The tree species composition of eastern North American forests has changed such that oaks are increasingly replaced by other species, threatening the ecosystem services these forests provide. Understanding the drivers of this change is critical for the development of effective management strategies. There are at least three different explanations for why and how eastern forests have changed, making it unclear which mechanism(s) is at play. First, fire exclusion has led to fuel beds that are less available and receptive to fire, which decreases the likelihood of burning and allows the encroachment of fire-sensitive tree species, relative to fire-adapted oaks. Second, atmospheric nitrogen deposition has increased soil nitrogen availability, which favors tree species that associate with arbuscular mycorrhizal fungi, relative to oaks, which associate with ectomycorrhizal fungi. Third, invasive earthworms increase soil nutrient availability, which increases the competitive ability of tree species that grow rapidly and have nutrient-rich litter, relative to slower-growing, nutrient-conservative oaks. In each case, a positive feedback loop propagates forest change, while simultaneously resisting a return to the former state. Our goal was to reconcile these distinct views of forest change into one cohesive framework. We sought to understand to what degree these cooccurring changes were related and involved the same tree species. First, we analyzed the dominant tree species of eastern North America and how they relate to fire adaptation, mycorrhizal association, and litter quality. Then, we used a bibliometric analysis to determine if published literature has considered one or multiple mechanisms for forest change. We found evidence that tree species change to eastern forests include multiple functional traits, including fire adaptations, mycorrhizal fungi, and litter chemistry. Our bibliometric analysis revealed that these disturbances have largely been considered in isolation from the others, despite the fact they often co-occur across the landscape. We present an integrated framework for mesophication that considers how multiple disturbances may reinforce an ecosystem state change through alterations to the forest floor. This framework will aid researchers and land managers in recognizing tipping points and identifying forests where restoration efforts are most likely to be successful.
