Distributional shifts of regional forest communities in the eastern United States

<p><strong>Aim</strong></p>

<p>Forest ecosystems have been impacted by climate change, resulting in species-level tree migration.  However, little is known about climate change impacts at the community level.  Here, we assess changes to forest communities at three scales: within community changes in species composition, individual community spatial shifts, and changes across all communities.</p>

<p><strong>Location</strong></p>

<p>Eastern United States</p>

<p><strong>Major taxa studied</strong></p>

<p>Forest tree species</p>

<p><strong>Methods</strong></p>

<p>Using a region-wide forest inventory dataset from the USDA Forest Service’s Forest Inventory and Analysis Program with over 70,000 plots, we identified forest communities using the Latent Dirichlet Allocation method. 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 distribution of forest communities across historic climate conditions to predict where the communities could migrate during the study period and compared climate-predicted shifts to observed community shifts.  Changes across all communities were modeled as a function of climate and non-climate variables using generalized linear mixed-effects models.</p>

<p><strong>Results</strong></p>

<p>We identified 12 regional forest communities of the eastern United States, 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<sup>-1</sup>).  Historic climate and changes in seasonal temperature variability were the best predictors of change across all communities. However, the distance and direction of individual community migration was poorly predicted by climate change, and the observed direction was often opposite of the predicted direction.</p>

<p><strong>Main conclusions</strong></p>

<p>Forest communities shifted their distributions over the last three decades, but climate change outpaced the rate of community migration.  Slow responses to climate change may indicate lags between climate change and responses of forest communities, leading to the inability to change at a rate commensurate with changing climate.</p>