine root production, LAI, and shrub biomass were found in response to as little as 0.1-0.4 cm yr-2 sedimentation. Soils in plots with the greatest sedimentation rates (i.e. highest sediment disturbance) were lower in nutrients, CEC, temperature, and moisture, and higher in bulk density, percent sand, and pH. Highly disturbed plots also had lower decomposition rates, net N mineralization rates, microbial C and N, and had higher NUE, PUE, and nutrient resorption proficiency and efficiency. A marked decline in nutrient circulation rates corresponded to current sedimentation rates between 0.1-0.4 cm yr-1. Measures of community composition revealed proportionally more annual, exotic, and upland species in the understory of plots receiving high sediment deposition (HD) compared with reference (RF) plots. Shade-intolerant and N-fixing species had greater biomass in the midstory of HD plots; suggesting an increase in early-seral and opportunistic species. Increased overstory mortality was associated with high sedimentation rates, though increased understory light levels associated with overstory mortality was not the main driver of species changes in the understory. Edaphic factors, such as soil texture, moisture, and temperature, correlated to species changes in all three forest layers, suggesting that changes in soil physical properties associated with sedimentation are substantial enough to drive changes in the entire forest community. Decreased sediment retention (measured as increased sediment export) was found with increasing sedimentation rates. This study demonstrated that sedimentation rates are linked to declines in forest productivity, nutrient circulation, and changes in the vegetative community of riparian forests. Increased sedimentation rates may compromise the water purifying function of these forests.