Abstract
Whether plant populations are limited by seed or microsite availability is a long-standing debate. However, since both can be important, increasing emphasis is placed on disentangling their relative importance and how they vary through space and time. Although uncommon, seed addition studies that include multiple levels of seed augmentation, and follow plants through to the adult stage, are critical to achieving this goal. Such data are also vital to understanding when biotic pressures, such as herbivory, influence plant abundance. In this study, we experimentally added seeds of a native thistle, Cirsium canescens, at four augmentation densities to plots at two long-term study sites and quantified densities of seedlings and reproductive adults over 9 years. Recruitment to both seedling and adult stages was strongly seed-limited at both sites; however, the relative strength of seed limitation decreased with plant age. Fitting alternative recruitment functions to our data indicated that post-dispersal mortality factors were important as well. Strong density-dependent mortality limited recruitment at one site, while density-independent limitation predominated at the other. Overall, our experimental seed addition demonstrates that the environment at these sites remains suitable for C. canescens survival to reproduction and that seed availability limits adult densities. The results thus provide support for the hypothesis that seed losses due to the invasive weevil, Rhinocyllus conicus, rather than shifting microsite conditions, are driving C. canescens population declines. Shifts in the importance of density-dependent recruitment limitation between sites highlights that alternate strategies may be necessary to recover plant populations at different locations.
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Acknowledgements
We appreciate access to the study sites provided by The Nature Conservancy, Nebraska Chapter, and the support provided by both TNC staff and the University of Nebraska, Cedar Point Biological Station personnel. Research participation was made possible by grants from The Nature Conservancy (David H. Smith Post-Doctoral Fellowship to T. A. Rand), and from the National Science Foundation, Division of Environmental Biology (DEB96-15299, DEB-0414777) and the U.S.D.A., National Research Initiative (OEP2000-00848) for the other participants.
Funding
This study funded in part by Grants from The Nature Conservancy (David H. Smith Post-Doctoral Fellowship to T. A. Rand), and from the National Science Foundation, Division of Environmental Biology (DEB96-15299, DEB-0414777) and the U.S.D.A., National Research Initiative (OEP2000-00848) for the other participants.
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SML conceived and designed the surveys and experiments. SML, TAR, FLR and NMW performed the surveys and experiments. NMW analyzed the data. TAR and NMW wrote the manuscript; SML and FLR revised the manuscript.
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Communicated by Sarah M. Emery.
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Rand, T.A., West, N.M., Russell, F.L. et al. Post-dispersal factors influence recruitment patterns but do not override the importance of seed limitation in populations of a native thistle. Oecologia 193, 143–153 (2020). https://doi.org/10.1007/s00442-020-04656-2
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DOI: https://doi.org/10.1007/s00442-020-04656-2