The aim of this study was to analyse and quantify the effects of the canopy of the native-invasive N-fixer woody shrub Retama monosperma in the dune ecosystem, affecting the structure and function of the dune environment as well as plant community, in the context of the facilitation mechanism. Air temperature and relative humidity; soil pH, electric conductivity, organic matter (OM) and nutrient content; above and below-ground vegetation biomass, litter mass, species richness and Shannon diversity were determined and compared from sampling plots below the R. monosperma canopy and in canopy gaps within a coastal dune system in SW Spain. The relationships between soil OM and nutrient contents and above and below-ground vegetation biomass, litter mass, species richness and Shannon diversity were also assessed. A predominance of positive interactions was confirmed. The canopy of R. monosperma ameliorated temperature extremes beneath, and soil OM and nutrient concentrations were increased by 188-466%, compared to those found in gaps. Plant biomass increased by 442% beneath the canopy and was composed almost exclusively of herbaceous annuals. Plant diversity was not affected. Plant communities were clearly structured as fertility islands, distributed in an environmentally stressful dune matrix characterized by scarce vegetation cover and low biomass.