ABSTRACT The northern Rocky Mountain Trench of eastern British Columbia is a broad valley mantled by glaciolacustrine terraces supporting a complex mix of mesic-temperate (“interior wet belt”) forests that are strongly affected by terrain and substrate. Neither the geomorphic history during early Holocene deglaciation nor the vegetation history of the origin of the Tsuga heterophylla (western hemlock) and Thuja plicata (western redcedar) populations in the interior wet-belt forest is well understood. Sediment cores were obtained from two lakes, 10 km apart and occupying different terraces (83 m elevational difference), and these were compared to existing fire-history and paleoclimate reconstructions. Radiocarbon dates and a mapped terrain classification indicate the upper terrace formed as a lacustrine and glaciofluvial kame terrace hundreds of years prior to the lower terrace, which was formed by glaciolacustrine sediments of a proglacial lake. The minimum limiting ages of these terraces correlate with dated jökulhlaup deposits of the Fraser River. The upper site’s first detectable pollen at >11.0 ka was dominated by light-seeded pioneer taxa (Poaceae [grasses], Artemisia [sagebrush], and Populus [aspen]) followed by a peak in Pinus (pine) and finally dominance by Betula (birch) at 10.2 ka. Pollen data suggest an earlier invasion of T. heterophylla (western hemlock) (by ca. 8 ka) than previously understood. Wetlands on extensive, poorly drained, glaciolacustrine soils promoted the persistence of boreal taxa and open forests (e.g., Picea mariana [black spruce]), while the better-drained upper kame terrace promoted development of closed-canopy shade-tolerant taxa. Invasion and expansion of mesic cedar-hemlock taxa progressed since at least the middle Holocene but was highly constrained by edaphic controls.

ABSTRACT Climate during the Last Glacial Maximum (LGM) varied substantially across North America, strongly influencing changes in plant and animal distributions and causing variations in the timing and relative magnitude of ice expansion and recession. The Olympic Peninsula is a mountainous maritime terrain in northwestern Washington, where the climate today is most strongly influenced by Pacific weather systems. However, what about during the LGM, when ice sheets covered most of northern North America? Fossil beetle assemblages of LGM age contain species that currently inhabit riparian and lacustrine habitats in the boreal zone of Canada and Alaska, and in higher elevations in the Cascades and Rocky Mountains. They include three Olophrum species that today are unknown from the Olympic Peninsula. Olophrum consimile is especially well represented, and its occurrence today above 1000 m elevation in the Cascades of northern Washington State indicates summers during the LGM would have been at least 4 °C cooler than today. The absence of wood-boring beetles, in contrast to assemblages from deposits correlating with marine isotope stage (MIS) 3, supports an open rather than a forested landscape. The insect fossils also include an undescribed species of a blind trechine ground beetle, likely endemic to the Pacific Northwest with biogeographic affinities to Asia. Pollen and plant macrofossil evidence for a Sitka spruce and mountain hemlock parkland with similarities to the vegetation of modern southeast Alaska also supports an interpretation of a climate with summer temperatures ~4 °C cooler than today. Both the vegetation and the insects provide evidence that the climate was wet with persistent snow cover and not as dry as has been reported from the Puget Lowland to the east. Glacial geology provides evidence that during the colder climate of the LGM, mountain glaciers advanced down the western valleys of the Olympic Peninsula to the lowlands but not as far as they had extended during MIS 3. The amount of climatic cooling on the Olympic Peninsula during the LGM was less than at similar latitudes in midcontinental or eastern North America, indicating a strong modulation of climate by the Pacific Ocean.