Reinvasion of restored California vernal pools reveals the importance of long‐term restoration planning

Ecological restoration often focuses on short‐term intervention efforts
with the goal of creating restored ecosystems that do not require
continuous human maintenance. Here, we ask: Do short‐term restoration
efforts result in self‐sustaining native assemblages, or do these restored
ecosystems require long‐term management to prevent reinvasion of exotic
species? We address this question using restored vernal pool wetlands in
coastal California. Restoration efforts in vernal pool ecosystems are
often hindered because many restored vernal pools exist within a grassland
matrix that is highly invaded by exotic annual grasses and forbs. To test
whether restored pools experienced reinvasion, we assessed plant species
abundance and diversity at varying times after intensive weeding had
ceased. The central bottom of pools, where inundation duration is the
longest, showed stable or even increasing native cover and no trends in
exotic abundance over time. However, exotic cover and richness increased
in the upland edges of the pools, where drier conditions allow exotic
grasses from the surrounding unrestored grassland to grow. Our findings
indicate that edges of restored ecosystems are susceptible to invasion
over time, but that this depends on abiotic and biotic conditions within
the ecosystem, such as pool shape and landscape matrix, that can
potentially be manipulated through initial planning (e.g., constructing
circular pools) and long‐term management (e.g., annual weeding). Our
findings highlight the importance of ongoing monitoring and adaptive
management and support a paradigm shift away from short‐term interventions
and toward viewing restoration as a longstanding relationship with the
land that may require continuous human management.