Distributional shifts of regional forest communities in the eastern United States

<p>Forest ecosystems across the globe have been impacted by human activities.  Many studies have analyzed species-level responses to climate change, but much less attention has been paid to community-level responses.  Identifying and quantifying changes in forest tree communities can reveal how forest ecosystem functions and services are impacted by climate change and other stressors.  Utilizing the Latent Dirichlet Allocation (LDA) topic model method we identified 12 regional forest communities of the eastern U.S. based on data from the Forest Inventory and Analysis Program (FIA).  In addition, we detected geographical shifts of these communities over the last three decades by evaluating movement of the community centroid and spatial expansion or contraction of these communities.  Of the 12 identified regional communities in the eastern U.S., 11 significantly shifted their centroids, with movement in all directions (four eastward and four westward; three northward and four southward).  Additionally, five of 12 communities had significant changes in area, with two contractions and three expansions.  The oak-pine-hickory community of the Southern Pine-Hardwood Region had the largest changes over the study period (26.5 km dec<sup>-1</sup> shift northward and eastward and 22,361 km<sup>2</sup> dec<sup>-1</sup> decrease in area), while the cherry-oak community of the Central Hardwood Region was the most stable (4.1 km dec<sup>-1</sup> shift northward and 142 km<sup>2</sup> dec<sup>-1</sup> increase in area).  Mixed-effects models revealed that forest-related (fire frequency, basal area, and nitrogen deposition) and climate-related (temperature, precipitation, and precipitation change) predictors of changes in community distribution were significant.  Our results may reflect the resilience of forest communities to climate change, but it may also indicate a lag between climate change and regional forest community responses. </p>