A core challenge in community ecology has been understanding temporal variations in plant communities. This becomes even more challenging with plant communities increasingly exposed to plant invasions and novel environmental conditions. Understanding these changes isn’t only important for conserving and restoring native diversity and controlling noxious invasives but is also important because changes in plant communities can influence multiple ecosystem services. The research presented here focuses on three main grass functional groups of California inland grasslands:- early-season naturalized exotic annual grasses that dominate most of California’s valley grasslands
- recent noxious invasive annual grasses that are late-season and decrease diversity and rangeland productivity,
- native perennial grasses that are the focus of restoration efforts, and are active throughout the growing season, into the early summer, overlapping with the noxious weeds in late-season phenology.
I assess the roles of plant-soil feedbacks, priority effects, and long-term changes in precipitation in impacting the community dynamics among these three groups using a long-term grassland field experiment in Davis, CA.
In Chapter 1, I investigate the importance of plant-soil feedbacks in California grasslands and whether they mediate interactions between the exotic annual (naturalized and noxious) and native perennial grasses. While short-term greenhouse studies suggest the exotic annual grasses of California grasslands negatively impact native perennial grass growth through PSFs, there is limited knowledge of how these results extend to a more natural setting. I performed a long-term field feedback experiment, assessing multiple soil properties and plant performance variables. I found that both native and exotic grass groups were structured by negative feedbacks, suggesting that soil amelioration may not be necessary for native grassland restoration in California. Feedbacks were only detected by measuring multiple traits (e.g., root biomass, seed production), and was not visible by looking at aboveground biomass alone. While feedbacks were observed in a variable field setting, growing natives in competition with exotics had a far stronger effect on native performance than did the feedback effect.
In Chapter 2, I examined the role of priority effects in determining long-term community composition in a system that experienced high annual variability in weather, including a multi-year drought. The three grass functional groups were planted alone and in mixtures with the other groups, with composition assessed annually over a twelve-year period. I hypothesized that all three groups, when seeded alone and given priority, would have greater cover than when grown in competition, and limit recruitment of another naturally colonizing functional group. I found that priority effects were strong among all three groups in the initial years but were disrupted by the multi-year drought, which reset the communities. Priority effects disappeared between the two exotic annual species groups, but the native perennial grasses were able to continuously exclude the late-season noxious weeds, likely because they share phenology and compete for late-season residual soil moisture.
Finally, in Chapter 3, I investigated how the prevalence of these three grass functional groups differed when exposed to sustained augmented or reduced precipitation, as long-term changes in precipitation can affect interactions between exotic and native species. I found that phenology influenced the exotics’ response to reduced precipitation- the noxious weeds were highly drought sensitive and disappeared while the naturalized exotics increased in abundance. The impact of drought on natives, however, depended on the initial vegetation community, increasing in noxious weed communities yet decreasing in their own communities. The experiment didn’t successfully achieve an increased rainfall treatment, since the treatment added water to ambient precipitation, and ambient conditions were so dry, that water augmentation still largely led to below-average precipitation. I also assessed the existence of edge effects as potential reservoirs for future invasion by comparing the cover of each grass functional group between the edges and core of plots they were planted in or recruited into. The cover of the natives and the noxious weeds were identical between the core and edges, but the naturalized exotics were generally higher in the plots’ core than edges. This indicates that edges do not serve as a reservoir for future invasion into the plot core.