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Master of Science (MS) in Biology
With over 99.9% of the Palouse prairie lost to land conversion, restoring native plant communities is crucial for ecological function, however, research on Palouse prairie restoration methods is sparse. Seed-based restoration uses a mix of seeded species to enhance competition against weeds, diversify vegetation, and adapt to environmental conditions. However, many factors can be varied, such as seed mix diversity and composition, the proportion of forbs to grasses, and seeding rate, and the most effective levels of each are not clear. Further, soil amendments, such as biochar, may benefit properties of tilled soils, but have not explored in Palouse Prairie systems. To address these knowledge gaps and inform Palouse Prairie restoration efforts, I conducted an investigation composed of 2 experiments. For experiment 1, my objectives were to test the hypotheses that (1) increasing levels of seed mix diversity, forb content, and biochar treatment amount would each increase native plant cover and species richness and decrease non-native plant cover and richness in the first-year after establishment, (2) biochar and topography would interact to increase native cover in dry areas with higher biochar treatment, and (3) moist topographic zones would improve restoration outcomes. For experiment 2, my objectives were to test the hypotheses that (1) higher seeding rate would increase native cover and reduce non-native cover and richness, and (2) species identity would interact with topography and seeding rate to affect native and non-native establishment. My experiments were established on a 15-acre pilot site of Eastern Washington University’s Prairie Restoration Project in Cheney, Washington. For experiment 1, 4 seed mixes were drill-seeded in a randomized block design across a topographic gradient. Biochar treatments of low (600 lbs/acre) and high (6000 lbs/acre) were overlaid onto the seed mixes. The four seed mixes included 1) 6 grasses, 2) 6 grasses with low diversity (8 spp.) forbs at 70% grass and 30% forbs, 3) 6 grasses with low diversity (8 spp.) forbs at 60% grass and 40% forbs, and 4) 6 grasses with high diversity (15 spp.) forbs at 70% grass and 30% forbs. For Experiment 2, I broadcast-seeded 4 common Palouse Prairie restoration species in monoculture, 1 equal parts blend of the species, and an unseeded control across 4 seeding rates: 861, 1615, 2368, and 3122 pure live seed per m (superscript 2). Sampling for both experiments took place in late June 2022. In Experiment 1, native species richness and cover, and nonnative cover were only affected by topography, but not other factors. Seed mix diversity and topography interacted to affect non-native species richness, which was lower in swales than north slopes across treatments but higher in north slopes than top/south in grass-only treatments. Topography was a dominant influence on first-year outcomes, which were not significantly affected by grass-forb composition. While biochar did not affect native or nonnative species richness or cover, it did affect community composition. As biochar treatment level increased, the difference in community composition between topographic groups also increased. In Experiment 2, higher seeding rates did not clearly improve restoration performance. Instead, species identity and topography played key roles. Treatments of C. pulchella and A. millefolium led to more native cover than G. aristata or L. lewisii. Surprisingly, the blend treatment did not result in more native richness than the single-species treatments. For both experiments, future sampling is needed to test the longer-term effects of these mixes. My results indicate that (1) topography should be used to inform seed blend composition, (2) approaches outside of seed mix strategy may be necessary for suppressing invasive species, (3) biochar interacts with dry topographic zones to influence species composition, and (4) high-rate plantings may not be necessary for every species.
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Johnson, Thurman, "Examining the effects of seed mix diversity and composition, biochar application, seeding rate, species identity, and topography on Palouse Prairie restoration" (2023). EWU Masters Thesis Collection. 867.