Sink patches, nurse shrubs and plant water-use strategies control the establishment of tree seedlings in Mediterranean-dry reclaimed mining hillslopes

Ecohydrologically suitable microsites, such as surface depressions and micro-topographical barriers acting as sink patches intercepting and infiltrating runoff, as well as nurse shrubs have largely been proposed as tools to improve the establishment of tree vegetation in Mediterranean reclaimed landscapes and other degraded dryland environments. We analyze the impact of sink patches and nurse (Genista scorpius) shrubs developed in Mediterranean-dry reclaimed mining hillslopes (Utrillas field site, central Spain) on seedling performance up to 8 years after plantation of two tree species with contrasted water-use strategies: Pinus nigra, a drought avoider species, and Quercus ilex, a drought tolerant species. Nurse shrubs enhanced early establishment of seedlings in shaded spots under its canopy. Further, sink patches ameliorated the survival of both species, although only increased plant growth during wet years that promoted source-to-sink transference of surface water resources as surface runoff. The survival and growth of P. nigra seedlings were strongly constrained during dry periods, resulting in a high cumulative mortality after 8 years regardless of microsite. Q. ilex showed a better performance during the experiment, keeping the positive effects of suitable microsites on plant survival after 8 years of plantation. Overall, our results encourage the use of ecohydrologically suitable microsites that concentrate water resources and nurse shrubs that ameliorate local conditions as key spots for introducing late-successional tree species in Mediterranean-dry reclaimed mining sites. Our results also indicate that seedling functional strategy to cope with drought is a critical factor conditioning plantation performance, therefore constituting a fundamental species selection criterion for restoration actions in Mediterranean areas, especially under effects of climate change.