Deglacial landforms and Holocene vegetation trajectories in the northern interior cedar-hemlock forests of British Columbia

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 wetbelt”) 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 and Thuja plicata populations in the interior wetbelt forest is well understood. Sediment cores were obtained from two lakes, 10 km apart and occupying different terraces (83 m elevational difference) and 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 a lower terrace 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, Artemisia, and Populus) followed by a peak in Pinus and finally dominance by Betula at 10.2 ka. Pollen data suggest an earlier invasion of T. heterophylla than previously understood. Wetlands on extensive poorly drained glaciolacustrine soils promoted the persistence of boreal taxa and open forests (e.g., Picea mariana) 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.