Natural disturbance affects each organism or population of organisms differently, therefore changing the strength of interactions between them. For many plants, reproduction depends on an animal moving pollen between individuals, an interaction which is liable to change greatly after severe disturbance. In the northern California coast range, wildfires are rare but regular and change both pollinator communities and characteristics of plants. The annual mint Trichostema laxum is a small plant in normal years, pollinated by small-bodied bees, and has a mixed-mating system. After fire, plants are markedly larger, with more flowers, and pollinator communities shift to larger-bodied bee species. In this study, we used pre-burn and post-burn data, as well as a matrix of burned and unburned sites resulting from two chaparral fires to assess the impacts of fire on pollinator communities, plant morphology, and outcrossing rate.
Results/Conclusions
Outcrossing rates were significantly lower in burned areas compared to adjacent unburned areas. However, this pattern was solely due to the far larger size of plants in burned areas (due to increased water, nutrients, and/or light); outcrossing rate was negatively correlated with plant size. The pollinator community in burned areas consisted of larger bodied bees, which visited more flowers per plant visit. Flowers visited per foraging bout increased with increasing plant size, suggesting increased geitonogamy caused by this pollinator behavior. Visitation rate on a per flower basis did not decrease with increasing plant size, as might be predicted, we have no evidence for autogamy increasing. In conclusion, these fires changed the morphology and pollinator community; these effects combined to decrease outcrossing rate of this annual plant following this natural disturbance.