UC Riverside

Invasive species are a major driver of species loss in the worldwide biodiversity crisis. Among them, invasive plants can have uniquely deleterious ecological impacts, as changes to vegetation cover can dramatically alter the frequency and intensity of major disturbances like wildfire or soil disruption. Furthermore, the control of invasive plants is often costly and disturbance-intensive in and of itself, which can cause unintended secondary impacts including reinvasion, secondary invasion, and incomplete recovery of biodiversity. There are many interacting factors influencing the dynamics of plant community recovery, making responses to management actions unpredictable. This has led many land management agencies to pursue a less costly strategy of invasive plant mitigation rather than more resource-intensive strategies of full ecological restoration. This can lead to a positive feedback loop of management-associated disturbance and reinvasion, without significant progress towards the restoration of native biodiversity. Fortunately, recent developments in ecological theory regarding the interaction of plant traits, disturbance intensity, and plant community responses may provide new avenues to improve the efficiency and efficacy of vegetation management. I constructed my dissertation to investigate the underlying causes of native plant community responses to plant invasions and disturbance-intensive land management. My dissertation includes a combination of observational studies and manipulative experiments designed to disentangle these factors, assisting land managers to integrate ecological theory with practice. My first chapter assessed the influence of fire roads and bulldozer lines on the spatial spread of different plant functional groups after wildfire, and whether plant seedling traits could design better seed mixes for postfire restoration. I found that bulldozer lines substantially increased the spread of invasive grasses from the fire road, but diversity was slightly higher in bulldozer lines than unbulldozed areas. While native species were less abundant in bulldozer lines the first year after fire, they regenerated to similar levels as unbulldozed areas by the second year. Additionally, seed treatments were largely ineffective in altering plant community composition in bulldozer lines. These results suggest that while disturbances associated with wildland firefighting can increase the spatial spread of invasive grasses, native plant communities can be highly resilient to multiple disturbances including fire and soil tillage. My second chapter leveraged data from a 15-year invasive plant removal effort combined with an observational study of native plant cover in one of the last few years of the project. I found that the disturbance-intensive manual removal of the target invader led to near-complete recovery of biodiversity and species richness in treated areas, with significant but relatively small impacts to plant community composition. My findings indicate that native plant communities can be highly resilient to long-term invasion, and that disturbance-intensive invasive plant control does not necessarily lead to negative secondary impacts like reinvasion or secondary invasion. My dissertation provides important examples of how native plant communities can be resilient to intense disturbance associated with land management activities, which can provide valuable context as agencies design and adapt vegetation management strategies to a changing world.