Effects of sediment deposition on aboveground net primary productivity, vegetation composition, and structure in riparian forests

Sediment filtration potential is well known as a key function of riparian forests; however, the capacity of riparian ecosystems to accumulate sediment without degradation is unclear. This study examined the effects of sediment deposition on productivity, vegetation composition, and structure in riparian forests of ephemeral streams at Fort Benning, Georgia, USA. Sedimentation occurs at Ft. Benning as a result of erosion from unpaved roads situated in sandy soils along slopes and ridges. Seven ephemeral streams were selected to represent a range of sediment deposition rates, and another two were selected as reference catchments. Within all nine catchments, paired plots were established with one plot being delineated in an upper portion of the catchment and another lower, nearer to the ephemeral stream. Upper plots of disturbed catchments showed evidence of sediment accumulation, such as buried tree bases and alluvial fans, while lower plots lacked those indications. Aboveground net primary productivity (ANPP), litterfall nutrient contents, leaf area index (LAI), species composition, and stand structure were compared within and among catchments. Decreases in litterfall, woody increment, ANPP, and LAI were observed with sediment accumulation rates near 0.2 cm yr−1, and an equilibrium response appeared to be reached near 0.5 cm yr−1. Nutrient contents of litterfall followed a similar pattern. Changes in species composition and structure were also observed. In general, reference catchments and lower plots were associated with closed overstory canopies, whereas upper plots had more overstory mortality and heavier densities of saplings and seedlings of shade-intolerant species. These results suggest that sedimentation rates commonly occurring in some riparian forests may alter productivity, structure, and composition. Consequently, riparian functions that are closely linked to forest integrity may be jeopardized as well.