A multi-compartmented glacial refugium in the northern Rocky Mountains: Evidence from the phylogeography of <Emphasis Type="Italic">Cardamine constancei</Emphasis> (Brassicaceae)

The age and origin of the mesic coniferous forest ecosystem of the Pacific Northwest of North America have long been the subject of debate by biogeographers. Cardamine constancei, an endemic of the Rocky Mountain segment of this ecosystem, was subjected to phylogeographic analysis to test explicit hypotheses on the age of the ecosystem. We have predicted genetic homogeneity among river drainages if C. constancei and other associated species migrated into the region after glaciation, in contrast to the genetic differentiation that may have accrued if the species and its ecosystem have long survived in the relatively warm river canyons south of glaciation. We detected 19 haplotypes with divergence up to 1.5%, and they comprise 4 well-differentiated cpDNA clades. These clades are allopatric except for two haplotypes from the lower Clearwater clade that appear to have dispersed north into partial sympatry with the clade endemic to St.␣Joe River. The divergence and distribution of these clades is consistent with the existence of a complex glacial refugium with at least four compartments. The surprisingly high cpDNA diversity within this species suggests that conservation of mesic coniferous forest ecosystems in the region warrant a conservation plan that accounts for the historically imposed spatial structure of genetic diversity. We are currently testing our phylogeographic hypotheses by the comparative analyses of a suite of plants, animals and fungi.