PARENT SESSION
Poster Session #41: Plant Demography.
Thursday, August 9, 2001. Presentation from 3:30 PM to 5:00 PM. Exhibition Hall
104
Pollen limitation, rainfall, and cyclical masting in a perennial wildflower, Astragalus scaphoides.
Crone, Elizabeth1, Lesica, Peter2, 1 2
ABSTRACT- In plant ecology, the importance of external resource availability in determining population and community dynamics is virtually dogmatic. A notable exception to this generalization is masting, that is, high variance in reproductive output, synchronized among individuals within a population. Here, we explore factors driving cyclical flowering of a long-lived herbaceous perennial wildflower, Astragalus scaphoides, endemic to SW Montana and adjacent Idaho. Time series analysis from a 14-year monitoring study shows marked alternation between years with high and low flowering. Drawing on life history theory for alternate bearing and masting in trees, we hypothesize that patterns of flowering are due to some combination of life history tradeoffs (between flowering effort in one year and ability to flower the following year) and external resource availability (that may sometimes allow plants to flower in successive years, and synchronize individuals within populations), possibly reinforced by interactions with insects (plants may be more pollen-limited or suffer greater seed predation if they flower asynchronously). Field experiments show statistically significant pollen limitation in low-flowering years and sites, apparently due to behavioral changes of a key pollinator, Bombus nevadensis. Supplemental watering also significantly increases individual growth and survivorship. However, neither of these effects alone is large enough to drive cyclical flowering. Using simple empirically-based models, we explore the significance of both processes combined.
KEY WORDS: masting, pollen limitation, water limitation, Astragalus scaphoides