Distance Decay in Vascular Plant Families of Eastern North America1

Distance decay, the decrease in biotic similarity with increasing geographic distance, has been observed across a broad phylogenetic spectrum where prior studies demonstrated the rate of decay was dependent upon a variety of biotic and physiographic factors. This study compared distance decay among eight large vascular plant families and examined the role of exotics and species geographic range size as contributors to distance decay. Floristic similarities, quantified using the Sørensen index, were calculated for eight of the largest vascular plants families in 20 local floras in eastern North America. To test for distance decay, Mantel permutation tests were conducted using the similarity and geographic distance matrices. To test for the effects of sample size and geographic scale on distance decay, a continental dataset, comprised of floras that spanned the distance from Alabama to Maine, was subdivided into a state dataset comprising 12 floras from the southern Appalachian Mountains of Tennessee and North Carolina and a reduced continental dataset comprising 12 floras that spanned the distance of the continental dataset. Distance decay was shown for all eight plant families but the rate of decay was strongest in the Ericaceae. The Ericaceae also had the smallest mean geographic range size. Exotic species had a larger mean range size compared to native species which in some plant families, may have led to homogenization of floras and reductions in distance decay.