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Forests of the southeastern United States play a critical role in carbon sequestration, contributing significantly to climate change mitigation. Understanding the factors influencing carbon storage in these forests is crucial for effective management and conservation. Prior studies suggest that structural diversity, reflected by variability in tree sizes, may be more closely linked to forest productivity than species diversity. However, the direct and indirect relationships between structural diversity, species diversity, aboveground carbon (AGC), belowground carbon (BGC), and total carbon storage (TCS) in SE US forests remain unclear. This study aimed to unravel the causal networks linking biotic and abiotic factors to AGC, BGC, and TCS in SE US forests by testing the complementarity effects hypothesis. This hypothesis posits that increased diversity, whether in species or structural complexity, enhances resource utilization and ecosystem functions, including carbon storage. Using structural equation modeling with data from 4908 Forest Inventory and Analysis (FIA) plots, we examined the impacts of structural diversity, species diversity, elevation, and climate on AGC, BGC, and TCS in mixed oak-pine forests. We hypothesized that (1) climate and elevation would positively influence AGC, BGC, and TCS; (2) species diversity would have a stronger effect than tree size inequality (i.e., structural diversity) on carbon storage due to niche complementarity; and (3) inequality of tree sizes would mediate the effects of climate and elevation on carbon storage across a large-scale ecological gradient. Our results revealed that the mean annual temperature had a significant direct negative effect on AGC, BGC, and TCS, while annual precipitation had a significant direct positive effect. Elevation indirectly increased AGC, BGC, and TCS by enhancing species diversity. Both structural and species diversity had direct positive effects on AGC, BGC, and TCS, but structural diversity emerged as a stronger predictor and mediator of climate and elevation effects. Maintaining high structural diversity levels, rather than focusing solely on species diversity, can boost carbon storage in mixed oak-pine forests under changing climatic conditions. • Structural diversity explained most of the variation in carbon storage. • Structural diversity enhanced aboveground, belowground, and total carbon. • Temperature had considerable direct negative impacts on carbon storage. • Structural diversity declined at higher elevations. • Managing for structural diversity can increase carbon storage in oak-pine forests.
Fatunsin et al. (Mon,) studied this question.