UC Santa Barbara

Vernal pools are temporary wetlands that only form when rainwater pools in depressions, often within a grassland landscape. Over 95% of California's vernal pool wetlands have been lost to land use change, and remaining pools are often invaded by European annual grasses. To counteract this degradation, efforts to restore vernal pools in Santa Barbara began in the 1980s, which involved intensive short-term restoration actions such as the excavation of pool basins and 1-3 years of seeding with endemic pool species. My research evaluates the resilience of native plant assemblages in restored pools over time, after short-term restoration actions were completed. In a survey of 69 restored pools along the coast Santa Barbara County, I found that native plants were abundant throughout pool bottoms, but that the edges of pools that had been restored decades ago had lower native plants and/or higher exotic plants than pools restored more recently. Vernal pool edges are often surrounded by exotic-dominated grasslands, and exotic grasses can invade restored pools over time in the absence of long-term management. After these exotic grasses are established, they produce large amounts of plant litter, or “thatch”. This thatch allows for the germination of more exotic grasses but suppresses the germination of native species in subsequent years. I designed a long-term experiment to reduce exotic grass thatch in pool edges via annual raking to disturb or remove thatch. I coupled thatch raking with the addition of native plant seed to test whether the extant seed bank was also limiting native plant populations. After three years of thatch removal and native seed addition, native plant abundance and diversity increased. However, not all seeded species were able to establish, suggesting that native seed germination or seedling survival was limiting populations of some native species. I hypothesized that the performance of native populations depends on characteristics of the source population. Intraspecific variation, e.g., phenotypic plasticity or local adaptation, may result in different populations responding differently to environmental stressors such as drought and invasive species. Invasive grasses are prevalent in and around Santa Barbara vernal pools, and Santa Barbara is also predicted to experience a shorter winter wet season and a longer summer/fall drought in the future. I set up a common garden greenhouse experiment to evaluate intraspecific differences in the drought and competition tolerance of the native rush, Juncus bufonius, from populations in Santa Barbara and San Diego Counties. When I subjected the populations to drought regimes and planted them in competition with the invasive species Festuca perennis, I found differences in plant performance among populations. Specifically, the Santa Barbara population exhibited the highest mortality and the lowest biomass production when subjected to drought, one San Diego population produced the greatest biomass under the drought and competition treatments, and the other San Diego population had the most significant negative effect on F. perennis when grown together in competition. This evidence of intraspecific plant performance can inform seed sourcing strategies for future restoration: because local Santa Barbara populations exhibit intolerance to drought and competition, and because drought and competition are prevalent threats in Santa Barbara, sourcing seed from a variety of other populations that exhibit higher tolerance, such as those from San Diego, may increase the performance of restored populations. Overall, my studies and research findings highlight the need for long-term monitoring and adaptive management strategies to track the performance of restored assemblages over time and to address ongoing threats. My research demonstrates the need and opportunity for investing in long-term stewardship actions to increase the persistence of native plant assemblages.