Structural diversity is an important predictor of forest productivity responses to drought

The response of forest productivity to drought depends on biodiversity and other stand attributes. Biodiversity can regulate productivity through niche complementarity (i.e. greater diversity better utilizes resources to determine ecosystem functioning) and mass-ratio effects (i.e. dominance drives ecosystem functioning). However, when these forest attributes occur with drought, stress results are mixed. One emerging biodiversity metric is structural diversity, the variation in vegetation size and form. There is a lack of understanding of the interactive effects of structural diversity and drought on forest productivity. Addressing these knowledge gaps can improve the projection of forest dynamics and structural diversity-based management for drought adaptation. We compiled multiple datasets including repeated survey data of 13,059 trees in 299 plots across southcentral US forests stressed by the 2010-2015 drought. We used the Random Forest algorithm to examine how structural diversity and other factors influenced three aspects of productivity measured by above-ground wood biomass: ingrowth, growth, and net change accounting for mortality. Our results showed that (1) Among the eight stand attributes studied, structural diversity was among the top three variables showing the greatest main and interactive effects of drought on forest productivity, except for the interactive effect on ingrowth; (2) The drought-related losses in forest growth and biomass net change were exacerbated in forests with greater structural diversity; (3) Compared with the species and functional diversity metrics, functional identities had greater interactive effects with drought on forest productivity (except for the ingrowth). Synthesis and applications: Productivity losses related to drought increased with greater structural diversity and decreased with functional identities characterized by conservative resource strategies, such as a high leaf carbon-nitrogen ratio. Thus, structural diversity and functional identity could be considered in projecting and managing forest dynamics under climate change. The comparisons between functional identity and the diversity metrics support the important role of the mass-ratio effect in forest productivity during drought.