Long-term monitoring reveals forest tree community change driven by atmospheric sulphate pollution and contemporary climate change

Diversity and Distributions published by John Wiley & Sons Ltd Aim: Montane environments are sentinels of global change, providing unique opportunities to assess impacts on species diversity. Multiple anthropogenic stressors such as climate change and atmospheric pollution may act concurrently or synergistically in restructuring communities. Thus, a major challenge for conservation is untangling the relative importance of different stressors. Here, we combine long-term monitoring with multivariate community modelling to estimate the anthropogenic drivers shaping forest tree diversity along an elevational gradient. Location: Camels Hump Mountain, Vermont, USA. Methods: We used Generalized Dissimilarity Modelling (GDM) to model spatial and temporal turnover in beta diversity along an elevational gradient over a 50-year period and tested for spatiotemporal shifts in density and elevational distribution of individual species. GDMs were used to predict community turnover as nonlinear functions of changes in elevation, climate and atmospheric pollution. Results: We observed significant shifts in elevational range and density of individual species, which contributed to an overall reduction in the elevational gradient in beta diversity through time. GDMs showed the combined effects of sulphate deposition and temperature as drivers of this temporal reduction in beta diversity. Spatiotemporal changes differed among species, with shifts observed both up- and downslope. For example, in a reversal of a previous upslope range contraction, red spruce (Picea rubens Sarg.) increased in density and shifted downslope since the 1990s, occupying warmer, drier climates. Main conclusion: Our results demonstrate that global change is impacting the stratification of forest tree diversity along elevational gradients, but the responses of individual species are complex and variable in direction. We suggest abiotic drivers may directly impact individual species while also indirectly altering species interactions along elevational gradients. Our approach modelling the drivers of compositional turnover quantifies the rate and amount of change in beta diversity along environmental gradients and serves as a powerful complement to studying species-specific responses

Insights from long-term ecological monitoring: determining the impacts of contemporary climate change and atmospheric pollution on forest communities

Temperature and atmospheric pollution have been shown to influence forest community composition, as well as the productivity and distributions of individual tree species. Empirical studies however, demonstrate conflicting individualistic and community level responses regarding elevational range shifts and often ascertain the importance of environmental drivers for individual species. In this study, we seek to investigate patterns of biodiversity along environmental gradients by modelling community turnover, or the rate of change in beta diversity, which can be quantified using pairwise changes in species composition (e.g. dissimilarity). We couple Generalized Dissimilarity Modelling (GDM) with a long-term forest tree inventory (years 1965-2015) on Camels Hump, VT to (1) characterize how the elevational gradient in forest community composition has shifted over a 50-year period, both in terms of beta diversity and the relative distribution of individual species, and (2) determine the relative importance of atmospheric pollution and climate change as drivers of temporal shifts in forest communities. The rate of compositional turnover along the elevational gradient was highest between 800-900m elevation, the area encompassing the boreal-deciduous ecotone, and decreased in lower and higher elevations. While the pattern of turnover was consistent over time, the total magnitude of community change was significantly reduced in the last census, reflecting a more homogeneous forest community. Notably, mid-elevation forests have shifted from high diversity with few dominant species to lower diversity dominated by red spruce, balsam fir, and American beech. At low elevations, red spruce first contracted its range (1965 to 1990) but has recently increased in abundance and shifted downslope occupying warmer, drier climates. We provide evidence to support the realized niche expansion of red spruce in low elevations, possibly as a result of competitive release due to sugar maple decline. Temporal models showed S pollution and mean annual temperature are significant drivers of temporal changes in forest communities, which corroborates previous findings of climate effects on northeastern forests, as well as the long-legacy effects of acid deposition