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The roles of exotic grasses and forbs when restoring native species to highly invaded southern California annual grassland

Abstract

Many semi-arid shrublands in the western US have experienced invasion by a suite of exotic grasses and forbs that have altered community structure and function. The effect of the exotic grasses in this area has been studied, but little is known about how exotic forbs influence the plant community. A 3-year experiment in southern California coastal sage scrub (CSS) now dominated by exotic grasses was done to investigate the influence of both exotic grasses (mainly Bromus spp.) and exotic forbs (mainly Erodium spp.) on a restoration seeding (9 species, including grasses, forbs, and shrubs). Experimental plots were weeded to remove one, both, or neither group of exotic species and seeded at a high rate with a mix of native species. Abundance of all species varied with precipitation levels, but seeded species established best when both groups of exotic species were removed. The removal of exotic grasses resulted in an increase in exotic and native forb cover, while removal of exotic forbs led to an increase in exotic grass cover and, at least in one year, a decrease in native forb cover. In former CSS now converted to exotic annual grassland, a competitive hierarchy between exotic grasses and forbs may prevent native forbs from more fully occupying the habitat when either group of exotics is removed. This apparent competitive hierarchy may interact with yearly variation in precipitation levels to limit restoration seedings of CSS/exotic grassland communities. Therefore, management of CSS and exotic grassland in southern California and similar areas must consider control of both exotic grasses and forbs when restoration is attempted.

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Acknowledgments

We thank Christine Moen, Tom Ash, Jud Monroe, and others at the Western Riverside County Multi-species Habitat Reserve. Several undergraduate and graduate students helped with plot setup and data collection. We are thankful to the UC Riverside Center for Conservation Biology for support. We also thank Jodie Holt, Richard Minnich, three anonymous reviewers, and the Journal’s subject editor for comments on previous versions of this manuscript. This project was funded in part by the Shipley-Skinner Endowment to the College of Natural and Agricultural Sciences by the Center for Conservation Biology, and by National Science Foundation DEB 04-21530.

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Correspondence to Robert D. Cox.

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Cox, R.D., Allen, E.B. The roles of exotic grasses and forbs when restoring native species to highly invaded southern California annual grassland. Plant Ecol 212, 1699 (2011). https://doi.org/10.1007/s11258-011-9942-y

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Keywords

  • Bromus
  • California annual grassland
  • Coastal sage scrub
  • Erodium
  • Grassland restoration
  • Removal experiment