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

Exotic annual grasses can dominate ecosystems by producing a thick layer
of dead plant litter, hereafter “thatch”, that 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. We measured vegetation
composition in a set of 180 permanent monitoring quadrats (12 per
experimental pool) over five years to quantify the effect of annual thatch
removal and native seed addition on thatch, bare ground, native plant
species cover and richness, and exotic plant species cover and
richness. 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 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.