Productivity estimates of nitrogen-saturated forests with different harvesting histories: An adaptation of the PnET-CN model

            Ecosystem nitrogen balances are coupled tightly with processes that control terrestrial productivity and carbon storage.  This study examined productivity of deciduous hardwood forests in the Central Appalachian region that have been subjected to different types of harvest strategies. The study sites, watersheds located in the Fernow Experimental Forest in Parsons, WV, have been characterized as nitrogen-saturated forests. Using dendrochronological techniques, we measured greater aboveground net primary productivity (ANPP) 40 years after a clear-cut harvest than ANPP in a ~100 year old watershed (p<0.05). Our goal was to validate an ecosystem productivity model against ANPP measurements so that we could use the model as a tool to estimate net ecosystem productivity (NEP). The PnET-CN model includes routines for harvest events but could not be validated for the experimental watersheds in the Fernow because of ecosystem process differences associated with nitrogen saturation. We compared the variation of localized foliar and soil nitrogen to the range of nitrogen concentrations observed in the surrounding region and conducted a sensitivity analysis of PnET-CN parameters that might be associated with such variation. We modified the N balance equations in the PnET-CN model to more accurately describe the nitrogen fluxes in this forest ecosystem by including changes to nitrogen retention, nitrification rates, inorganic nitrogen pools, and relative foliar growth. We validated the new model, PnET-CN_sat, against independent ANPP estimates so that we could more confidently predict NEP and carbon storage potentials of nitrogen-saturated forests with different harvest histories.