Increasingly dense: spatiotemporal trends in forest relative density across the US since 1999

Christopher W. Woodall, Northern Research Station, USDA Forest Service, Durham, NH and Aaron Weiskittel, Center for Research on Sustainable Forests, University of Maine, Orono, ME

Background/Question/Methods
Tree size-density dynamics can inform key trends in forest productivity along with opportunities to optimize adaptive management actions to increase ecosystem resiliency within the context of global change. Here, we employ a novel yet effective incorporation of forest type, climate, and additional site factors for a comprehensive assessment of tree relative density (range 0-1) using the coterminous US national forest inventory between 1999 and 2020.
Results/Conclusions
Results suggest an increase in relative density over this time period with a majority of observations now reaching a biologically-relevant threshold of self-thinning induced mortality (relative density>0.6), particularly in areas prone to future drought conditions. Notably, the area of the most over-stocked stands (relative density>0.75) has increased over 200%, while the least stocked stands (relative density<0.3) have decreased 2%. Overall, the two ends of the coterminous US forest relative density distribution suggest opportunities not only to increase live tree stocking in understocked stands but also density management to address tree mortality in increasingly dense forests.