Realized climatic niches of deciduous trees: comparing western Eurasia and eastern North America

Aim  Assessment of cross-continental similarities and differences in climatic limiting values for deciduous tree species and of the possible deterministic influence of past and present climatic differences on the modern tree flora in two regions.
Location  The deciduous forest regions of western Eurasia and eastern North America.
Methods  Based on species distribution data (range maps) and climate site data, the realized climatic niches of 137 deciduous tree species from the two regions were quantified using climatic envelopes. To compare these envelopes on the two continents, a hierarchical cluster analysis was performed, and principal components analysis was used to check cluster consistency.
Results  Significant differences do exist for upper limits of winter temperatures and for lower limits of summer temperatures between Western Eurasia and eastern North America. Lower limits for the annual water balance also appear different, suggesting that the deciduous trees may be more drought-tolerant in western Eurasia than in eastern North America. Climatic range types generated by the cluster analysis can be characterized, according to the distribution of the species, as boreal-temperate, northern temperate, temperate, southern temperate, and Appalachian. Five of the eight clusters contain trees from both regions, but three groups consist only of American species that have no European counterparts.
Main conclusions  Differences in temperature limitations can be explained by location on the east versus west side of the continents and by the almost complete lack of warm moist areas in western Eurasia. The difference in drought tolerance, on the other hand, is more likely to be the product of a deterministic sorting process that occurred during the Plio-Pleistocene.