Reference conditions for restoration of heterogeneous Mediterranean wetland are best defined by multiple, hydrologically diverse sites

In a Mediterranean climate, the vegetation of embanked salt marshes can vary considerably in time and space to obscure the definition of reference condition for restoration purposes. The aim of our study was to find a basis for defining reference vegetation for the reinstatement of a wetland hydrological regime on abandoned agricultural land. We investigated five reference sites surrounding a 2,668 ha restoration site in the Donana National Park (southwest Spain). Environmental conditions were monitored on a sampling grid for 7 years: surface elevation and hydroperiod were mapped (using Light Detection and Ranging [LIDAR] and satellite imagery, respectively) and rainfall, soil salinity, and soil pH were recorded. The reference sites collectively encompassed the range of elevation and environmental conditions at the restoration site, although none individually was representative. The vegetation at the reference sites was sampled annually at fixed grid points. Hierarchical cluster analyses identified assemblages of perennial and annual species that were differently distributed among the reference sites. BIO-ENV analysis showed that the distribution of perennial assemblages was determined by elevation, annual hydroperiod, and salinity. More labile annual assemblages were loosely associated with particular perennial ones. Species composition fluctuated over 7 years, in concert with rainfall and hydroperiod, but showed no directional change. Understanding the hydrochemical drivers of spatiotemporal variation in vegetation across multiple sites has established a rationale for defining reference conditions for large, heterogeneous wetland restoration sites.