RELATIVE CONTRIBUTIONS OF SHORT-TERM CANOPY AND LONG-TERM SOIL EFFECTS OF A NATIVE SHRUB UNDER ARID BIOCLIMATE: A CASE STUDY FROM TUNISIA

The objectives of the study, conducted in the National Park of Bou Hedma, were to examine: (1) the relative contributions of short-term canopy (STE) and long-term soil effects (LTE) of a shrub species in explaining differences in biomass, species diversity (richness) and species density of understory plants (i) between shrubs and open areas, (ii) between shrubs and removed shrubs; and (2) the role of grazing in driving changes in direction of short-term and long-term effects in shrub/understory species interactions. We measured environmental conditions (soil nutrients and soil water) in plots that represented different neighborhood conditions (in open areas between shrubs, amongst intact shrubs and among removed shrubs), which were either fenced or exposed to grazing by large herbivores. We also studied understory species biomass, richness, density survival rate of a target species in plots represented different neighborhood conditions. Differences in species richness, density and biomass of understorey communities between shrub removed and open areas were mostly due to long-term soil effects whereas differences below shrubs and shrub removed were due to short-term effects in particular on soil water content. Our study provides additional evidence that savannas shrubs have the potential to increase the diversity of arid systems at the landscape level. Additionally, grazing by large herbivores influenced negatively the dynamic of vegetation under arid bioclimate.