Abstract
We measured seed germination and seedling survivorship of spotted knapweed, Centaurea stoebe, in a series of laboratory and field experiments to evaluate the efficacy of seed limitation as a management focus. This work was initiated 6 years after introduction of several biological control agents. The soil seed bank of the site used in this study contained a mean density of 5,848 seeds/m2 (ranging from 0 to 16,364 seeds/m2), and 92% of the seeds isolated from soils were shriveled, discolored, and/or partially decayed. Additionally, none of the intact seeds germinated, suggesting that the viable seed bank at our field study site has been exhausted. Centaurea stoebe seeds were planted into pots under a range of soil nitrogen (N) availability, with half of the pots containing a single density of previously established seedlings of a native cool-season grass, slender wheatgrass (Elymus trachycaulus). A watering regime mimicking local precipitation was applied. Spotted knapweed exhibited large biomass responses to N addition, but the presence of grasses suppressed the ability to exploit this N. Surprisingly, low soil N conditions improved knapweed survivorship in the presence of grasses. Nevertheless, recruitment and biomass were still far below the levels reached in the absence of competition. To evaluate the effect of density on successful recruitment, Centaurea stoebe seed was introduced into a meadow at three densities matching reduced levels of seed production under the constraints of seed predators. These densities were sown with or without a seed mixture of native species, into an existing plant community lacking C. stoebe, and seedling recruitment was recorded over 2.5 years. Across all plots and densities sown (568–2,272 seeds m−2 year−1), seedling recruitment was less than 1%. The invasion potential of spotted knapweed was greatly diminished when realistic levels of plant competition and biological control limit seed production. We therefore conclude that a combination of seed limitation and shortage of ‘safe sites’ within undisturbed vegetation can limit densities of C. stoebe.
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Acknowledgments
We thank Christine Fairbanks, Kali Blevins, Justin Feis, and Dr. Nataly Ascarrunz for field and lab help. Drs. Deane Bowers, William Bowman, Carol Wessman, and Susan Beatty provided valuable input on drafts of the manuscript. We also thank Linda and Sergio Sanabria for use of their land to conduct research, and Tom Lemieux for assistance in the greenhouse. This work has been funded by the National Research Initiative of the USDA Cooperative State Research, Education and Extension Service, grant number 06-03618.
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Knochel, D.G., Flagg, C. & Seastedt, T.R. Effects of plant competition, seed predation, and nutrient limitation on seedling survivorship of spotted knapweed (Centaurea stoebe). Biol Invasions 12, 3771–3784 (2010). https://doi.org/10.1007/s10530-010-9769-9
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DOI: https://doi.org/10.1007/s10530-010-9769-9