The coastal-disjunct mesic flora in the inland Pacific Northwest of USA and Canada: refugia, dispersal and disequilibrium
cordilleran ice sheet, dispersal barrier, glacial refugium, Idaho, Thuja plicata, Tsuga heterophylla
Abstract Aim Understanding the history of the mesic-adapted plant species of eastern British Columbia and northern Idaho, disjunct from their main coastal distribution, may suggest how biotas reorganize in the face of climate change and dispersal barriers. For different species, current evidence supports establishment of the disjunction via an inland glacial refugium, via recent dispersal from the coast, or via a combination of both. In this study, the modern distributions of the coastal-disjunct vascular plants are analysed with respect to modern climate to examine how refugia and/or dispersal limitation control regional patterns in species richness. Location North-west North America. Methods The distributions of nine tree and 58 understorey species with a coastal-disjunct pattern were compiled on a 50-km grid. The relationship between species richness and an estimate of available moisture was calculated separately for formerly glaciated and unglaciated portions of the coastal and inland regions. Growth habit and dispersal mode were assessed as possible explanatory variables for species distributions. Results Species richness shows a strong relationship to climate in coastal-unglaciated areas but no relationship to climate in inland-glaciated areas. In inland-glaciated areas, richness is c. 70% lower than that expected from climate. Species with animal-dispersed seeds occupy a larger portion of coastal and inland regions than species with less dispersal potential. Main conclusions Modern patterns of diversity are consistent with both refugia and dispersal processes in establishing the coastal-disjunct pattern. The inland glacial refugium is marked by locally high diversity and several co-distributed endemics. In the inland-glaciated area, dispersal limitation has constrained diversity despite the nearby refugia. Onset of mesic climate within only the last 3000 years and the low dispersal capacity of many species in the refugium may explain this pattern. This study suggests that vascular plant species will face significant challenges responding to climate change on fragmented landscapes.