Variation in Overstory Nitrogen Uptake in a Small, High-Elevation Southern Appalachian Spruce-Fir Watershed

High-elevation red spruce (Picea rubens Sarg.) – Fraser fir (Abies fraseri (Pursh) Poir.) forests of the southern Appalachians exhibit considerable spatial heterogeneity in structure, and possibly in N uptake, because of a combination of natural disturbances and heavy fir mortality caused by infestations of the exotic balsam woolly adelgid (Adelges piceae Ratz). The objectives of this study are to determine spatial variability in tree N uptake in a small high-elevation catchment in the Great Smoky Mountains National Park, compare outcomes among calculation methods, and assess the influence of stand and landscape properties on N uptake. Tree N uptake is estimated for fifty 20 × 20 m plots in the Noland Divide Watershed (NDW). Components considered in the calculations are stem growth, foliage increment, and mortality of spruce, fir, and yellow birch (Betula alleghaniensis Britt.) from 1993 and 1998 stand inventories; throughfall N flux measured in summers 1998 and 1999; litterfall N return for 1 year in a subset of 12 plots; tissue N analyses; and atmospheric N deposition and root turnover estimates from the literature. Overstory N uptake varies spatially within NDW, with a CV of 9–41% depending on the calculation method. Variability among methods is even higher, with an almost 15-fold difference between the smallest and largest average overstory uptake estimate (5 vs. 74 kg·ha–1·year–1). Only 5 and 3 kg·ha–1·year–1 of N is sequestered in wood and foliar increment, respectively, while 36 kg·ha–1 of N returns annually as aboveground litterfall. Uptake and its components are correlated with measures of stand structure but not with elevation or aspect