Intraspecific variation in plant reproductive schedules: Individual flowering times, population-level flowering curves, and pollinator visitation patterns

Background/Question/Methods

Many plant populations consist of individuals that flower at different times. However, relatively little is known about the way in which individual plants compose a population-level flowering curve. These patterns have important ecological and evolutionary implications, such as understanding selection regimes for flowering time (stabilizing vs. directional selection), life history trade-offs involved in the timing of reproduction, and the ability of a population’s flowering phenology to adapt to climate change. I followed the individual flowering schedules of three plant species in subalpine meadows of the Colorado Rocky Mountains over the summer 2013: Potentilla pulcherrima, Linum lewisii, and Heterotheca villosa. These species were selected because they are abundant in subalpine meadows in the study area. Additionally, receipt of outcross vs. self pollen results in higher reproductive success in these species, suggesting pollinators may play an important role in their flowering schedules. We counted the number of flowers on tagged individuals every other day and observed pollinator visits to flowers of each species during 10min observation sessions throughout the growing season. A common expectation is that higher floral abundances attract proportionally more pollinators (e.g. at peak flowering), in which case pollinator visitation rates should increase with intraspecific floral abundance. 

Results/Conclusions

Flowering curves were generally consistent at individual and population levels. Potentilla flowering curves were irregular at both levels, and Heterotheca flowering curves were bimodal at both. Linum population-level flowering curves were positively-skewed; many individual curves were positively-skewed and some were normally distributed. For all three species, earlier-flowering individuals flowered for longer time periods than later-flowering individuals, inconsistent with the common hypothesis that early individuals may have limited reproductive capacity. The maximum number of flowering individuals coincided with the population-level peak in floral abundance in Linum and Potentilla. In contrast, the highest number of Heterotheca individuals in flower occurred during the second population-level peak in floral abundance, suggesting that chances for outcrossing may be highest later in the flowering period (18.0 ± 13.6 days between respective peaks, N = 6 plots). Contrary to the common expectation, visitation rates did not increase with increasing intraspecific floral abundance for any of the three plant species. Instead, pollinators may become saturated at higher floral abundances. These results are suggestive of a trade-off between the probability of outcrossing and reproductive assurance for individual plants. A temporally explicit framework for intraspecific variation will improve our understanding of ecological and evolutionary dynamics within populations and communities.