Postglacial record of diatom assemblage changes related to climate in an alpine lake in the northern Rocky Mountains, Canada

We relate changes in diatom assemblages to Holocene climate since ca. 10 300 cal. years BP at a small (4 ha), alkaline, alpine tundra lake in the Rocky Mountains of northeast British Columbia. A complex of small benthic Fragilaria-type taxa generally dominated the fossil diatom communities reflecting shallow, alkaline, and likely low light conditions resulting from minerogenic turbidity or extensive ice cover throughout the Holocene. Shifts in diatom assemblages occurred with changes in local vegetation, inferred from pollen and stomate records, and shifts in charcoal-inferred fire frequency (e.g., lowering of tree line and reduced fire frequency (approx. 7500 cal. years BP) and establishment of modern alpine tundra (approx. 3500 cal. years BP). Importantly, striking changes in diatom communities (e.g., during the mid-Holocene (approx. 5500 to approx. 3500 cal. years BP) and Medieval Warm Period (approx. AD 800 to approx. AD 1250)) occurred without evidence of vegetation shifts. During these times, diverse periphytic diatoms dominated suggesting improved benthic habitat availability with longer growing seasons and probably reduced alkalinity and turbidity with warmer- and wetter-than-present climates. Diatom analysis from this alpine lake provides a sensitive record of climate-related limnological responses, but also refines our previous understanding of Holocene climate changes in the northern Rocky Mountains inferred from terrestrial-based paleoecological records.Key words: alkalinity, alpine lakes, climate change, diatoms, Holocene, paleolimnology.

Postglacial spread of hemlock (Tsuga) and vegetation history in Mount Revelstoke National Park, British Columbia, Canada

To investigate postglacial environmental changes in southeastern British Columbia, subfossil pollen was analyzed from Eagle Lake, Mount Revelstoke National Park. Palynological evidence suggests that during the early Holocene, Eagle Lake was surrounded by an open forest. The pollen assemblage indicates that climatic conditions were warmer and drier than at present with high alder pollen values. Spruce and fir were common in local forests throughout the Holocene. Western hemlock pollen first occurs at approximately 3500 14C years BP. The migration path of western hemlock into this region is currently unclear. Data from palynological records appear insufficient to resolve the migration pattern, although several routes are possible. The available data support northward migration into southeastern British Columbia. The arrival of this conifer may be in response to the late Holocene increase in precipitation, as inferred from many lower elevation and coastal sites. Mountain hemlock pollen does not appear until about 2100 14C years BP. Mountain hemlock's migration route into southeastern British Columbia is also unresolved. In the late Holocene, the presence of mountain hemlock, and increases of Ericales and Valeriana sitchensis pollen, suggest a shift to the more open, cool, and moist conditions characteristic of the modern-day subalpine parkland. These climatic inferences correspond closely to those derived from other palynological and geological evidence throughout southern British Columbia.Key words: pollen, vegetation, Tsuga, hemlock, British Columbia, Holocene, climate.

Phylogeography of the tailed frog (Ascaphus truei) in British Columbia

The tailed frog (Ascaphus truei) is highly philopatric, with naturally disjunct populations. To infer the historical pattern of range expansion and contraction in A. truei of British Columbia (B.C.), we conducted a randomly ampli fied polymorphic DNA (RAPD) survey, using 16 primers, of 469 individuals sampled from 22 populations in four regions. Genetic distances, based on 169 putative RAPD loci, clustered populations into three clear groups: mid/north-coastal B.C., south-coastal B.C., and interior B.C. The interior populations were the most distinct group, but their genetic distance from other groups equals that expected from isolation by physical distance alone, as opposed to taxonomic differentiation. FST values within coastal regions were moderately low (0.02-0.05) but were high in the interior (0.12) and across all populations (0.18). Within the mid/north-coastal and south-coastal regions, genetic distance showed no relationship with physical distance, while among regions and within the interior region there was a strong relationship. Gene diversity varied significantly among regions, with both the interior and south-coastal populations showing about 25% less diversity than the mid/north-coastal populations. These data indicate a complex history of geographic restrictions to multiple refugia, followed by various types of range expansion. The implications of these results for conservation efforts in this species are discussed.