Long-term changes in plant community diversity in the human-dominated landscape of southeastern Vancouver Island, Canada

Background/Question/Methods

The Douglas-fir forests and oak savannahs of Southeastern Vancouver Island are among the rarest vegetation types in British Columbia, and they have been extensively disturbed by logging, agriculture and development. A long-term perspective is needed to understand the effect of human disturbance and other potential drivers (e.g. climate change, nitrogen deposition) on plant communities. We resurveyed 184 vegetation plots 40 years after they were originally surveyed and compared plant community diversity and composition between the two time periods in three community types. We predicted that the spread of exotics across the landscape would lead to an increase in plot-level species richness, and a decrease in beta diversity (biotic homogenization). We also compiled data on several plant traits: specific leaf area, geographic range position, seed weight, disturbance tolerance, deer palatability, and nitrogen regime preference. We tested for significant shifts over time in average abundance-weighted values of these traits in each plot.

Results/Conclusions

Paired t-tests indicate an average increase in plot-level species richness of 38% (eight species) over the past four decades. Regional diversity (the total number of species across all plots) has increased by 32%, while compositional differences across plots (beta diversity) decreased significantly. The species that increased in frequency were significantly more likely to be exotic species, but, surprisingly, native species richness also increased, particularly in Douglas-fir forests. There were significant shifts in plot-averaged traits towards higher disturbance tolerance and higher nitrogen tolerance/preference. Our results point to human-mediated disturbance as the dominant driver of plant community changes in recent decades in this region, but – contrary to similar studies in other regions – the net result has actually been an increase in richness, both locally and regionally, and for both natives and exotics, despite some degree of biotic homogenization.