Reconstruction of Holocene Changes in Alpine Vegetation and Climate, Susie Lake, British Columbia, Canada

Abstract

A ca. 9000-yr pollen, macrofossil, and sedimentological record from a laminated sediment core obtained from Susie Lake, British Columbia, reveals changes in vegetation and inferred climate during the Holocene. The pollen record indicates that a shrub and herb assemblage (ca. 9000–7800 yr BP) was rapidly replaced by a spruce (Picea) and subalpine fir (Abies lasiocarpa) forest (ca. 7800–4000 yr BP) under the influence of warmer-than-present climate conditions. The macrofossil record indicates that an elevated treeline persisted from ca. 5200-2000 yr BP Pine pollen percentages increase significantly from ca. 5000—4000 yr BP and likely reflect the migration of P. contorta into the area. The disappearance of spruce (ca. 4000 yr BP) and fir (ca. 2000 yr BP) needles from the record indicates a gradual lowering of treeline below the elevation of Susie Lake and the establishment of the modern cold and moist climate. A pronounced increase in extralocal western hemlock (Tsuga heterophylla) pollen occurs at ca. 2000 yr BP and is coincidental with an increase in the number of debris flow–derived, fining-upward graded beds in the core. These are probably a consequence of landscape destabilization associated with treeline depression and an increase in storm frequency. Increased storm frequency may be a consequence of inland penetration of the moist Pacific air mass and the displacement of the drier continental air mass. This study suggests that substantial variations in vegetation assemblages can result from past changes in regional circulation patterns.