Unraveling the Mixed Patterns of Structural Diversity--Forest Productivity Relationships Across Continental-Scale Stand and Climate Gradients

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, to investigate the mixed patterns of structural diversity – productivity relationship (SDPR) by analyzing the growth of existing trees under the main and interactive effects of SD 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 relationship. Our findings revealed: 1) SDPR was negative at low SD but became positive at high SD; 2) SDPR became more positive with greater basal area, and the effect of basal area on productivity strengthened with SD; 3) SDPR was more positive with stand development; and 4) SDPR was more positive with higher precipitation at low SD but became negative at high SD. The negative SDPR in wetter conditions aligns with published studies showing higher precipitation in negative SDPR cases. Our findings identified the drivers of the mixed patterns of SDPR to enhance the understanding of forest dynamics from a SD perspective.