Do exotic trees threaten southern arid areas of Tunisia? A case study.
This study was conducted in an afforested Stipa tenacissima steppe with aims to compare the effects of exotic and native planted trees (Acacia salicina and Pinus halepensis, respectively) on the understory vegetation and soil properties. For each tree species, two sub-habitats were distinguished: the canopied sub-habitat (under the tree crown) and the un-canopied sub-habitat (open grassland). Soil moisture was measured in both sub-habitats at 10 cm depth. In parallel to soil moisture, investigated the effect of tree species on soil fertility. Soil samples were collected from the upper 10 cm soil, excluding litter and stones. The nutrient status of soil (organic matter, total N, extractable P) was significantly higher under A. salicina compared to P. halepensis and open areas. This tendency remained constant with the soil water content which was significantly higher under trees compared to open sub-habitats. For water content, there were no significant differences between studied trees. Total plant cover, species richness and the density of perennial species were significantly higher under the exotic species compared to other sub-habitats. Among the two tree species, Acacia salicina had the strongest positive effect on the understory vegetation. It seems to be more useful as a restoration tool in arid areas and more suitable to create islands of resources and foster succession than the other investigated tree species.