Drivers, climate sensitivity, and management of functional diversity in northeastern North America

The functional diversity (FD) represented by plant traits is fundamentally linked to the ecosystem’s capacity to respond to changes in the environment. Thus, there is an ongoing debate about including FD considerations in management plans to safeguard forests and their services to the society under climate change. However, incomplete scientific knowledge and difficulties to understand the concept of FD hinder the implementation of FD-based management approaches. Our study fills these knowledge gaps by (i) mapping the current distribution, (ii) analyzing the drivers, and (iii) testing the sensitivity of FD to projected increases in temperature and precipitation in northeastern North America. Following the stress-dominance hypothesis, we expected the strongest effect on FD from environmental filtering (i.e., climatic conditions) within our study region.

We combined a literature and database review of 44 traits for 43 tree species with terrestrial inventory data of 48,426 plots spanning an environmental gradient from northern boreal to temperate biomes. Employing multiple non-parametric models, we evaluated the impacts of 25 covariates on FD. Subsequently, we conducted a climate sensitivity analysis. The effect of rarity and commonness were tested for all outcomes using Hill numbers with different abundance weightings.

We identified FD hotspots in temperate forests and the boreal-temperate ecotone east and northeast of the Great Lakes. Forest stand structure explained most of the variation in FD. Elevated temperature increased FD in boreal, but lowered FD in temperate forests. Different species abundance weightings affected trait diversity distributions and drivers only marginally.

As environmental filtering was of secondary importance behind forest structure in explaining the trait diversity distribution of tree species in northeastern North America, our study provides only partial support for the stress-dominance hypothesis. Forest management can increase FD by promoting structural complexity. In addition, mixing species from functionally different groups identified in this study can enhance the response diversity of forests to an uncertain future.