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Questions
Do Acacia tortilis trees facilitate herbaceous species development under arid conditions? Do tree–herb interactions change with water stress and grazing intensity?

Location
National Park in a Tunisian arid area (34°39?N, 9°48?E).

Methods
Soil water content, soil nutrient concentration, light intensity and microclimatic data were collected under the canopy and in open areas to evaluate the relative roles of A. tortilis (Forssk.) subsp. raddiana (Savi) trees in structuring vegetation composition and diversity patterns in a arid pseudo-savanna. Data were collected inside and outside Bou Hedma National Park, both during a dry and a wet year.

Results
In both unprotected and protected areas, the influence of the tree canopy on the herbaceous strata can be considered as positive, even though it is more significant inside the park. This positive interaction increased with higher abiotic stress conditions and decreased with grazing intensification. Soil nutrients (organic matter, total N, extractable P) and moisture availability was greater under tree canopies than in the open. Species composition of the herb layer also differed between Acacia trees and open areas. Some palatable species were found under trees, and were replaced by less desirable species in the open. Microclimate measurements showed significant differences in light intensity reaching the soil, mean air temperatures and relative humidity.

Conclusions
These results suggest that the optimal association of climate factors under the canopy would combine with a high soil fertility mediated by litter decomposition to increase plant cover under tree canopies.