This dissertation focuses on the influence of planting conditions on the restoration of prairie communities, both in the applied context of improving restoration outcomes, and as a test of our understanding of mechanisms of community assembly. Ecological restoration is incredibly important in this era of human-induced landscape degradation, but restoration projects do not always result in outcomes that match practitioners’ goals. A possible factor in this variation is the weather conditions that occur at the outset of a restoration project that are typically referred to as “year effects.” However, we lack strong tests of year effects that disentangle them from site-to-site variation and other confounding factors, such as management. Additionally, there has been no investigation of the mechanisms for year effects; current evidence for this phenomenon is correlational and lacks direct experimentation. Inter-annual variation in precipitation is often assumed to be a major factor causing year effects, though there are countless potential drivers that could vary inter-annually. In addition to implications for restoration, evidence for year effects would suggest assembly is driven by stochastic mechanisms (i.e. random variation due to weather) as opposed to deterministic mechanisms (i.e. drivers with consistent, repeatable effects such as site soil characteristics or prescribed fire management). To address these challenges, I investigated the causes and consequences of planting-year variation on community assembly in a tallgrass prairie restoration system using two main methods: (I) an experimental manipulation of precipitation as part of a long-term prairie restoration at Michigan State University’s W. K. Kellogg Biological Station (KBS), and (II) an analysis of differences in species composition among 83 restored tallgrass prairies established in different years across the Midwestern United States of America. I first explored how year effects on seedling emergence and survival. I created 6 replicate prairies in three different years, using identical methods at a single large site. In each year, experimental plots within each restored prairie received water manipulation treatments for the first nine weeks after sowing. In these plots, rainfall was blocked with rain-out shelters and then watered low (1.39 cm/month), mean (9.91 cm/month), or high (17.81 cm/month) amounts compared to the 30-year average in the area. I found large differences in seedling emergence across the three planting years as well as across watering treatments. Initial community composition differed across years but not watering treatment. I continued to monitor these communities to determine if initial differences would persist. I found that some initial community differences dissipated while others persisted, suggesting the operation of both deterministic and stochastic assembly mechanisms in this system. In the second study, I looked for year effects in existing restored prairies across a wide geographic and temporal range. In summer 2016, colleagues and I visited 83 prairies in Illinois, Indiana, and Michigan that had been restored between 2000-2015 and determined their current plant community compositions. Using these data, I evaluated the importance of planting conditions using historical weather data on restoration outcomes, also accounting for different management histories and locations. I found that planting year precipitation had lasting effects on the richness and abundance of non-sown species. These signatures explained as much variation in the community as known drivers of assembly such as soil characteristics, fire frequency, and seed mix richness. These data provide additional support that planting year conditions can have profound effects on communities in sown restoration projects which can persist through assembly over time. Documentation of persistent year effects will be helpful for understanding the mechanisms that drive variation in restoration outcomes and could be used to inform tailored remedial management that addresses contingencies that arise after different first year conditions.
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