Relative role of understory and overstory in carbon and nitrogen cycling in a southern Appalachian spruce–fir forestAES Publication 7863. Utah Agricultural Experiment Station, Utah State University, Logan, Utah.

Abstract

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 × 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.