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




Brown, Kendrick Jonathan

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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 Pimis 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 subalpine 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. At this time closed Picea, T. heterophylla, and possibly Alnus forests grew in the wettest part of southern Vancouver Island at Whyac Lake. At high elevations, forests consisting of T. heterophylla and Pseudotsuga coupled with Alnus expanded during the early Holocene. Fires occurred frequently in lowland forested ecosystems during this interval, although East Sooke Fen in a dry, open region experienced less fire. At high elevations, charcoal increased somewhat from the late Pleistocene, indicating slightly more fires and reflecting continued moist conditions at high elevations. The mid and late Holocene was characterized by increasing precipitation and decreasing temperature respectively. Mid Holocene lowland forests were dominated by Pseudotsuga with T. heterophylla and Alnus in southeastern regions, T. heterophylla and Thuja plicata (western red-cedar) in southern regions, and T. heterophylla and Picea in southwestern regions. An overall decrease in charcoal influx suggests a decrease in lowland fires, although locally isolated fire events are evident in most sites. Quercus garryana (Garry oak) stands spread westward during the mid Holocene, attaining maximum extent between East Sooke Fen and Pixie Lake, approximately 50 km beyond their modem limit. Lowland sites record a general decrease in fires at this time. At high elevation, mid Holocene forests were dominated by T. heterophylla, Picea, and Abies with Alnus. An overall increase in charcoal influx at high elevations may reflect an increase in the number of charcoal fragments entering the basins by overland flow as opposed to an increase in fire incidence because climate was moister. 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 (THMl) pollen ratios.



Geology, Stratigraphic, Quaternary, Holocene, Vancouver Island (B.C.)