Several recent studies have shown that during the Pleistocene ice ages (>11,200 years BP) species distributions were not uniformly shifted to the south, but that several northern “cryptic” refugia for warm-adapted species occurred proximal to the ice sheets. In the Clearwater drainage of northern Idaho, modern distributions and genetic studies of several herbaceous plants and amphibians support the existence of refugia for mesic-adapted species. The Clearwater is unique, however, because most mesic-adapted species in this region are disjunct from their main coastal distribution, and therefore alternative hypotheses for this disjunction involve persistence in refugia or long-distance dispersal from the coast. No paleoecological studies exist in the unglaciated Clearwater Refugium from which to assess regional vegetation changes. In August 2008, a 10-m-long sediment core was extracted using a Livingstone piston corer and a surface corer from Dismal Lake located 120 km south of the maximum extent of the Cordilleran ice-sheet. Dismal Lake is a small (2.9 ha) deep (20.9 m) cirque basin located at 1630 m elevation in Tsuga mertensiana (mountain hemlock) forest. Measurements made on the core include pollen analysis, magnetic susceptibility, loss-on-ignition, charcoal, and four AMS radiocarbon dates.
Results/Conclusions
Initial pollen analysis on the ca. 16,500-year-long record suggests that nearly all of the tree species present at the site today increased in abundance enough to be detected in the pollen record by 13,000 years ago. However, T. mertensiana appears to be a recent component of the forest, arriving only about 700 years ago in northern Idaho. The timing of the T. mertensiana arrival from this sediment core in consistent with another paleoecological record further north (in British Columbia) with a slightly earlier arrival (ca. 1000 years ago). These two lines of evidence suggest that T. mertensiana increased and expanded its distribution relatively recently. Today, T. mertensiana is associated with high snowfall and is closely associated spatially with the low-elevation mesic-adapted species that may have comprised the Clearwater Refugium. The timing of its increase in the Dismal Lake core is inconsistent with T. mertensiana existing within the Clearwater Refugium. Rather, the pollen evidence suggests an early-successional cold dry forest consisting of Pinus contorta (lodgepole pine) and Artemisia. Reconciling this vegetation history with the evidence of refugia will likely involve assessing how small pockets of mesic-adapted species could persist during glacial- and late-glacial climates.
