A 10,000 cal yr vegetation history of Douglas-fir forests on Pender Island, south-coastal British Columbia

Background/Question/Methods

Paleoecological tools such as fossil pollen and charcoal analyses are important for describing the long-term natural variability in the composition, structure and disturbance regimes of vegetation communities. In turn, this information can be used to guide modern ecosystem management by providing a variety of baselines for potential restoration, particularly in the context of climate change. The primary goal of this research is to determine Holocene vegetation dynamics and fire regimes on Pender Island, which is situated in dry Pseudotsuga menziesii-dominated forests in south-coastal British Columbia. This long-term vegetation and fire history can then be used to aid forest management and prescribed burning strategies in this region. A 9 m sediment core was collected from a small lake (Roe Lake) on Pender Island to document local changes in vegetation communities using pollen analysis and past fire frequencies using charcoal analysis. A chronology for the sediment core was established using four AMS radiocarbon ages and the Mazama tephra deposited ~7600 cal yr BP.

Results/Conclusions

Pollen analyses reveal a long-term vegetation history that is relatively stable in terms of composition: despite changes in climate, forests on Pender Island have been dominated by Pseudotsuga menziesii and Alnus rubra for the last 10,000 cal yr. Pinus contorta, Cupressaceae, Tsuga heterophylla, and Alnus viridis have also been continuously present, although at much lower abundances. Pollen percentages and accumulation rates suggest that forests have been moderately dense for the entire period of record, but with a somewhat more open canopy during the warm, dry early Holocene. The relatively stable long-term vegetation history of this island is marked by the expansion of Quercus garryana populations between about 7000 and 5500 cal yr BP. Charcoal analyses from the last 1300 cal yr indicate that 14 local fire events occurred, with the data recording fire suppression practices of the recent past (1770-present). The mean fire-return interval for the period before recent fire suppression is 85 years between fires. This information should be considered when devising strategies for prescribed burning on Pender Island.