| Summary: | Terrestrial plant communities are complex systems, and major storehouses of global biodiversity. The composition of a plant community today is contingent on conditions that have occurred throughout its history. Therefore, an understanding of current plant community structure requires an understanding of its origin and its variability over time. In this thesis, I investigate the history of plant communities on southeastern Vancouver Island, Canada, at two time scales. These communities are now highly fragmented and threatened by human disturbance, but they also have a long history of management by indigenous peoples.
To quantify changes due to recent urbanization, I resurveyed 184 vegetation plots originally surveyed in 1968. I documented striking increases in plot-level and total species richness, but a decline in the variation in plant community composition between plots, a phenomenon called ‘biotic homogenization’. Exotic species were more likely than natives to increase over time, but exotic colonizations were not correlated with biotic homogenization or native declines. Plant life history traits predicted colonizations based on landscape context within 500m of a plot, but extirpations were rare and much less predictable, suggesting time lags in plant community response to landscape disturbance and fragmentation, and a potential extinction debt.
I used plant microfossils called phytoliths extracted from soil to investigate changes in plant communities prior to European settlement. I established that the ratio of asterosclereid phytoliths produced in the needles of Douglas-fir (Pseudotsuga menziesii) to the rondel phytoliths produced by most grasses can accurately distinguish between Douglas-fir dominated forests and Garry oak (Quercus garryana) savannah habitats today. I then examined changes in this ratio with depth at seven local sites, finding that infilling by Douglas-fir forest first began at different times, depending on the site. However, some savannah sites have supported grassy vegetation for at least two thousand years. Active management to maintain open conditions will be necessary to preserve rare species that evolved in these conditions.
These investigations demonstrate that examining the history of plant communities can reveal surprises and challenge assumptions about how they respond to disturbance. This knowledge can improve ecological theory, and inform management and conservation strategies. === Science, Faculty of === Botany, Department of === Graduate
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