UC Davis

Grasslands cover 40% of the world’s land surface and provide a large range of ecosystem services including carbon storage, nutrient cycling, and habitat for diverse species. However, these systems and the services they provide are threatened by global environmental changes, including species invasions, drought, and atmospheric nitrogen (N) deposition. Grasslands often require management practices that support the diverse plant communities driving ecosystem functions. In many grasslands, this involves the use of disturbances such as herbivory through introducing large grazers (or mowing/clipping as a proxy) and fire through the application of prescribed burns. Overcoming biodiversity losses may also involve planting native species for restoration of degraded grasslands. However, how these grassland management and restoration practices affect ecosystem functions, particularly in the face of climate variability and rising atmospheric N deposition, remains underexplored. A large body of research now focuses on plant traits and the functional diversity of plants in a community to understand how they can affect grassland resilience to environmental changes. The goals of this research are to understand how grassland restoration practices affect grassland communities and functions under climate changes, and to explore plant trait diversity as a regulator of critical grassland ecosystem functions.Chapter 1 uses long-term data in an invaded California grassland experiment to evaluate clipping management effects on grassland communities and ecosystem functions in an experimental drought. Results revealed that drought did not affect the relative abundances of native and invasive plants, but it increased aboveground plant biomass and decreased root biomass allocation. Clipping management increased root biomass post-drought, but this disturbance also facilitated the spread of noxious invaders, highlighting the complexity of restoration strategies and their outcomes. However, neither clipping, experimental drought, nor invasive species altered soil organic carbon. Chapter 2 investigated the impact of rising atmospheric N deposition on grassland ecosystems using a long-term N fertilization experiment in the same California grassland. Like drought effects, long-term N fertilization did not significantly alter the relative abundance of native or invasive plant groups, but it did influence ecosystem functions like increasing soil N cycling rates and belowground biomass. Clipping management did not alter plant community or ecosystem responses to N deposition. Plant community composition was more strongly influenced by priority effects than by N deposition or clipping management. Chapter 3 examined how prescribed fire and grazing affect soil ecosystem functions in mesic tallgrass prairies and to what extent this may be mediated by plant taxonomic or functional trait diversity. The direct impacts of these management practices on soil properties were significant, with bison presence increasing soil N and recent fires decreasing carbon mineralization rates. However, many soil properties like microbial biomass carbon, microbial biomass N, and soil organic matter, remained unaffected. Older restoration sites tended to have reduced soil nitrate levels, and this was slightly mediated by higher functional trait diversity in older plots. However, plant diversity did not mediate grazing or fire effects on soil and did not have strong impacts on any other soil functions measured. Finally, Chapter 4 used a biodiversity-ecosystem function experiment to explore the relationship between plant diversity (functional and taxonomic) and soil extracellular enzyme activities (EEAs) in a tallgrass prairie. Plant diversity did not significantly influence most of the EEAs studied, except for the activity of N-acetyl-β-D-glucosaminidase (NAG), an enzyme that degrades chitin. This suggests other factors besides plant taxonomic or functional trait diversity may play a more crucial role in regulating these activities, but plant diversity might interact with fungal abundance in a way that affects chitin availability as a substrate for these enzymes and the microbes that produce them. Collectively, these studies improve our understanding of plant-soil relationships and underscore the complexity of grassland restoration and the need for nuanced management strategies to enhance ecosystem functions and resilience especially in the face of global environmental changes.