Species, structural, and functional diversity in relation to live aboveground biomass in mature and late-successional northeastern temperate forests

Determining the role of ecological diversity in forest biomass dynamics is critical to advancing climate change mitigation and forest adaptation goals amidst global change. Species diversity and structural complexity are typical benchmarks for adaptive forest management which are widely considered to sustain ecosystem productivity. However, the relationship between functional trait diversity and forest aboveground biomass remains underexplored. We combined continuous forest inventory data with local functional trait observations from mature and late-successional forests in western Massachusetts to improve understanding of how different types of ecological diversity influence aboveground biomass stocks in northeastern temperate forests. We define a linear model fit within a broader Bayesian hierarchical framework to predict site-level live aboveground biomass (AGB) stocks as a function of functional diversity (incorporating local trait information), species diversity, and structural complexity. After accounting for stand density, we found that structural complexity explains most of the variability in AGB, while species and functional diversity explain only a small proportion of variability across 626 inventory plot observations. Of the structural complexity variables considered, tree diameter diversity explains the largest amount of variation in AGB, highlighting the important legacy of small non-stand-replacing disturbances characteristic of mature and late-successional mixed hardwood forests in the Northeast. Our results underscore the critical role of structural complexity in forest carbon management in mixed hardwood forests and the need for more robust datasets to better understand the role of functional diversity in shaping forest biomass stocks across different scales.