Impacts of invasive species removal: How does restoration approach affect salt marsh functional recovery?

Background/Question/Methods

The goal of restoration is often the return of ecosystem services or functions.  However, the interaction between different restoration approaches and multiple ecosystem functions is poorly understood.  Within the San Francisco estuary, non-native cordgrass, Spartina alterniflora, hybridized with native S. foliosa producing an ecosystem engineer that converted unvegetated tidal mudflat to vegetated salt marsh habitat and displaced native plants.  This study utilizes the landscape scale restoration currently being implemented by San Francisco Estuary Invasive Spartina Project to assess the impacts of restoration approach on the recovery of ecosystem functions. We hypothesized active replanting would speed time to recovery.  We chose four categories of sites, representing four different restoration states: 1) invaded marsh: invasive hybrid Spartina present, no current restoration; 2) passive restoration: hybrid Spartina removed, no active revegetation; 3) active restoration: hybrid Spartina removed, revegetated with native S. foliosa; 4) uninvaded marsh: S. foliosa present, never displaced by hybrid Spartina.  Each restoration state is replicated two times.  Ecosystems functions assessed include: primary production (above and belowground plant biomass and algal biomass), secondary production (epifaunal and infaunal community), and biogeochemical cycling (soil carbon storage).  Soil environmental parameters (temperature, water content, pore water salinity) were also assessed. 

Results/Conclusions

The passive restoration marshes were distinct from the other three restoration states in many of the ecosystem function assessed (ex: aboveground biomass, ANOVA p=0.000).  We hypothesize this effect will become less pronounced as passively restored sites experience full decomposition of belowground hybrid Spartina biomass and as natural recruitment of native vegetation progresses.  For several ecosystem functions (ex: aboveground biomass, belowground biomass) actively restored sites represent intermediate values between native S. foliosa and hybrid Spartina invaded sites.  Alternatively, other functions (soil water content, algal biomass, pore water salinity) were more similar between actively and passively restored areas.  Secondary production differed between restoration states, while invertebrate community structure differed on the site level.  These data indicate that certain ecosystem functions respond more quickly to active restoration than others.  However, when examining restoration states from a multi-functional perspective, we found passively restored sites differed from both invaded and native S. foliosa marshes, indicating that active restoration may influence both speed and trajectory of functional ecosystem recovery.