Carbon sources and sinks in high-elevation spruce-fir forests of the Southeastern US

This paper examines carbon (C) pools, fluxes, and net ecosystem balance for a high-elevation red spruce-Fraser fir forest [Picea rubens Sarg./ Abies fraseri (Pursh.) Poir] in the Great Smoky Mountains National Park (GSMNP), based on measurements in fifty-four 20 m x 20 m permanent plots located between 1525 and 1970 m elevation. Forest floor and mineral soil C was determined from destructive sampling of the 0 horizon and incremental soil cores (to a depth of 50 cm) in each plot. Overstory C pools and net C sequestration in live trees was estimated from periodic inventories between 1993 and 2003. The CO2 release from standing and downed wood was based on biomass and C concentration estimates and published decomposition constants by decay class and species. Soil respiration was measured in situ between 2002 and 2004 in a subset of eight plots along an elevation gradient. Litterfall was collected from a total of 16 plots over a 2-5-year period. The forest contained on average 403 Mg C ha(-1) almost half of which stored belowground. Live trees, predominantly spruce, represented a large but highly variable C pool (mean: 126 Mg C ha(-1), CV = 39%); while dead wood (61 Mg C ha(-1)), mostly fir, accounted for as much as 15% of total ecosystem C. The 10-year mean C sequestration in living trees was 2700 kg C ha(-1) year(-1), but increased from 2180 kg C ha(-1) year(-1) in 1993-1998 to 3110 kg C ha(-1) year(-1) in 1998-2003, especially at higher elevations. Dead wood also increased during that period, releasing on average 1600 kg C ha(-1) year(-1). Estimated net soil C efflux ranged between 1000 and 1450 kg C ha(-1) year(-1), depending on the calculation of total belowground C allocation. Based on current flux estimates, this old-growth system was close to C neutral. (c) 2006 Elsevier B.V. All rights reserved.