Modeled vegetation community trajectories: Effects from climate change, atmospheric nitrogen deposition, and soil acidification recovery
Forest understory plant communities in the United States are diverse and often sensitive to changes in climate and atmospheric inputs of nitrogen (N) caused by air pollution. Recently, understory vegetation response functions (expressed as Probability of Occurrence of Plant Species model for the United States [US-PROPS]) have been developed based on observations of forest understory and grassland plant species presence/absence and associated abiotic characteristics. We investigated methods development and application of US-PROPS to evaluate the potential impacts of atmospheric N deposition, and climate change on forest ecosystems of Great Smoky Mountains National Park (GRSM) located in the southern Appalachian mountains of the United States. Species response and critical loads (CLs) of N deposition were determined. Individual and synergistic effects of expected future air temperature and soil pH on habitat suitability and CLs of N were also evaluated. CLs were estimated to be low (< 2 kg N/ha/yr) under current and expected future conditions across broad regions of GRSM and these CLs were exceeded at a generally similar spatial extent. Northern hardwood, yellow pine, and chestnut oak forests were among the most N-sensitive vegetation map classes found within GRSM. Future scenarios of increases in soil pH and air temperature generally led to CLs that were unattainable. This suggests that the original community composition may be unattainable even if N deposition were reduced to zero. The methodology for establishing regional CLs for understory species is transferable to other National Parks any location for which the US-PROPS models are applicable and can be informed by the required input data.