Limitations to the use of facilitation as a restoration tool in arid grazed savanna: a case study

QuestionThere is increasing interest in using facilitation by nurse plants for restoring dry degraded ecosystems. However, most studies concern semi-arid areas, and the efficiency and importance of facilitation in arid grazed areas is still poorly understood. The goal of this study was to determine to what extent positive interactions among plants could be used as a restoration tool in an arid grazed region (southern Tunisia). In particular, we examined variations in plant interactions, along with water stress, grazing intensity and the type of nurse plant (grass vs chenopod shrub). LocationAcacia tortilis open woodland community of the National Park of Bou-Hedma, Tunisia (33 degrees 30N, 9 degrees 38E). MethodsSurvival of transplants of Acacia was monitored during two growing seasons, with and without herbivores, in the presence/absence of a chenopod shrub and a perennial grass, and in three communities of contrasting water availability. We quantified the importance of biotic interactions and their variation across the community, grazing and neighbour type treatments. ResultsCompetition was the only statistically significant interaction occurring between Acacia seedlings and both neighbours in all water and grazing conditions. The negative effect of neighbours was mostly for light and water. However, competition was important only at the start of the experiment during a relatively wet period, in the wettest community and in the absence of herbivores. Drought stress and disturbance were the most important drivers of target tree survival. ConclusionsFacilitation cannot be used as a tool for ecological restoration in this arid system, mainly because the only available target tree species is too shade-intolerant and drought-tolerant to benefit from neighbours. In addition, the importance of plant-plant interactions vanished due to the overwhelming importance of stress and disturbance. Our study highlights the fact that nurse-based ecological restoration should not be an automatic process, but must be tailored to the species and environment in question.