The mesic coniferous forests of the Pacific Northwest are divided into coastal/Cascadian and northern Rocky Mountain segments, which are isolated from each other by the arid Columbia River Basin. Many plant and animal species have a disjunct distribution between the coastal and inland mountains, whereas other species are endemic to one region. As part of a larger effort to test hypotheses on the phylogeography of the region, we have begun to analyze species with both endemic and disjunct distributions. We are using molecular genetic data to test hypotheses on whether Rocky Mountain mesic forests are the result of ancient vicariance and subsequent survival in river canyons south of glaciation. Alternatively, species may have dispersed into the northern Rockies in the last few thousand years since glaciation. The Clearwater River drainage in the northern Rockies lies south of glaciation, and has the highest diversity of disjunct and endemic species. One of the disjunct species we are examining is the dusky willow (Salix melanopsis), which has three ecologically distinct chloroplast races. The race closely associated with mesic forests is being used to test phylogeographic hypotheses of disjunct species. A second species, Cardamine constancei, is endemic to the mesic forests of the northern Rockies. Chloroplast DNA sequence data revealed a divergent clade of the mesic race of dusky willow with a distribution centered on the Clearwater River drainage, but also the existence of the coastal haplotype in the glaciated northern latitudes of the Rocky Mountains. Cardamine exhibited chloroplast differentiation among Clearwater River tributaries. These data support an ancient vicariance model with survival of populations during Pleistocene in the dissected river canyons south of glaciation in the northern Rocky Mountains.

Key words: Cardamine, Mesic forests, Pacific Northwest, Phylogeography, Salix