An ongoing critical challenge in invasion ecology is the ability to predict invaderresponses to environmental change and their associated impacts on the native resident communities, especially in historically uninvaded systems such as North American drylands. Biological invasions occur over distinct stages such as introduction (i.e.,, species introduced into novel ranges), establishment (i.e., species establishing self-sustaining populations), and spread (i.e., species dispersing and expanding their ranges), and different factors may mediate invasion dynamics within each of these stages. Thus, I designed my dissertation to focus on identifying the drivers and impacts of invasive species establishment and spread, as well as investigating effective strategies to manage biological invasions in temporally variable environments. My dissertation uses a combination of observational, experimental, and modeling-based approaches to elucidate the mechanisms that allow invaders to establish, spread and persist in dryland systems. Chapter 1– spread – assessed the role of environmental niche shifts in facilitating the spread of a winter annual invasive plant, Brassica tournefortii (sahara mustard) in North America. Through a biogeographic approach, I discovered that despite having a relatively unchanged and consistent environmental niche (including climate and soil conditions), Brassica tournefortii still has a significant amount of available and unused habitat in its introduced North American range. These findings indicate that the invader may persist in spreading throughout North America and highlight potential regions where preventative measures can be taken. Chapter 2 – impact – leveraged a long-term dataset to understand how attributes of an invasion regime influence community synchrony and stability in a stabilized sand dune system. I found that both boom frequency and the dominant magnitude of invader boom are strong predictors of community synchrony and stability, becoming more asynchronous and less stable as the intensity of invasion increases. Further, the underlying invasion level mediates the dynamics that a system experiences. These findings move beyond basic invader abundance-native diversity metrics and shed light on additional factors that may be contributing to invader impacts in these historically uninvaded environments. Chapter 3 – management – addressed the role of seasonality during herbicide applications for the control of an emerging invasive winter annual forb, Oncosiphon pilulifer (stinknet). I found that seasonal herbicide efficacy is contingent on the density of invasive species stored within the soil seed bank, highlighting the importance of incorporating above and below-ground metrics of community composition when evaluating strategies for invasive plant management. For instance, in areas that have a long history of invasion (i.e., saturated invader seed bank), using pre-emergent herbicides provides higher control compared to post-emergents. These findings allowed me to make recommendations to land practitioners on when to apply herbicides to limit the spread of invasive species. Overall, the results from my dissertation provide insight into the mechanisms that allow invasive species to spread and persist within historically uninvaded landscapes.

With increasing alterations to disturbance regimes, understanding recovery mechanisms is vital to prevent habitat degradation. Plant traits can link responses from individuals across multiple levels of ecological processes to predict community recovery, but this may be contingent on selecting the right trait at the right life stage and scale of environmental gradient or the diversity of traits and functional strategies. Regeneration traits (early life stage traits), which often differ from adult traits, may be more indicative of successful establishment post-disturbance, yet are not used to predict recovery or used in restoration of degraded habitats. Therefore, I aimed to assess how regeneration traits mediate post-fire recovery and restoration in chaparral shrublands. I asked 1) how regeneration and adult traits differ and which is more predictive of recovery, 2) how spatial scale influences trait filtering and variation at different life stages, and 3) how regeneration trait functional strategies and restoration timing influence restoration success. To assess life stage trait differences, I collected functional traits for regenerating plants within multiple burn scars and for adults in nearby unburned areas in Southern California. To link traits to recovery, I measured community composition and survival over multiple years. To test the influence of spatial scales, I used the trait same sampling design across a regional scale elevation gradient and local scale aspect. To test chaparral restoration methods, I planted shrubs with different regeneration trait functional strategies (i.e., acquisitive, conservative, diverse) into burn scars at different times since fire. I found regeneration traits were more resource-acquisitive than adult traits, and the functional diversity of regeneration traits, but not adult traits, predicted recovery (Q1). Regeneration traits were more strongly filtered at the regional scale and had different drivers of trait variation compared to adults (Q2). The conservative regeneration trait functional strategy had higher survival one year post-planting, and planting sooner after fire improved restoration success (Q3). Overall, this work first highlights how regeneration traits can be used to improve recovery predictions, then provides experimental evidence to improve post-fire restoration outcomes using a trait-based approach.

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.

Understanding the potential mechanisms that influence invasion resistance and coexistence in plant communities has been a central tenet of invasion ecological research during the past few decades. My dissertation used observational and experimental approaches to understand what processes influence whether a community is invaded, resists invasion, or results in species coexistence within a California grassland. Chapter 2 reviewed the impacts that alien plant species may have on communities and provided a framework for how to identify when invader impacts lead to recovery constraints for the native community and integrate these constraints into restoration efforts. Chapter 3 investigated how species effects on resource availability can result in differing invasion dynamics in native versus exotic dominated grasslands. I found that while exotic and native species differentially alter the availability of light and nitrogen in a community, nitrogen availability is key in determining invasion of an exotic into a native grassland as well as the invasion of a native into an exotic dominated community. Chapter 4 investigated how propagule pressure after an extreme disturbance can result in the invasion of intact native grasslands. I found that the recovery of native grassland stands after an extreme disturbance (fire+drought) can be stalled by an influx of exotic propagules from the surrounding matrix. Chapter 5 addressed how the strength of plant-soil feedbacks for a native and exotic may change with soil resource availability changes on soil communities and with a competitor. I found a negative effect of exotic conditioned soil on native growth and no effect of native conditioned soil on exotic growth, suggesting that plant-soil feedbacks may facilitate the establishment of the exotic as well as its dominance. Lastly, Chapter 6 investigated how seed addition and soil amendments management efforts affected native recovery after an extreme disturbance. I found that seed additions and soil N reductions were able to increase the establishment and fitness of some natives, but may not be sufficient to promote full native recovery. This work provides a tool to understand not only why native resident communities are invaded but also how to reduce the resistance of invaded communities and increase the resistance of native communities. Additionally this work allowed me to integrate the impacts that exotic species have on communities to make general predictions about the recovery of native communities after an extreme disturbance or control efforts. Overall, I observed that native communities and populations are vulnerable to invasion after a large disturbance and with nitrogen enrichment. From low to moderate nitrogen availability, native and exotic species should coexist due to niche partitioning, but not as a result of density dependent negative plant-soil feedbacks. Lastly, I found that an exotic species is able to maintain its dominance due to its strong competitive effect on native species, particularly at high nitrogen availability and its ability to culture a soil community that negatively impacts the growth of native species.

Ecosystems are undergoing unprecedented rates of change with severe consequences for biodiversity loss, yet natural resilience and restoration of ecosystems provides hope to mitigate these losses. Despite the potential for natural recovery, there are thresholds and feedback mechanisms that inhibit recovery that are often driven by invasive species. Consequently, understanding how invasive species interact with their surrounding communities and how they respond to restoration efforts is crucial information for effective management and successful restoration. My dissertation seeks to understand how the impact of invasive plants prevents effective restoration through a combination of field experiments with paired greenhouse components to capture robust processes in the field and disentangle the underlying mechanisms in a controlled greenhouse setting. My first two chapters investigate how a local and regionally obnoxious novel invasive forb, Oncosiphon pilulifer, responds to the common management technique of prescribed fire, and how Oncosiphon interacts with soil biota to inhibit native plant growth. My final chapter focuses on how the multiple factors of seed limitation, invasive litter accumulation, and soil symbiont depletion constrain restoration success in a Northern Californian annual grassland. Taken together, my dissertation projects aim to both address fundamental questions in community ecology and produce actionable science for land management practitioners.