Late quaternary vegetation, climate, fire history, and GIS mapping of Holocene climates on southern Vancouver Island, British Columbia, Canada.

摘要

Pollen and microscopic charcoal fragments from seven sites (East Sooke Fen and Pixie, Whyac, Porphyry, Walker, Enos, and Boomerang lakes) were used to reconstruct the post-glacial vegetation, climate, and fire disturbance history on southern Vancouver Island, British Columbia, Canada. A non-arboreal pollen and spore zone occurs in the basal clays at Porphyry Lake and likely represents a tundra or tundra-steppe ecosystem. This zone precedes the Pinus contorta (lodgepole pine) biogeochron that is generally considered to have colonised deglaciated landscapes and may represent a late Wisconsinan glacial refugium. An open Pinus contorta woodland characterised the landscape in the late-glacial interval. Fires were rare or absent and a cool and dry climate influenced by “continental-scale katabatic” easterly winds dominated. Closed lowland forests consisting of Picea (spruce), Abies (fir), Tsuga heterophylla (western hemlock), and Tsuga mertensiana (mountain hemlock) with P. contorta and Alnus (alder) and sub-alpine forests containing Picea, Abies, and T. mertensiana with P. contorta replaced the P. contorta biogeochron in the late Pleistocene. Fires became more common during this interval even though climate seems to have been cool and moist. Open Pseudotsuga menziesii (Douglas-fir) forests with Pteridium (bracken fern) in the understory and Alnus in moist and disturbed sites expanded westward during the warm dry early Holocene. In the late Holocene, closed T. heterophylla and T. plicata forests became established in wetter western regions, Pseudotsuga forests occupied drier eastern portions, and T. mertensiana and Cupressaceae, likely Chamaecyparis nootkatensis (Alaska yellow cedar), forests were established in sub-alpine sites. Lowland fires were infrequent in wet western regions but frequent in drier eastern regions. A slight reduction in charcoal influx generally occurs at high elevations, implying fewer fires. A recent increase in charcoal influx at East Sooke Fen and Whyac, Walker, Enos, and Boomerang lakes may reflect anthropogenic burning. Holocene paleoclimates were reconstructed at 1,000 year intervals through a geographic information system (GIS) using contemporary climate data and surface and fossil pollen assemblages by establishing empirical regression equations that calibrated contemporary precipitation and temperatures to present day Douglas-fir-western hemlock (DWHI) and T. heterophylla-T. mertensiana (THMI) pollen ratios. (Abstract shortened by UMI.)