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Pollen and water limitation in Astragalus scaphoides, a plant that flowers in alternate years

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Abstract

Mast seeding is common in plant populations, but its causes have rarely been tested experimentally. We tested mechanisms of alternate-year flowering and fruit set in an iteroparous, bee-pollinated, herbaceous plant, Astragalus scaphoides, in semi-arid sagebrush steppe. Patterns of reproduction from 1986 to 1999 indicated that spring precipitation was a cue for synchronous flowering, and that increased pollination in high-flowering years was a fitness advantage of synchrony. We tested these patterns by adding supplemental water and pollen to plants in high- and low-flowering sites and years. Supplemental water had no effect on flowering or seed set, so water is not a proximate cue for reproduction, though it could be important over longer (>3 year) time scales. Supplemental pollination increased fruit set in low- but not high-flowering years, indicating that synchronous flowering increases pollination success. Many shorter-term studies also report increased fruit set after pollen supplementation, but not after resource addition. This pattern may reflect the fact that plants can store and reallocate resources, but not pollen, across multiple years. For animal-pollinated herbs such as these, uniting theories about pollination ecology and mast seeding may promote an understanding of the mechanisms that determine patterns of reproduction over time.

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Acknowledgments

T. Alampi, P. Askey, G. Barr, J. Harris, A. Krause, K. Moffatt and S. Reed patiently assisted with hand pollinations and pollinator observations, H. Attowe and S. Miles watered apparently bare sage steppe plots in March, and V. Anglin provided logistic support for our field camp in Tendoy, ID. L. Harder and N. Williams contributed insight throughout this research, and A. Sala, L. Polansky, D. Inouye, the Maron and Crone lab groups at the University of Montana, and five anonymous reviewers provided valuable comments on early versions of this manuscript. This research was funded by an NSERC operating grant to E. Crone and by the University of Montana. Writing and analysis were conducted while E. Crone was supported by NSF DEB 02-40963 and NSF DEB 02-36427. To the best of our knowledge, these experiments comply with the laws of the United States of America.

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Correspondence to Elizabeth E. Crone.

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Communicated by Louis Pitelka.

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Crone, E.E., Lesica, P. Pollen and water limitation in Astragalus scaphoides, a plant that flowers in alternate years. Oecologia 150, 40–49 (2006). https://doi.org/10.1007/s00442-006-0506-0

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