Community‐based long‐term management to address reinvasion of restored grassland vernal wetlands

Abstract Exotic annual grasses can dominate ecosystems by producing a thick layer of dead plant litter, hereafter “thatch,” which promotes the regeneration of exotic grasses and inhibits native plants. Vernal pool wetlands within a grassland matrix are threatened by these exotic annual grasses, meriting the need for long‐term management. We utilized the investment of local community members to test the efficacy of long‐term thatch management on urban vernal pool plant assemblages. We recruited over 40 undergraduate students to perform manual annual summer thatch removal around the edges of 15 urban vernal pools for four years. We coupled thatch removal with annual native seed addition because our analysis of environmental DNA (“eDNA”) in the soil seed bank revealed a lack of native plant species and an abundance of exotic plant species. Our annual thatch removal treatment successfully reduced thatch accumulation and increased bare ground, but it did not result in a consistent decrease in exotic plant cover or an increase in native plant cover. Instead, the effects of thatch manipulation on plant composition were modulated by annual precipitation, with exotic species increasing during dry years and native species increasing during wet years. The addition of native plant seed caused an increase in native plant species richness, but only after three years of annual thatch removal and native seed addition. Our results indicate that the restoration of native vernal pool plants can be limited by invasive species, native seed availability, and annual precipitation. Our findings show how engaging the local community in the long‐term restoration of urban ecosystems can address the persistent threat of invasion and build up capacity for native plant populations to increase over time.