Shifts in forest composition in the eastern United States.
Forest ecosystems in the United States (U.S.) are facing major challenges such as climate change, exotic species invasions, and landscape fragmentation. It is widely believed that forest composition in the eastern U.S. is transitioning from shade-intolerant, fire-tolerant species to shade-tolerant, fire-intolerant species, but most evidence is anecdotal or localized. No comprehensive studies exist to quantify the shifts in forest composition across multiple genera at a regional scale. Here, we examined the genus-level compositional changes in eastern U.S. forests to: (1) quantify the extent and magnitude of this transition, and (2) assess the influence of shade and fire tolerance traits on abundance change. Genus-level data were compiled from the Forest Inventory and Analysis (FIA) database across 37 states in the eastern U.S. for the last three decades. We analyzed shifts in forest composition with three metrics - stem density, basal area, and importance value - for 10 of the most abundant genera (Acer, Betula, Carya, Fraxinus, Nyssa, Pinus, Populus, Prunus, Quercus, and Ulmus). In addition, we estimated density-weighted fire and shade tolerances for each genus using species-level published data, assessed the shifts in spatial patterns of these traits, and analyzed the associations between these traits and county-level abundance changes. In general, Acer, Fraxinus, Pinus, and Prunus increased in abundance during the study period. Acer experienced the largest increase in abundance across the study area. In contrast, Carya, Nyssa, Quercus and Ulmus decreased in abundance in the majority of the study region, with Quercus having the largest and most extensive decline. Although density-weighted shade and fire tolerances were correlated at the genus level, shade tolerance was a better predictor of genus-level abundance change than fire tolerance. Traits of fire and shade tolerance are not always interchangeable when used to predict the dynamics of a genus, and management decision making based on traits should focus at the species level when possible. Our analyses provide evidence that forest composition has shifted in the last three decades in the eastern United States across multiple genera, and the shifts are more closely related to species' shade tolerance than fire tolerance.