Efficacy of tarping, herbicide, soil removal, and mulching as strategies for restoring California coastal prairie grasses

Background/Question/Methods

Restoration of Mediterranean grasslands is strongly impeded by lack of native propagules and competition with many species of exotic grasses and forbs.  We report on a study testing several non-chemical methods for exotic plant control combined with planting native grasses for restoring prairies in coastal California.  Specifically we compared tarping (shading out recently germinated seedlings with black plastic) once or twice, topsoil removal, herbicide, and no treatment in factorial combinations with or without wood mulch as methods to reduce exotic plant cover.  Into each of the ten treatments we planted three native grass species. Elymus glaucus, Hordeum brachyantherum, and Stipa pulchra.  We monitored survival and cover of individual planted grass seedlings, and cover of native and exotic plant guilds for three growing seasons.

Results/Conclusions

Survival of grass species was high in all treatments, but significantly lower in unmulched soil removal and control treatments in the first two years.  In the first two growing seasons, mulch, tarping, and herbicide were all effective in enhancing native grass cover and reducing exotic grass cover, but by the third growing season the treatment effects had mostly converged. Soil removal was not effective in enhancing native establishment likely because it altered the microtopography in this system, which resulted in flooding of the plots during the first growing season. Our results suggest that tarping once in the fall following the first rains and mulching are effective methods for reducing cover of exotic grasses during the early stages of coastal prairie restoration, but that longer term monitoring is needed to evaluate the efficacy of restoration efforts. The most appropriate approach to controlling exotics will depend on relative costs and site constraints (e.g. chemical use restrictions).