Forest Densification and Pollution Rival Climate in Driving US Tree Demographic Change.

Forests play critical roles in biodiversity, climate regulation, and human well-being, yet their responses to simultaneous environmental changes remain poorly understood. Across the United States, forest demographic rates are influenced by climate change, atmospheric pollution trends, and systematic changes in forest density. In spite of long-term interest in these facets of global change, their relative importance has not previously been quantified and compared. Here, we apply a framework that decomposes demographic trends into sensitivity (species' responsiveness to environmental variables) and exposure (magnitude of environmental change experienced). Using nationwide forest inventory data from 2002 to 2024, we analyzed how recruitment, growth, and mortality of 100 tree species responded to six environmental variables representing climate, pollution, and forest density. This analysis showed that, despite substantial research attention on climate impacts, changes in forest density and atmospheric pollutant deposition (particularly sulfur) showed stronger associations with all three demographic rates, driven by both higher exposure and greater sensitivity to these factors. We also found that, of the demographic processes, recruitment probability was the most sensitive to each of the six environmental drivers, emphasizing the importance of regeneration in shaping forest responses to global change. The results also document a general slowdown in forest dynamics over recent decades, with declining recruitment, growth, and mortality on average across the United States, albeit with substantial geographic and taxonomic variation. This comprehensive analysis provides a framework for understanding forest responses to complex environmental change, identifying the most influential drivers of demographic processes, and developing targeted conservation and management strategies in an era of rapid change.