Structural diversity shifts from negative to positive associations with forest productivity via basal area, stand age, and precipitation thresholds

Understanding the relationship between biological diversity and ecosystem productivity is a central challenge in ecology. Structural diversity—the variation in size, height, and spatial arrangement of individuals within communities—has been proposed as a key driver of productivity, yet previous studies have reported mixed results, leaving its role in ecosystem functioning unclear. Clarifying the structural diversity and productivity relationship (SDPR) is essential not only for advancing ecological theory but also for informing biodiversity conservation, resource management, and climate adaptation strategies across ecosystems. Here, we utilized the USDA Forest Service Forest Inventory and Analysis (FIA) database, encompassing 95,602 trees within 3,307 plots throughout most types of forest ecosystems in the contiguous U.S. We investigated the variation of SDPR by analyzing the growth of existing trees under the main and interactive effects of structural diversity with basal area, stand age, and annual precipitation, with other factors. We also compiled the studies of SDPR published during 1974 - 2022 and compared the mean annual precipitation between studies showing the negative and positive SDPR. We found that: 1) SDPR was negative under the low range of structural diversity, but became positive under the high range; 2) SDPR was more positive with greater basal areas; 3) SDPR was more positive with stand development, and 4) SDPR was more positive with greater annual precipitation in the range of low structural diversity but switched to be more negative in the high range, and the negative SDPR with greater precipitation aligns with the analysis of published studies showing greater precipitation in studies with negative SDPR than the positive ones.