Environmental drivers for riparian restoration success and ecosystem services supply in Mediterranean agricultural landscapes

Riparian forests nestled in agricultural landscapes represent a small proportion in crop-intensive areas, while contributing remarkably to their biodiversity. This biodiversity supports several ecological processes crucially involved in the supply of ecosystem services (ES) complementary to that provided by agricultural lands and also relevant for designing biodiverse and multifunctional landscapes. Riparian forest is one of the most threatened ecosystems due to land-use intensification and associated water extraction, especially in Mediterranean semi-arid areas, and proper evaluation of the success of riparian restoration projects is usually lacking. Furthermore, there is little empirical evidence of the effects of riparian restoration on ES supply. In this study, we first investigated the effect of hydrological and soil features on survival and growth of saplings planted in degraded riparian areas in two Mediterranean watersheds. Then, we evaluated how riparian restoration affected the supply of ES, comparing nine regulating and supporting ES on these restored areas with other riparian areas spanning a gradient of conservation status, and with other natural and agricultural land-uses in the same watershed. We found that restoration success mainly depended on water table depth, soil salinity and soil nutrients (namely Mg⁺² and Olsen P). Moreover, we detected an antagonistic interaction between the latter two, and a synergetic interaction between water table depth and soil salinity. Forest patches provided meaningful regulating and supporting ES in agricultural landscapes. In particular, riparian restoration zones increased the supply of regulating and supporting ES (water purification, habitat provision, microclimate regulation and soil C storage) in comparison with degraded natural land-uses and crops. Nevertheless, they were still far from the magnitude and range of ES provided by mature riparian forests. These results highlight the importance of focusing management practices on conserving riparian forest patches and restoring the degraded ones to reconcile agricultural production with the maintenance or enhancement of ES in agricultural Mediterranean landscapes.