Forest Structure
There were strong differences in three-dimensional structure between many of the North American forest ecosystems (Also see Carrasco et al., 2019). Maximum canopy height was one of the strongest structural predictors of species richness and functional diversity in our models, and it was correlated with most other structure metrics; therefore, we generalize maximum canopy height to represent structural differences between forest ecosystems (see Table S4 for other comparisons). The maximum canopy height was significantly different across forest ecosystems (X 2Kruskal-Wallis(14) = 213.07, p-value < 0.001) (Figure 4). The lowest canopy heights were recorded in spruce/fir forests with a mean height of 11.6m (SD = 5.8m), which was significantly shorter than 8 of the 14 forest ecosystems (p<0.05). The tallest canopy heights were recorded in the California mixed conifer forests with an average maximum height of 46.9m (SD = 8.8m), which was significantly taller than any other forest ecosystem (p<0.05) (Figure 4).