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

Abstract

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