Relative role of understory and overstory in carbon and nitrogen cycling in a southern Appalachian spruce-fir forest

This study investigated aboveground pools and fluxes of biomass, carbon (C), and nitrogen (N) in the overstory and understory of a southern Appalachian red spruce (Picea rubens Sarg.) -- Fraser fir (Abies fraseri (Pursh) Poir.) forest, following adelgid-induced fir mortality and spruce windthrow. Using fifty 20 m x 20 m plots, stratified by elevation (1700-1900 m), we estimated standing biomass and fluxes of all growth forms from periodic stand inventories (1998-2003), vegetation surveys, and existing or derived allometric equations. Total C and N pools and fluxes were calculated from plant- and tissue-specific C and N concentrations. Total aboveground biomass attained predisturbance values, ranging from 313 Mg·ha-1 at the lower elevations to 204 Mg·ha-1 at the upper elevations. Overstory biomass production (5650 kg·ha-1·year-1) and N uptake (11-15 kg·ha-1·year-1) exceeded earlier reported values, indicating forest recovery. Woody understory accounted for 3% of aboveground biomass, 10% of annual productivity, and 19% of total N uptake (~7 kg·ha-1·year-1). Herbaceous vegetation, which comprised only 1% of total biomass, took up 18-21 kg N·ha-1 annually, >50% of total ecosystem N uptake (37 kg·ha-1·year-1). This suggests that N-rich understory vegetation plays an important role in N cycling.