Effects of climate change and atmospheric nitrogen deposition on forest understory vegetation communities in selected U. S. national parks

Abstract

Climate change and elevated atmospheric nitrogen (N) deposition can have adverse effects on biodiversity, leading to acidification of soil and surface waters, reductions in tree growth and survival, alteration of aquatic food webs, and changes in biodiversity across landscapes. The objectives of this study were to implement the United States Probability of Occurrence of Plant Species (US-PROPS) model to evaluate species-level CLs and responses to N deposition under three potential future climate scenarios at eight national parks located across the continental United States. Model application results were used to make comparisons of CLs and magnitudes of change in species responses across parks. The eight NPS units included in this assessment were Acadia NP (ACAD), Glacier NP (GLAC), Grand Teton NP (GRTE), Joshua Tree NP (JOTR), Rocky Mountain NP (ROMO), Sleeping Bear Dunes National Lakeshore (SLBE), Theodore Roosevelt NP (THRO), and Yosemite NP (YOSE). Critical loads of N deposition and changes in occurrence probability were determined for current climate conditions and three potential future climate scenarios to reflect change in occurrence probability under simulated air temperature increases of +0.5, +1.0, +1.5 ?C. Under a future air temperature scenario of +0.5 ?C, occurrence probability of the most sensitive species to air temperature changes at ACAD and SLBE decreased by at least 50% for approximately half of the map classes at ACAD and SLBE. Four other parks (GLAC, ROMO, THRO, and YOSE) showed a 50% reduction in occurrence probability for the most sensitive species at approximately half of the map classes under a warming scenario of +1.0 ?C. Some species may either be extirpated or nearly extirpated (i.e., more than 95% reduction in occurrence probability) with a 1.5 ?C future increase in air temperature at ACAD, GLAC, SLBE, and YOSE. Critical loads of atmospheric N deposition were less than 5 kg N/ha/yr for the vast majority of vegetation species and map classes included in this study. Protection of the most sensitive species within vegetation classes typically requires N deposition to be lower than 2.5 kg N/ha/yr. Median CLs across map classes were nearly always indeterminable under climate change scenarios (i.e., maximum occurrence probability under ambient climate was no longer attainable even if annual N deposition was reduced to zero). This indicates that at least one-half of the map classes at each NPS unit are expected to experience reductions in occurrence probability from climate change that cannot be compensated for by reductions in N deposition. Although these adverse effects of climate change are expected to outweigh benefits of decreased N pollution, reductions in N deposition would still be expected to benefit vegetation species that experience deposition above the designated CL.