Coweeta LTER

Human Disturbance Decreases Ability of Forested Ecosystems to Retain Nitrogen

Ecosystem nitrogen dynamics gain international attention as the demand for clean drinking water increases and nitrogen saturation threatens stream quality. Investigators at the Coweeta Hydrologic Laboratory LTER site have recorded nitrogen dynamics of streams and precipitation since 1972. We analyzed these data for N dynamics in an extensive network of mature mixed hardwood-covered watersheds in the southern Appalachian Mountains of North Carolina to assess trends and dynamics of N in baseline ecosystems. We also assessed watershed N dynamics’ responses to management practices such as clearcutting, selection cutting--with and without tree removal, conversion of native hardwood to coniferous forest, and old-field succession, with natural disturbances on reference watersheds. Reference watersheds were characterized as highly conservative of inorganic N. However, reference watersheds appeared to be in a transition phase between stage 0 and stage 1 of watershed N saturation. Evidence for stage 3 of N saturation, where the watershed is a net source of N rather than a N sink, was found for the most disturbed watershed at Coweeta. Compared to other intensive management practices, prescribed burning had little effect on stream water NO3- concentrations, and stream NO3- losses associated with natural disturbances are small and short-lived.

Human Activities Affect Biodiversity in Southern Appalachian Streams

Human activities, particularly habitat destruction and species introductions, are resulting in increased homogenization of once unique biogeographic regions. In southeastern United States streams, highland streams flow into lower elevation systems which are often inhabited by more widespread, generalist fish species adapted to warmer, more turbid, fine-sediment-rich, and nutrient-rich conditions. Deforestation and other common land use practices degrade stream habitats and reduce habitat diversity, which is often correlated with taxonomic and ecological diversity.

In a 10-year long study, we assessed the relative importance of environmental variation, interspecific competition for space, and predator abundance on assemblage structure and microhabitat use in a stream fish assemblage inhabiting Coweeta Creek, North Carolina. We observed a total of 16 species (e.g. rosyside dace, mottled sculpin, longnose dace) during our investigation, and the total number of species was significantly higher in drought than in pre-drought samples. Potential predators (e.g. rainbow trout, rock bass, and creek chub) were present in the site in only 9 of 16 seasonal samples and never were abundant. Our results demonstrate that variability in both mean and peak stream flows had a much stronger effect on the structure and use of spatial resources within this assemblage than either interspecific competition for space or predation.