Historical flowering phenology, invasion by exotic grasses, and pollinator resources

Background/Question/Methods   Patterns of flowering phenology in plant communities determine the sequence and timing of resource availability for pollinators. When native plant abundance is reduced in invaded communities, seasonally specific gaps in resources may be created, especially if there is little synchrony among blooming species; seasonal gaps could influence pollinator populations and feed back to plant populations in future generations. Alternatively, if communities show extensive flowering synchrony among plant species, gaps may be less important. I used herbarium specimens to construct theoretical flowering-time (FT) curves for 20 of the most common biotically pollinated native and exotic plant species in the Willamette Valley/Puget Trough prairies of the Pacific Northwest. Most herbarium records covered the past 100-120 years. I associated geographically and temporally specific historical climate data with each herbarium specimen and regressed climatic variables against dates of flowering. I used this relationship to project contemporary FT curves for all species at 10 prairie sites along a climatic gradient, which vary in their degree of invasion. I weighted FT curves by contemporary species abundances to seek potential seasonal community-wide gaps in floral resources.

Results/Conclusions   Most prairies showed extensive overlap in flowering phenology and significant FT overlap between native and exotic flowers. This overlap suggests that community-wide gaps in floral resources, which might be problematic for pollinators, has not resulted from invasion. However, the abundance of biotically-pollinated native forbs is reduced in grass-invaded prairies, implying an overall decrease in pollinator resources. Thus, host-specific pollinators on native forbs will be especially affected by grass invasion.