Evaluating Restoration Methods across a Range of Plant Communities Dominated by Invasive Annual Grasses to Native Perennial Grasses

Abstract Prairies are imperiled habitats, with remnants being generally small and often existing in isolation. Invasive plants have the potential to invade not just the edge of small remnants but also the interior because smaller remnants experience greater edge effects than do large, contiguous prairies. Additionally, invasive plants limit recruitment of native plants, which can arrest secondary succession. We proposed to assess techniques for restoration that included removing annual grasses and supplementing native species recruitment with seeding of native grass and forb species. We also assessed the effect of specific factors affecting recruitment: soil moisture and seed predation. Treatments included broadcast, spot, or no application of the herbicides imazapic and glyphosate and with or without seeding plus mulch. With treatments nested within each of three plant communities, ranging from annual- to perennial-dominated communities, in four blocks per community, plant characteristics (percentage of cover and plant density), soil moisture availability, and seed-predation losses were measured along a plant community gradient within one season at two locations. A combination of broadcast herbicide application and seeding with mulching was found to be more effective in reducing annual grasses and enhancing the establishment of native grass species in predominately annual and mixed communities (annuals and perennials). Spot herbicide application was effective in predominately perennial communities, whereas only seeding native species did not improve recruitment. Although seed predation reduced seedling recruitment, mulch provided seed protection and enhanced soil moisture retention. Plant community response to imposed treatments differed among communities, suggesting that a decision support tool would facilitate management decisions tailored for each plant community. The decision tool would be useful to ensure that appropriate treatments are applied and that specific factors affecting recruitment, such as seed predation and soil moisture, are addressed. Nomenclature: Glyphosate; imazapic. Interpretive Summary: Restoration of grasslands in the Pacific Northwest from annual, nonindigenous plant–dominated communities to native, perennial plant–dominated communities is impeded by loss of viable native perennial seed reserves and by competition from the nonindigenous annuals. Addressing those challenges within the framework of a decision mechanism that considers initial plant community composition would allow restoration strategies tailored to emphasize techniques appropriate to the challenges faced. Our study assessed techniques for restoration within Palouse Prairie that included removing annual grasses by broadcast and spot-herbicide applications and supplementing native species recruitment with seeding of native grass and forb species across a plant community gradient. We also assessed seed predation and soil moisture as specific factors affecting plant recruitment. The results showed that response to treatment differed for annual and perennial grasses and forbs among the plant communities. High seed predation, ranging from 40 and 85% was observed. Although seed predation was associated with the observed low recruitment of native perennial grasses, mulch seemed to provide seed protection and enhanced soil moisture retention. The results enabled us to develop a decision mechanism that suggests (1) predominately perennial plant communities do not benefit from added seed, but spot herbicide treatment can reduce annual grass cover; (2) broadcast herbicide applications are required in mixed plant communities, even when foliar cover of perennial grasses approaches 20%; and (3) predominately annual plant communities should be seeded along with the use of a broadcast herbicide application. Decision mechanisms focused on foliar cover and density of dominant plant species can inform restoration efforts within the Palouse Prairie and are likely useful within other Pacific Northwest grasslands as well.