Overcoming ecological barriers to dryland restoration: innovative techniques and community-engaged approaches across spatial scales and environmental gradients

Abstract

Drylands are arid and semi-arid ecosystems characterized by water limitation. These landscapes are often overlooked, yet cover over 40% of the global terrestrial surface, support billions of people, and face high rates of degradation. Under anthropogenic climate change, drylands are expected to expand and increase in aridity. Restoration of plant communities in these water-limited systems is inherently challenging but identifying effective strategies to enhance restoration success is increasingly urgent. Given the large spatial extent of dryland areas in need of revegetation and associated cost constraints, seed-based restoration approaches are the most common. However, many restoration projects in drylands result in little or no establishment of desirable species. Major barriers to emergence include abiotic stress, invasive species dominance, degraded soils, and interannual climate variability. This dissertation evaluates how active restoration interventions targeting dispersal limitation, harsh abiotic conditions, and biotic interactions influence restoration outcomes across dryland systems in the western United States. The experiments included in the four chapters of this dissertation range in scale from a greenhouse study to a multi-site networked trial, and each uses a precision restoration framework, designed to address site-specific ecological barriers to restoration that vary across space and time. A further unifying theme is the use of community-engaged approaches throughout to develop and frame each study and to provide actionable insights to our research partners and the broader restoration community. Across these studies, climatic variability and competitive pressure constrained seed-based restoration outcomes. However, even small gains in the emergence and establishment of desirable species can have meaningful ecological impacts in degraded drylands. Key findings include, increasing propagule availability through active seeding led to increases in desirable species. Microsite climate modification strategies designed to retain moisture and, in some cases, reduce soil surface temperatures, showed promise when paired with sufficient precipitation. However, many of our more intensive restoration interventions, including soil inoculation, targeted livestock treatment, and seed pellets, had limited or context dependent effects, particularly under persistent drought conditions. We also found using herbicide as a restoration tool to control invasive species resulted in both target and non-target impacts, emphasizing the importance of careful consideration of site history, existing community composition, and the potential for secondary invasion following the removal of a target species. Pairing preemergent herbicide application with seed pellets made with activated carbon showed promise as a strategy to reseed desirable species under reduced competitive pressure. Ultimately, restoration outcomes are constrained by interacting ecological filters, including propagule availability, biotic, and abiotic constraints. Strategies to address these filters can lead to gains in restoration success but are often mediated by climatic conditions at the time of intervention and during early plant life stages. Successful restoration of drylands under climate change will depend on aligning interventions targeting site-specific ecological barriers with favorable episodic climatic conditions, using adaptive and iterative invasive species management, and continuing to test novel interventions.