Potential for using facilitation by grasses to establish shrubs on a semiarid degraded steppe

In arid and semiarid environments, isolated vegetative patches establish is lands of fertility in which facilitation is a dominant interaction between plant species. These patches may provide favorable microsites for revegetat ion with desirable species in areas where traditional revegetation procedur es fail. Alpha grass (Stipa tenacissima) steppes are widely distributed wit hin the semiarid areas of southern Europe and northern Africa and represent a degraded stage of climax vegetation. In this study, we analyzed the effe cts of S. tenacissima tussocks on the survival, growth, and ecophysiologica l features of experimentally planted seedlings of Medicago arborea, Quercus coccifera, and Pistacia lentiscus in three sites in a semiarid region in s outheastern Spain. Our main objective was to test whether S. tenacissima wa s able to facilitate shrub establishment in semiarid degraded steppes. Soils under S. tenacissima tussocks had higher organic matter content and w ater availability than those from open areas. Stipa tenacissima significant ly reduced photosynthetically active radiation and soil temperature. One ye ar after planting, shrub survival was significantly higher near S. tenaciss ima ("tussock" microsite) than on the open areas ("open" microsite). Predaw n water potentials of the shrub seedlings measured before and after the sum mer were significantly higher in the tussock microsites, with differences r anging from 22% to 33% and from 17% to 38% before and after the summer, res pectively. Leaf biomass for seedlings harvested before and after the summer was significantly higher for seedlings planted on tussock microsites in co mparison to the open microsites. Our results suggest a direct facilitative effect of S. tenacissima on introduced shrubs. This study indicates that po sitive interactions in semiarid steppes can be of particular importance for effective restoration in degraded semiarid ecosystems.