Can native shrubs facilitate the establishment of trees under arid bioclimate? A case study from Tunisia

Abstract

The scarcity of tree regeneration is a major concern for the long-term conservation of tree cover in the arid areas of Tunisia and the causes of this shortage are poorly understood. The objectives of the study, conducted in the National Park of Bou Hedma, were to examine the direction of interaction between the nurse shrub Retama raetam on the survival of the keystone tree species Acacia tortilis subsp. raddiana in the arid bioclimate of Tunisia.The survival of transplants of Acacia was monitored during two years in the presence/absence of Retama in two communities of contrasting water availability. We quantified the intensity of biotic interactions and their variation between the two communities and among three treatments (in areas without woody vegetation, among intact Retama shrubs, and among cropped Retama shrubs).Contrary to the predictions of established models of plant-plant interactions, the results always showed negative interactions (competition or allelopathy) between plants along the natural water stress gradient. 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.