Integrating nitrogen and carbon cycling into LANDIS-II/PnET-Succession to improve forest landscape modeling: methods and sensitivity analyses

Carbon (C) and nitrogen (N) cycles are closely coupled in forested ecosystems. Studies of interactions between nitrogen cycling and forest succession have been challenging due to the broad spatial and temporal scales involved in feedbacks between successional dynamics and N cycling mechanisms. In this study, algorithms were added to a forest landscape model (LANDIS-II/PnET-Succession extension) to account for nitrogen cycling and its effects on growth and competition. In the revised model, PnET-CN-Succession, with coupled C and N cycling, forest species compete for not only light and water resources, but also for nutrients. Parameterized and validated for both deciduous and evergreen (hemlock) stands at Harvard Forest, Massachusetts, USA, along with a sensitivity analysis, the PnET-CN-Succession model replicated empirical C and N cycling patterns in deciduous and coniferous forests under a N constraint. Predicted C and N cycles were faster in deciduous forests, but more tightly coupled in evergreen forests. Analysis of harvesting scenarios demonstrated that N constraints reduced rates of biomass accumulation in early to mid-successional stages, compared to results from simulations with a version of PnET-Succession that did not simulate N cycling. Predicted changes in post-harvest species composition agreed with the observed successional patterns at Harvard Forest. By accounting for an important constraint on tree growth and competition, PnET-CN-Succession is a valuable tool to improve our ability to predict forest dynamics in response to various scenarios of forest management and global changes.