Forest ecosystems across the U.S. have been impacted by climate change, resulting in species-level tree migration. However, little is known about how forest communities—groups of commonly co-occurring species—are responding to climate change. Here, we assess changes to forest communities at three scales: (1) within community changes in species composition, (2) individual community spatial shifts, and (3) changes across all communities. We used the Latent Dirichlet Allocation (LDA) method to identify forest communities in a dataset from the USDA Forest Service’s Forest Inventory and Analysis Program with over 70,000 forest inventory plots across the Eastern U.S., spanning three decades of change. We analyzed species composition changes within communities and assessed community-level spatial shifts over the last three decades to quantify individual community responses to climate change. We utilized the frequency distribution of forest communities across various climate conditions to predict where the communities were expected to migrate during the study period and compared the climate-predicted shifts to the observed community shifts. Changes across all communities were measured with Jensen-Shannon Distance and modeled as a function of climate and non-climate variables using generalized linear mixed-effects models (GLMMs).
Results/Conclusions
We identified 12 regional forest communities of the Eastern U.S. with LDA, which varied in their stability of species composition over the study period. All communities experienced a relatively short yet significant shift in their spatial distribution (median = 8.0 km dec-1). Climate change failed to predict the spatial shifts of the communities. GLMMs revealed historic climate and shifts in seasonal temperature variability were significant predictors of change across the communities. Although forest communities have shifted their distributions over the last three decades, climate change outpaced the rate of community migration. Slow response to climate change may indicate a lag between climate change and responses of forest communities, leading to the inability of forest communities to change at a rate commensurate with changing climate.