Restoration in Mediterranean-climate grasslands is strongly impeded by lack of native propagules and competition with exotic grasses and forbs. We report on a study testing several methods for exotic plant control combined with planting native grasses to restore prairies in former agricultural land in coastal California. Specifically we compared tarping (shading out recently germinated seedlings with black plastic) once, tarping twice, topsoil removal, herbicide (glyphosate), and a control treatment in factorial combinations with or without wood mulch. Into each treatment we planted three native grass species (Elymus glaucus, Hordeum brachyantherum, and Stipa pulchra) and monitored plant survival and cover for three growing seasons. Survival of native grass species was high in all treatments, but was slightly lower in unmulched soil removal and control treatments in the first 2 yr. Mulching, tarping, and herbicide were all effective in reducing exotic grass cover and enhancing native grass cover for the first 2 yr, but by the third growing season cover of the plant guilds and bare ground had mostly converged, primarily because of the declining effects of the initial treatments. Mulching and tarping were both considerably more expensive than herbicide treatment. Topsoil removal was less effective in increasing native grass cover likely because soil removal altered the surface hydrology in this system. Our results show that several treatments were effective in enhancing native grass establishment, but that longer term monitoring is needed to evaluate the efficacy of restoration efforts. The most appropriate approach to controlling exotics to restore specific grassland sites will depend not only on the effectiveness, but also on relative costs and site constraints.
Nomenclature: Glyphosate, blue wild rye, Elymus glaucus Buckley, meadow barley, Hordeum brachyantherum Nevski, purple needlegrass, Stipa pulchra Hitchc.
Management Implications: Restoring California grasslands requires extensive efforts to reduce competition with exotic grasses and forbs and reintroduction of native species. Herbicides are frequently used to control exotic species, but given various concerns about their negative effects, cost-effective non-chemical strategies are needed. We compared five strategies for controlling exotic species in former agricultural lands to restore California coastal prairie species: tarping (shading out recently germinated seedlings with black plastic) once, tarping twice, topsoil removal, glyphosate herbicide, and wood mulch. Into each treatment we planted three native perennial grass species (blue wild rye, meadow barley, and purple needlegrass) and monitored plant survival and cover for three growing seasons. Our results show that using black plastic tarps to shade out recently germinated seedlings and applying wood mulch are both effective non-herbicide methods for reducing exotic grass cover and enhancing native grass cover in the initial stage of grassland restoration. Both approaches, however, are considerably more expensive than using herbicides unless they can be done with volunteer labor. Overall, tarping twice was no more effective than tarping once, so we do not recommend a second tarping. Removing topsoil resulted in a sufficient change in hydrology that the sites experienced extended flooding in the first year which reducing native grass survival, but this approach may be more effective in sites with different topography. The effectiveness of our restoration treatments declined substantially by the third year, highlighting the importance of ongoing monitoring to compare the efficacy of restoration strategies, combined with continued exotic control. Herbicide, tarping, and mulch were all effective in reducing exotic grasses for the first 2 yr of restoration. The most cost-effective strategy for reducing ex