Species response to environmental gradients
GDD and water balance impose a limit on the maximum abundance of most North American trees (Fig 4; plots for all the species are available in Appendix S8). In 74% of tree species, including the predictors GDD and water balance were at least 100 times better supported by the data than the null model (see Appendix S4). GDD had significant limiting effects on 73 tree species (p -value < 0.05). Likewise, water balance was significantly limiting in 72 species (p -value < 0.05). Model likelihood, measured by ∆AIC, was higher for species with larger area of occupancy (Spearman r = 0.79, p<0.001), larger extent of occurrence (Spearman r = 0.52, p<0.001), and larger environmental breadth (Spearmanr = 0.36, p<0.001; Fig S8, Appendix S9).
In 83% of bird species, including GDD and water balance predictors improved QR models by a hundredfold or more (see Appendix S5). GDD and water balance had a significant limiting effect on the maximum abundance of 46 and 33 bird species, respectively (see Fig 5 and Appendix S10 - S11).