Positive, negative, and net effects in grass-shrub interactions in mediterranean semiarid grasslands

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.