Does the climatic variability hypothesis explain the longitudinal range size gradient in North American trees?

Background/Question/Methods

Latitudinal range size gradients have long been documented for North American tree species, and recent work has shown that latitudinal range size gradients are consistent with the climatic variability hypothesis. While longitudinal range size gradients for North American tree species have also been documented, it is uncertain whether this pattern can also be explained by the climatic variability hypothesis. In this study, I evaluated whether the longitudinal gradient in geographic range sizes can be explained by climatic variability.

Using digital representations of range maps from E.L. Little’s Atlas of North American Trees, information from the USDA Plant Database, and climatic data from WorldClim, I performed a principal components analysis on 19 bioclimatic variables extracted from the geographic limits of each species’ range, to ascertain how geographic range size fluctuated with multidimensional climatic variability. I also used the PCA results to identify which bioclimatic factors most influenced climatic variability over geographic ranges, and tested how those combined factors varied with range size.

Results/Conclusions

Results show that variation in PCA 1 was driven by factors related to temperature, while variation in PCA 2 was driven by factors related to precipitation. PCA 1 exhibited a moderately statistically significant negative relationship with range size for all species (r2= 0.3675, p = 0.0). The pattern was somewhat weaker for gymnosperms (r2= 0.2483, p < 0.0001) but stronger for angiosperms (r2= 0.4222, p < 0.0001). PCA 2 showed only a moderately statistically significant positive relationship with range size for gymnosperms (r2= 0.2127, p < 0.0001). A regression model using five bioclimatic factors most influential in determining climatic variation across species’ ranges was statistically significant, explaining 66% of the variation in range size (r2= 0.66, p < 0.0001).

This study shows that the longitudinal range size gradient for North American trees is consistent with the climatic variability hypothesis. Species with small geographic ranges sample a climatic space with more variable temperature ranges and precipitation regimes than large range species. The results also seem to contradict the notion that species’ geographic range sizes are driven by whether they are climatic specialists or generalists. Large range species may not be climatic generalists; they may be simply exploiting a more homogenous climate.