Disturbance and exotic plant species are important factors structuring plant communities. Within the intermountain west of the USA, many plant communities have been disturbed by grazing, logging, and wildfire. Exotic plants, and their interactions with disturbance, have the potential to further transform these altered communities. The magnitude of change, however, is dependent upon the plant community’s resistance and resilience. Existing communities may (1) resist change, (2) be resilient (quickly return to a community with similar structure and function), (3) change to a new, but similar steady state, or (4) be transformed to a novel system.
We tested the resistance and resilience of previously altered communities to different forms of disturbance, and how the presence of an exotic species would alter community response. We studied three Linaria vulgaris-invaded plant communities in close proximity to each other: a logged forest, a previously burned forest, and a meadow. Treatments of digging (representing native animal disturbance), fire, herbicide to control the L. vulgaris, and no disturbance (control) were imposed, and sites were monitored (percent cover of each species) for changes in plant community six times over an eight year period. Resistance (1-Bray dissimilarity [yr1:yr2]) and resilience (1-Bray dissimilarity [yr1:yr_subsequent]) were calculated.
Results/Conclusions
The initial cover of L. vulgaris did not affect community resistance to disturbance, but the response to disturbance varied. Logged and meadow communities were least resistant to digging, while burning, herbicide, and control were equally resistant. At the previously burned site, the community was least resistant to herbicide, with no differences between the other treatments. Resilience was not affected by initial L. vulgaris cover at the logged or meadow sites, but it was at the burned site. At both the logged and meadow sites, digging plots were initially less resilient than other treatments, with the plant community taking several years to recover. At the burned site, community resilience decreased with increasing L. vulgaris cover for the burning, digging, and herbicide treatments, and these treatments resulted in lower resilience than the control for at least the first three years. However, for all sites there were no disturbance treatment effects on resilience by year eight, indicating that disturbance did not create novel plant communities. Overall, even the control plots showed a lack of stability, with resilience values ranging from approximately 0.26 to 0.72. These results indicate that natural inter-annual variation may have greater impact on plant community composition than single disturbance events.