We divide plant taxa into two groups based on their temperature and precipitation preferences. We calculate the climatic niche centroids of plant taxa from the last 18,000 y (SI Appendix, Table S8). We then calculate the ratio of MAP to MAT for each plant taxon, where the temperature and precipitation are scaled separately to the range 0–1 (Fig. 2B, Inset). We divide plant taxa into two groups based on the precipitation/temperature ratios: Tsuga, Abies, Alnus, Picea, Betula, and Fagus compose a cold and wet taxa group for which the precipitation to temperature ratio is higher than 0.5. Fraxinus, Quercus, Pinus, Cupressaceae, Ulmus, Cyperaceae, Salix, Poaceae, Artemisia, and Amaranthaceae compose the warm and dry taxa group (Fig. 2A). We plot the niche overlap values for cold/wet and warm/dry plant taxa over time to evaluate whether one group showed consistently higher niche overlap than the other (Fig. 2B).