Current theoretical models and field evidence suggest that facilitation and interference act simultaneously in the field, but there is little information on their joint dynamics under varying abiotic conditions. We evaluated spatial and temporal variations in the positive, negative, and net effects of the tussock grass Stipa tenacissima on the shrub Pistacia lentiscus in Mediterranean semiarid grasslands. We performed a field experiment in which positive (microclimatic amelioration and water inputs from runoff) and negative (belowground competition) effects were experimentally manipulated under contrasting levels of stress. The environment provided by S. tenacissima (tussock treatment) improved the survival and physiological status of planted P. lentiscus seedlings. The elimination of shade and competition from S. tenacissima significantly decreased and increased, respectively, seedling performance as compared to the tussock treatment. We found evidence that microclimatic amelioration is the main positive factor involved in the interaction under study, and that it can balance the negative effects of belowground competition. The latter was also important, especially after major rainfall events. In contrast with previous studies, both the water inputs from runoff and the soil chemical fertility were found to have a relatively weak effect in the interaction under study. The net effect of S. tenacissima on P. lentiscus was always facilitative, but the magnitude of this effect increased with the harshness of the environmental conditions in both space and time. Despite this, the mechanisms underlying plant–plant interaction responded unequally to small‐scale environmental variation. Our results agree with models suggesting that the magnitude of facilitation may increase under stressful conditions, and provide evidence of the dynamic nature of positive and negative interactions in the field.

No abstract

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.

The importance of the spatial pattern of vegetation for hydrological behavior in semiarid environments is widely acknowledged. However, there is little empirical work testing the hypothetical covariation between vegetation spatial structure and hillslope water and sediment fluxes. We evaluated the relationships between vegetation structural attributes (spatial pattern, functional diversity), soil surface properties (crust, stone, plant, and ground cover, and particle size distribution) and hillslope hydrologic functioning in a semiarid Mediterranean landscape; in particular, we tested whether decreasing patch density or coarsening plant spatial pattern would increase runoff and sediment yield at the hillslope scale. Runoff and sediment yield were measured over a 45-month period on nine 8 • 2-m plots that varied in vegetation type and spatial pattern. We grouped vegetation into functional types and derived plant spatial pattern attributes from field plot maps processed through a GIS system. We found that there was an inverse relationship between patch density and runoff, and that both runoff and sediment yields increased as the spatial pattern of vegetation coarsened. Vegetation pattern attributes and plant functional diversity were better related to runoff and sediment yield than soil surface properties. However, a significant relationship was found between physical crust cover and plant spatial pattern. Our results present empirical evidence for the direct relationship between the hydrologic functioning of semiarid lands and both the spatial pattern and the functional diversity of perennial vegetation, and suggest that plant spatial pattern, physical crust cover, and functional diversity may be linked through feedback mechanisms.