Climate and vegetation in a putative Pleistocene refugium in northern Idaho inferred from sediment records

Background/Question/Methods

The mesic forests of northern Idaho’s Rocky Mountains are unique because they support > 100 vascular plant species disjunct from their main distribution along the Pacific Northwest coast. The uplift of the Cascade Mountains during the Miocene led to the development of a pronounced rain shadow to the east and the development of a separate interior mesic climate, but it is less clear when and how the associated disjunctions were established. Specifically, it is unknown whether most species with an inland disjunction, including forest-dominants Thuja plicata and Tsuga mertensiana, survived the glacial periods of the Pleistocene in their present locations or whether they more recently dispersed from coastal areas. Phylogeography and modern distribution studies suggest that several species of plants and animals may have persisted in the proposed Clearwater Refugium of northern Idaho during the last glaciations. There are currently no climate or vegetation records that extend back to the last interglacial in this region to assess regional vegetation changes. Four sediment cores were extracted from three cirque lakes (ca. 14,000 years old), and one meadow (> 120,000 years old) in the proposed refugium. Measurements made on these cores include pollen analysis, magnetic susceptibility, loss-on-ignition, charcoal, and AMS radiocarbon dates.  

Results/Conclusions

Our study focused on two mesic tree species: Thuja plicata and Tsuga mertensiana. Pollen recovered from the sedge meadow is dominated during the late Pleistocene by Pinus sp., Picea, Larix/Pseudotsuga and Abies pollen types. During the mid Holocene T. plicata pollen appears in this sediment record, indicating a relatively recent arrival into to the southern region of the interior mesic forest. All three sediment records recovered from cirque lakes also show changing dominance of Pinus sp., Picea, Abies, Alnus, and Larix/Pseudotsuga pollen types from ca. 14,000 to present with T. mertensiana arriving into the region only 700 years ago. The timing of the T. mertensiana arrival from these sediment cores in consistent with another paleoecological record further north (in British Columbia) with a slightly earlier arrival (ca. 1000 years ago). 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 T. plicata and T. mertensiana increase in the sediment cores is consistent with a recent dispersal of T. plicata and T. mertensiana in northern Idaho.