Coastal rainforest connections disclosed through a Late Quaternary vegetation, climate, and fire history investigation from the Mountain Hemlock Zone on southern Vancouver Island, British Colombia, Canada

Abstract

The post-glacial vegetation and fire history of high-elevation regions on southern Vancouver Island is described using palynological and charcoal records from Porphyry and Walker lakes. A zone consisting mainly of Artemisia, Poaceae, and ferns occurs in the basal clay at Porphyry Lake and may represent a non-arboreal ecosystem in a late-Wisconsin glacial refugium. At both sites, a fire-free Pinus contorta zone occurs before ca 14 160 calendar years before present (cal BP). Climate at this time is interpreted as being cool to cold and dry. Mixed conifer forests of Picea, Abies, Tsuga mertensiana and Pinus contorta replaced the Pinus contorta woodlands after ca 14 160 cal BP. Fires are recorded for the first time. Climate is interpreted as cool and moist. Forests of Abies, Picea, Tsuga heterophylla, Pseudotsuga menziesii, and Alnus developed and expanded during the early-Holocene from ca 11 400–9910 to 7700–7300 cal BP as climate warmed and dried. Charcoal increased during this interval, indicating only slightly more fire activity and reflecting continued moist conditions at high elevations. In the mid-Holocene from ca 7700–7300 to 5200–4900 cal BP, Tsuga heterophylla pollen values increased as forests became dominated by Tsuga heterophylla, Picea, and Abies with Alnus in response to increased moisture. The increase in charcoal influx at this time may reflect an increase in slope wash and erosion resulting from a wetter climate rather than an increase in fire incidence. Starting at ca 5200–4900 cal BP, a further increase in Tsuga heterophylla combined with an increase in Tsuga mertensiana and Cupressaceae pollen suggest that the late-Holocene was characterised by increasing moisture and decreasing temperatures. Late-Holocene forests consisted predominately of Tsuga heterophylla, Tsuga mertensiana, Cupressaceae, and Pinus contorta. A slight reduction in charcoal influx at ca 4600 cal BP implies fewer fires. A recent increase in charcoal at Walker Lake at 1700 cal BP may reflect anthropogenic burning. The timing of events and response of taxa on southern Vancouver Island are comparable to other coastal sites in northwestern North America, suggesting that past ecosystems were widespread and contemporaneous. Palaeoecosystem changes detected in one region of the Pacific Northwest likely reflect a widepsread response to climate throughout the ∼2500 km long zone, a zone that today is home to half of the world’s remaining coastal temperate rainforest.