Potential for Using Facilitation by Grasses to Establish Shrubs on a Semiarid Degraded Steppe

In arid and semiarid environments, isolated vegetative patches establish islands of fertility in which facilitation is a dominant interaction between plant species. These patches may provide favorable microsites for revegetation with desirable species in areas where traditional revegetation procedures fail. Alpha grass (Stipa tenacissima) steppes are widely distributed within the semiarid areas of southern Europe and northern Africa and represent a degraded stage of climax vegetation. In this study, we analyzed the effects of S. tenacissima tussocks on the survival, growth, and ecophysiological features of experimentally planted seedlings of Medicago arborea, Quercus coccifera, and Pistacia lentiscus in three sites in a semiarid region in southeastern Spain. Our main objective was to test whether S. tenacissima was able to facilitate shrub establishment in semiarid degraded steppes. Soils under S. tenacissima tussocks had higher organic matter content and water availability than those from open areas. Stipa tenacissima significantly reduced photosynthetically active radiation and soil temperature. One year after planting, shrub survival was significantly higher near S. tenacissima ("tussock" microsite) than on the open areas ("open" microsite). Predawn water potentials of the shrub seedlings measured before and after the summer were significantly higher in the tussock microsites, with differences ranging from 22% to 33% and from 17% to 38% before and after the summer, respectively. Leaf biomass for seedlings harvested before and after the summer was significantly higher for seedlings planted on tussock microsites in comparison to the open microsites. Our results suggest a direct facilitative effect of S. tenacissima on introduced shrubs. This study indicates that positive interactions in semiarid steppes can be of particular importance for effective restoration in degraded semiarid ecosystems.