Decadal Change in Forest Carbon Stocks in the Delaware River Basin

Forest carbon dynamics at different scales are controlled by different factors, which may alter the forest structure and processes. Long-term measurements of biomass and soil carbon stocks in a nested watershed DRB provide good opportunity for monitoring forest carbon dynamics at multiple scale, calibrating a regional forest process model, and exploring the carbon-water interaction. The Delaware River Basin (DRB) is an ideal watershed for forest carbon cycle research because the basin features diverse forest types and land-use history, and includes physiographic provinces representative of the eastern US. In 2001-2003, the Delaware River Basin Monitoring and Research Initiative established 66 forest plots in three intensive monitoring research sites (nested sub-watersheds in DRB) using Forest Service inventory protocols and enhanced measurements. Mean biomass carbon density was 235.7 × 93.7 Mg C ha-1 in French Creek, 193.2 × 83.9 Mg C ha-1 in Delaware Water Gap, and 264.7 × 74.4 Mg C ha-1 in Neversink River Basin. Soil carbon density (including forest floor and mineral soil to depth of 20 cm) was 80.1 Mg C ha-1, 85.4 Mg C ha-1, and 88.6 Mg C ha-1, respectively. These plots were revisited and re-measured in 2012-2013. In French Creek, where the biomass remeasurement was conducted in fall 2012, results show that, the average biomass carbon density increased by 17.9 Mg C ha-1 over the past decade. Changes in live biomass (live tree, sapling, shrub, herb etc.) and dead biomass (dead tree, coarse woody debris, litter, duff etc.) contribute equally to the total biomass change. However, in a few plots total biomass carbon density decreased by 7.6 to 43.1 Mg C ha-1 due to disturbance from logging or invasive species. Based on the preliminary result, the different effects of climatic, topographic and geological factors on carbon stocks could be detected among the small watersheds. But within a watershed, changes in biomass and soil carbon stocks may depend mainly on forest dynamics, disturbances and edaphic factors. The soil carbon and nitrogen contents in the plots were measured by two different sampling methods, the standard soil core method used in the Forest Service Forest Health Monitoring Program and the quantitative soil pit method which can precisely measure soil bulk density particularly for rocky soils. The biomass and soil carbon stocks data in DRB will be used to calibrate PnET model, extrapolate into a watershed scale, estimate the carbon and nitrogen output in the ecosystem and then assesse the effects of forest C uptake on water quality and water balance in the Delaware River Basin.