Posted: Sat Dec 27, 2014 12:39 pm
by Lee Frelich

Great idea, to compare species richness in areas with similar temperature and size, since we know that both of those factors are strongly related to species richness. I am not surprised to see that the relationships between continents you found are similar to previous studies, since there is a lot of evidence that geologic and evolutionary history contributes in a major way to the differences among occurrences of temperate forest on different continents. And as you say, in Asia there are more opportunities for suptropical species to evolve over time to survive and spread into temperate areas. The opportunities include the contiguous spatial arrangement of the gradient from subtropical to temperate at large (continental) and local (mountains) spatial extents, and millions of years for species to evolve. An then there is the relatively small impacts of glacial advances in eastern China compared to Europe. One of the best studies of tree species richness I have seen was done for Europe (Svenning et al. 2010, Geography, topography and history affect realized-to-potetial tree species richness patterns in Europe, Ecography 33: 1070-1080). This study clearly shows how history affects tree species richness on 50 x 50 km grid cells throughout Europe, with mountain ranges and large water bodies interfering with migration of tree species after departure of the continental glaciers, leading to many areas with fewer species than the climate could support. These impacts are clearly larger in Europe than N. America or Asia.