Monitoring the development of nurse plant species to improve the performances of reforestation programs in Mediterranean areas

In the Mediterranean basin, a millenarian history of overexploitation has lead to the loss of most primeval forests and an increase of the surface area covered by shrublands that represent stages of degradation of mature forests. In this situation, and since environmental characteristics act as barriers to succession, human intervention is usually necessary to improve recovery of woodlands. Reafforestation is a common practice in Mediterranean areas to achieve this aim but its performances are very low with high rates of early mortality making this practice unprofitable in ecological as well as in economic terms. In degraded semiarid ecosystems, shrub and tall-grass species grow following a patchy distribution. Traditionally, shrubs growing near to newly planted trees are considered heavy competitors, and are consequently removed before planting. However, the vegetation patches usually constitute "fertility islands" or "resource islands" which could promote the tree species development. It has previously been assessed that some native plant species could act as "nurse plants" through their positive impacts on soil abiotic characteristics (i.e., soil nutrient contents), but they also exhibit a positive influence on soil microbiota, especially on symbiotic microorganisms including rhizobia and mycorrhizal fungi. In this chapter, an attempt is made to assess the beneficial effects of plant nurses on the growth of Mediterranean tree species like Cupressus species, and on the bio-functioning of soils. Furthermore, the potential benefits of native plant species in the rehabilitation of degraded areas especially in stressful conditions is reviewed and discussed. (Résumé d'auteur