COS 182-2 - Palouse Prairie Restoration in Eastern Washington: First-year Plant Cover and Richness Largely Driven by Topographic Variation, Not Seed Mix Diversity

Abstract: With over 99.99% of Palouse prairie lost to land conversion, restoring native plant communities is essential for regional ecosystem function. Little research exists on Palouse prairie restoration methods compared to other North American steppe communities, hindering the design of region-specific planting strategies. Seed-based restoration often uses species blends to bolster competition against weeds, diversify established vegetation, and hedge against environmental stochasticity. Increasing blend diversity could aid in establishing native species and hampering non-natives, but these advantages may not be present in the first growing season. Our research objectives were to test the hypotheses that increasing blend diversity would increase native plant cover and species richness and decrease non-native plant cover and richness in first-year establishment, with each factor also being influenced by topographic position. To test these hypotheses, we drill-seeded 3 diversity blends of native Palouse prairie species into 24 5.6 x 200 m strips on a 15-acre pilot site of Eastern Washington University’s Prairie Restoration Project in Cheney, Washington in October 2021. The 24 strips were seeded in a randomized-block design, with each strip containing 6 1-m2 plots distributed across a topographic gradient, totaling 144 plots. Blend treatments included 6 native bunchgrass species, the 6 grasses with low diversity (8 spp.) forbs, and the 6 grasses with high diversity (15 spp.) forbs. In June 2022, we recorded each plot’s plant species and their percent cover. We compared native and non-native cover and richness and community composition in each diversity treatment across three topographic positions: hilltop/south slope, north slope, and swale using mixed linear models. Blend diversity and topography interacted to affect non-native richness, which was lower in swales than north slopes across treatments, but higher in north than top/south in grass-only treatments and not different between top/south and swales. Our hypothesis that blend diversity would increase first-year native richness and cover and reduce non-native cover was not supported. Instead, we found that topography had a larger effect, with swales and north slopes having lower native richness and cover, and higher non-native cover than top/south. We expect the dynamics between blend diversity and resulting vegetation will become clearer with more time. Our results indicate that (1) topographic variation should be considered when structuring Palouse prairie seed blends, and (2) increasing blend diversity does not necessarily provide first-year competitive advantages but does interact with topography to affect community composition.