We used a new ecohydrological model, Visualizing Ecosystems for Land Management Assessments (VELMA), to analyze the effects of forest harvest on catchment carbon and nitrogen dynamics. We applied the model to a 10 ha headwater catchment in the western Oregon Cascade Range where two major disturbance events have occurred during the past 500 years: a stand-replacing fire circa 1525 and a clear-cut in 1975. Hydrological and biogeochemical data from this site and other Pacific Northwest forest ecosystems were used to calibrate the model. Model parameters were first calibrated to simulate the postfire buildup of ecosystem carbon and nitrogen stocks in plants and soil from 1525 to 1969, the year when stream flow and chemistry measurements were begun. Thereafter, the model was used to simulate old-growth (1969–1974) and postharvest (1975–2008) temporal changes in carbon and nitrogen dynamics. VELMA accurately captured observed changes in carbon and nitrogen dynamics before and after harvest. The interaction of hydrological and biogeochemical processes in the model provided a means for interpreting these changes. Results show that (1) losses of dissolved nutrients in the preharvest old-growth forest were generally low and consisted primarily of organic nitrogen and carbon; (2) following harvest, carbon and nitrogen losses from the terrestrial system to the stream and atmosphere increased as a result of reduced plant nitrogen uptake, increased soil organic matter decomposition, and high soil moisture; and (3) the rate of forest regrowth following harvest was lower than that after fire because post-clear-cut stocks and turnover of detritus nitrogen were substantially lower than after fire.
Abdelnour, Alex; McKane, Robert B.; Stieglitz, Marc; Pan, Feifei; Cheng, Yiwei. 2013. Effects of harvest on carbon and nitrogen dynamics in a Pacific Northwest forest catchment. Water Resources Research. 49: 1292-1313.