Spatial and temporal heterogeneity of nitrogen, carbon and vegetation in the Wisconsin River and its floodplain, Wisconsin, USA.

摘要

Understanding the roles of spatial and temporal heterogeneity in ecosystems is a frontier in ecology. In fluvial systems, flow regime generates spatial and temporal heterogeneity, yet many ecological consequences of this heterogeneity are not well studied. I examined the role of flow regime and spatial heterogeneity on aspects of biogeochemical cycling, soil organic matter and vegetation in reaches of the Wisconsin River and its floodplain. Lateral connections between the river and floodplain are largely intact, but flow is highly regulated by mainstem dams. On vegetated and unvegetated riverine sandbars, I measured hyporheic water chemistry and potential denitrification in months with varying river flow rates. Vegetation did not facilitate potential denitrification in Wisconsin River sandbars, but unvegetated sandbars were hotspots of nitrate processing. Unvegetated sandbars shifted from nitrate sources to sinks as the hydrograph declined. However, nitrate sinks were more prominent in a high discharge year, while sources predominated in a low discharge year. Anthropogenic modifications to the flow regime result in reduced flood peaks, which may limit nitrogen retention in this system.; In the floodplain, I used geostatistics to analyze the spatial pattern and content of soil organic matter along a flood frequency gradient. In all floodplain sites, a cyclic spatial pattern of soil organic matter was present. These repeating patterns may be generated by microtopography, specifically by differences in litter composition between ridges and swales. Overbank flood events appeared not to affect sediment sorting across the flood frequency gradient.; Lastly, I assessed how flow regime, landscape configuration and landscape quality predicted the presence, abundance and productivity of invasive shrubs (Lonicera and Rhamnus spp. and the native Zanthoxylum americanum) in the floodplain. Habitat configuration and quality associated with forest edge habitats were important predictors of invasives. Flooding decreased the presence of Lonicera and Rhamnus, but flood frequency increased the presence, abundance and productivity Z. americanum. Invasives were more also frequent in reaches where flooding was restricted. Overall, my research indicates that flooding and anthropogenic modifications to flow regimes influence spatial and temporal patterns of biogeochemical cycling, soils, and vegetation patterns in river-floodplain ecosystems.