Patterns of Forest Phylogenetic Community Structure across the United States and Their Possible Forest Health Implications

The analysis of phylogenetic relationships among co-occurring tree species offers insights into the ecological organization of forest communities from an evolutionary perspective and, when employed regionally across thousands of plots, can assist in forest health assessment. Phylogenetic clustering of species, when species are more closely related than expected by chance, suggests a process of evolutionary niche conservatism. Because such communities share much evolutionary history and an affinity for similar environmental conditions, they may be particularly susceptible to threats such as insects and diseases and shifting climate conditions. Meanwhile, a pattern of phylogenetic evenness, in which the species are less closely related than by chance, may indicate competitive exclusion or interspecies facilitation. The ecological integrity of such communities may be less at risk because they may encompass a wider variety of evolutionary adaptations. Using a network of more than 100,000 forest inventory plots across the conterminous United States, we tested whether community phylogenetic structure was significantly clustered or even at multiple scales. Clustering predominated across most of the study area, indicating the widespread significance of evolutionary niche conservatism, except in areas of the west. Phylogenetic structure varied along environmental gradients, suggesting that clustering predominates in more favorable locations and evenness predominates in areas with harsher environments. These results have implications for broad-scale forest health monitoring.