Functional and Phylogenetic Implications of Canopy Mesophication in Temperate Hardwood Forests

Forest mesophication describes the shifting of plant communities from fire-adapted, drought tolerant systems to those dominated by species better adapted to shaded, closed-canopy environments. While commonly characterized from a taxonomic perspective, the phylogenetic underpinnings and functional attributes of forest mesophication have received less quantitative testing. To address this, we applied phylogenetic and functional trait data to legacy datasets of changing forest composition in Wisconsin, USA from the latter half of the 20th century. In both the 1950s and 2000s, forests demonstrated a phylogenetically clustered structure, with closely related species occupying similar sites. The inclusion of functional traits into mixed models accounted for 40.8% of the variance explained by this clustering in the 1950s and just 19.6% in the 2000s. This diminished explanatory value of functional traits corresponds with significant functional and phylogenetic turnover, with the majority of sites demonstrating substantial temporal functional beta diversity. Contemporary trait syndromes were defined by broad declines in drought tolerance, fire tolerance, and litter flammability, as well as an increase in leaf traits associated with acquisitive resource strategies. Such shifts identify the diminished capacity of traits to explain phylogenetic clustering, but species-level trends and landscape-level functional trajectories also give stewards explicit targets to meet land management goals. While future studies on the influence of intraspecific trait variation and trait-environment relationships would further clarify functional shifts in temperate deciduous forests, our findings provide quantifiable support for the hypothesized functional implications of mesophication.