Disentangling the contributions of ecological conditions to biomass in longleaf pine forests

Theoretical and empirical studies have suggested that climate, soils, and topography are the primary drivers of aboveground biomass in forests. Yet, the direct effects of these drivers may be mediated by indirect effects, such as species diversity and structural diversity. This study investigates the relationships between climate, topography, soil fertility, species diversity, structural diversity, and aboveground biomass (AGB) using Structural Equation Modeling (SEM) to distinguish indirect and direct causal relationships. We conducted this study in longleaf pine (Pinus palustris)-dominated forests in the southeastern United States (SEUS), using United States Department of Agriculture Forest Service inventory data from 2015 to 2019. The longleaf pine ecosystems of the SEUS are of great importance due to their rich biodiversity and unique ecological functions, but they also provide an opportunity for scientific studies across a large ecological gradient because they exist across a wide range of edaphic conditions. However, studies in longleaf pine have primarily focused on stand structure, regeneration processes, prescribed fire practices, and groundcover restoration, leaving a knowledge gap regarding AGB in this ecosystem. We hypothesized that (1) climate, topography, and soil fertility would influence AGB through positive indirect effects; (2) structural diversity rather than species diversity would strongly mediate the response of AGB to climate, topography, and soil fertility; and (3) species diversity and structural diversity would be positively correlated, with structural diversity positively impacting AGB across coarse scale ecological gradients. Structural diversity could be important in predicting AGB because it reflects the horizontal complexity of the forest stand. Our results show that mean annual temperature and slope had considerable direct negative and positive impacts on AGB, respectively. Additionally, soil fertility, elevation, and precipitation indirectly impacted AGB by affecting species diversity. Specifically, AGB decreased in highly fertile soils, whereas elevation and precipitation led to an increase in tree species diversity. Structural diversity had a direct positive influence on AGB, while species diversity played an indirect role by promoting structural diversity. While there are diverse objectives for managing longleaf pine, management that promotes high levels of stand structural diversity may strengthen the stock of longleaf pine forest AGB, which could be especially important in the face of changing climatic conditions. Our findings emphasize the importance of integrating climate resilience and carbon storage goals into forest management practices.