Niche expansion after competitor extinction? A comparative assessment of habitat generalists and specialists in the tree floras of southeastern North America and southeastern Europe

Methods We compared co-occurrence-based distributions of habitat specialization of tree species in two geographic regions that are ecologically similar but differ in species pool size. We applied two methods. First, we used a rank-ordering of species along a gradient of estimated niche breadth that is based solely on species co-occurrence information derived from vegetation databases from each region. To compare niche-breadth distributions of different datasets we developed a procedure that standardizes expected values of species co-occurrences independently of the size of the species pool. Second, we calculated species turnover along an elevational gradient for both regions, estimated as the rate of decay of compositional similarity with elevation distance.