Long-term effects of management and climate on California’s grassland flora and rare plant species

The California floristic province is home to a rich diversity of plant species, and the ecosystems they compose have a long and complex history of human management and anthropogenic disturbance. This is especially true of native grassland habitats, which have been burned, grazed, and replaced by agriculture and housing, and are now present in only a small fraction of the area they once covered. More recently, restoration and management activities increasingly seek to maintain and improve the plant diversity of California grasslands, but the effectiveness of typical strategies may alter under a changing climate. In this dissertation, I examined evidence of climate change effects on California grassland communities, the long-term effectiveness of livestock grazing as strategy to conserve native species, and the lessons that several decades of rare plant reintroductions have for future projects. In the first chapter, I used eight datasets collected over periods of 12 to 33 years to examine whether global climate change has altered California grassland vegetation communities. I used a metric known as the Community Temperature Index (CTI), which draws on historical species distribution records and spatial climate data to measure the relative dominance of species adapted to warmer and cooler temperatures within a location. I found evidence of long-term (1950-2019) increases in temperature and vapor pressure deficit at the sites I analyzed, though shorter-term study-period weather patterns were more variable. Six of the eight sites showed significant shifts in community composition towards warmer-climate species over time, and these increases occurred at faster rates than has been measured in other systems. Overall, the results suggest that some California grassland communities are shifting towards greater dominance by species adapted to warmer climates, but that these changes must be understood and interpreted within the history of abiotic conditions, long-term climate and weather history, and past land-use context of a site, as shorter-term weather patterns may not align with longer-term climate change and site conditions and past land management may exert a strong influence over community trajectory. My second chapter is focused on long-term grazing as a management strategy to maintain the diversity of native annual forbs in California coastal prairies in light of a recent historic drought and increasing temperatures. I resampled paired transects in eleven grazed and ungrazed sites from Monterey to Sonoma counties, California, 15 years after the original study. I found evidence to support the continued use of grazing to maintain higher native annual forb richness in coastal prairies, but also found that native annual forb richness had declined over 15 years in grazed prairies. Grazing continued to maintain low vegetation heights and thatch depths, and prevented shrub encroachment. I used circumstantial evidence from wetland indicator status and specific leaf area to support the hypothesis that severe drought and increasing aridity may be driving the declines in native annual forb richness that I measured, and explore how management and climate may interact to affect plant communities. In my third chapter, I synthesized lessons learned from reintroduction efforts for 14 listed plant species in California. Introductions and reintroductions of listed plant species are likely to be increasingly necessary in the future, so understanding how practitioners view their work and identifying persistent resource mismatches are key to the long-term viability of listed species. I interviewed practitioners to understand their definitions of recovery; how likely they felt recovery was; the advice they would share with other practitioners; and the resources they thought were lacking but that could make future projects more successful. I found that practitioners were generally guided by sound ecological theory and wanted to invest significant time and resources into understanding species biology and ecology, but that there were often barriers to success in the form of funding, time, and social constraints. Rare plant reintroductions are complicated by mismatches in timing and goals, but some individuals have been able to successfully navigate these challenges