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Surface samples from Vancouver Island, Canada, were used to assess the relationship between discrete seasonal temperate rainforest (STR) plant communities and their corresponding pollen signatures. Pollen from 10 sediment cores was further used to evaluate the post-glacial development of these communities. Principal components analysis (PCA) of the surface data revealed the distinctiveness of the modern pollen rain, with samples from the Coastal Douglas Fir (CDF) zone, the dry Coastal Western Hemlock (CWH) zone, the wet CWH sub-zones and the Mountain Hemlock (MH) zone clustering distinctly. PCA of the fossil data revealed early-seral open canopy, wet rainforest, subalpine rainforest and late-glacial plant associations and showed that the STR has changed markedly through time. Pinus woodlands with low palynological richness prevailed in the early late-glacial period, only to be supplanted by mixed conifer forest with increased pollen richness. In the early Holocene, STR vegetation differentiated spatially as early-seral open canopy forests expanded, though a non-analog Picea-dominated forest persisted on the moist outer coast. Generally high pollen richness is attributed to the expansion of dryland habitat coupled with the development of a firemaintained vegetation mosaic. In the mid and late Holocene intervals, open canopy communities persisted in eastern areas, eventually developing into modern CDF and dry CWH forest. In contrast, moist and oceanic CWH rainforest developed on central and western Vancouver Island, whereas subalpine forest established at high elevation. Pollen richness declined in the mid Holocene concomitant with increased precipitation and a general reduction in the incidence of fire, though this trend was offset somewhat in the late Holocene by paludification.

The vegetation and fire history at the Coastal Douglas-fir (CDF) and Coastal Western Hemlock (CWH) boundary on east-central Vancouver Island was reconstructed. A basal non-arboreal assemblage at the most inland site likely represents an open parkland community during the cold early late-glacial interval. Widespread Pinus woodland followed under cold, dry climate, yielding to closed-canopy mixed conifer forest as climate moistened. While fire disturbance was initially rare, it increased in the late-glacial mixed conifer forest. In the early-Holocene, dry coastal temperate forest replaced the mixed-conifer assemblage. Fire disturbance was widespread, characterised by frequent, possibly lower severity burns. These CDF-like forests expanded northward and westward under warm, dry conditions, attaining maximum extent 9500-10,500 cal yr BP, retreating thereafter towards the modern CDF-CWH boundary, which established ~6000 years ago as climate moistened. Quercus garryana communities were scattered along the coast at this time, maintained by surface fire. Throughout the remainder of the record, the forest canopy closed as modern forests developed and a mixed-severity fire regime developed. The extended early Holocene range of CDF-like forest suggests that the existing CWHxm biogeoclimatic subzone may be replaced by CDF stands with climate change.

No abstract