Data from: 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.