Biodiversity, climate anomalies and human activities drive structural diversity–productivity relationships

Abstract A forest contains trees of varying sizes and shapes—its structural diversity—which can strongly influence ecosystem functioning, sometimes even more than biodiversity. However, how the relationship between structural diversity and ecosystem functioning varies in the face of heterogeneous abiotic and biotic factors, particularly under environmental stressors linked to global change, remains unclear. Here, we aim to clarify the complex mechanisms by which structural diversity affects forest productivity under multifaceted ecological and environmental stressors. We use over a decade of productivity data together with structural diversity data from 447 geographically widespread forest plots to examine the influence of structural diversity and multiple dimensions of biodiversity on productivity, as well as how several biotic and abiotic factors modulate these relationships. We find that the positive effect of structural diversity on productivity is stronger than that of biodiversity and that this effect increases at higher levels of biodiversity. Biodiversity can indirectly affect productivity through its direct effect on structural diversity. Stand age and other environmental factors do not interact with structural diversity, but exert indirect effects on productivity through structural diversity. Several environmental stressors (e.g. drought frequency, temperature variation, human footprint and nitrogen deposition) weaken the positive effect of structural diversity. Synthesis . Our study highlights the importance of considering the intricate relationships between several factors that modulate the relationship between structural diversity and ecosystem function. These findings will help develop management strategies aimed at enhancing forest ecosystem function through structural adjustments to forest management in the face of complex global changes.