Increasing carbon storage in forests presents a key opportunity to mitigate climate change. In addition to research about the general environmental conditions that influence forest carbon storage, substantial work has explored relationships between biodiversity and ecosystem functions and services, in part to assess when the twin goals of sustaining biodiversity and maintaining ecosystem services may be aligned. While much research has been conducted about the influence of biological diversity on ecosystem functioning, fewer studies have examined the influence of structural diversity—which in forests can refer to the variation in tree heights, sizes, density, clumping, canopy cover, and/or spatial heterogeneity in arrangement. In this paper, we capitalize on NASA’s new Global Ecosystem Dynamics Investigation (GEDI) data and on-the-ground forest inventory data from 1128 forest plots across the western and northern regions of the USA to assess the relative influences of the structural and biological diversity of live trees on aboveground carbon storage.
We found that structural diversity explained far more of the variation in carbon storage than species richness. Our plot-based metric of structural diversity (a composite index based on tree diameter classes and height classes) outperformed GEDI-based metrics (e.g. foliage height diversity). However, the strength of diversity-carbon storage associations was mediated by forest type and forest composition. Diversity-carbon storage relationships were positive in natural forests and non-significant in planted forests (except for plot-based measurements of structural diversity), perhaps due to substantial human influence in planted areas. Structural diversity exhibited more positive relationships with carbon storage in conifer forests than in mixed or broadleaf forests. Overall, plot-based structural diversity metrics refine our understanding of drivers of forest carbon balances at the fine scale of monitoring plots, while remotely sensed structural indices such as from GEDI offer the opportunity to extend that understanding to regional/national scales in a spatially continuous manner. These results suggest that incorporating structural diversity into management strategies could potentially help increase carbon storage and mitigate climate change.