Hillslope nutrient dynamics following upland riparian vegetation disturbance
We investigated the effects of removing nearstream Rhododendron and of the natural blowdown of canopy trees on nutrient export to streams in the southern Appalachians. Transects were instrumented on adjacent hillslopes in a first-order watershed at the Coweeta Hydrologic Laboratory (35°03'N, 83°25'W). Dissolved organic carbon (DOC), K+, Na+ Ca2+, Mg2+, NO3-N, NH4+-N, PO43--P, and S042- , were measured for 2 years prior to disturbance. In August 1995, riparian Rhododendron on one hillslope was cut, removing 30% of total woody biomass. In October 1995, Hurricane Opal uprooted nine canopy trees on the other hillslope, downing 81% of the total woody biomass. Over the 3 years following the disturbance, soilwater concentrations of NO3--N tripled on the cut hillslope. There were also small changes in soilwater DOC, SO42- Ca2+, and Mg2+. However, no significant changes occurred in groundwater nutrient concentrations following Rhododendron removal. In contrast, soilwater NO3--N on the storm-affected hillslope showed persistent 500-fold increases, groundwater NO3--N increased four fold, and streamwater NO3--N doubled. Significant changes also occurred in soilwater pH, DOC, SO42-, Ca2+, and Mg2+. There were no significant changes in microbial immobilization of soil nutrients or water outflow on the storm-affected hillslope. Our results suggest that Rhododendron thickets play a relatively minor role in controlling nutrient export to headwater streams. They further suggest that nutrient uptake by canopy trees is a key control on NO3--N export in upland riparian zones, and that disruption of the root-soil connection in canopy trees via uprooting promotes significant nutrient loss to streams.