Summary1. The stress-gradient hypothesis (SGH) predicts an increasing importance of facilitative mechanisms relative to competition along gradients of increasing environmental stress. Although developed across a variety of ecosystems, the SGH's relevance to the dynamic tree-grass systems of global savannas remains unclear. Here, we present a meta-analysis of empirical studies to explore emergent patterns of tree-grass relationships in global savannas in the context of the SGH. 2. We quantified the net effect of trees on understorey grass production relative to production away from tree canopies along a rainfall gradient in tropical and temperate savannas and compared these findings to the predictions of the SGH. We also analysed soil and plant nutrient concentrations in subcanopy and open-grassland areas to investigate the potential role of nutrients in determining grass production in the presence and absence of trees. 3. Our meta-analysis revealed a shift from net competitive to net facilitative effects of trees on subcanopy grass production with decreasing annual precipitation, consistent with the SGH. We also found a significant difference between sites from Africa and North America, suggesting differences in tree-grass interactions in the savannas of tropical and temperate regions. 4. Nutrient analyses indicate no change in nutrient ratios along the rainfall gradient, but consistent nutrient enrichment under tree canopies. 5. Synthesis. Our results help to resolve questions about the SGH in semi-arid systems, demonstrating that in mixed tree-grass systems, trees facilitate grass growth in drier regions and suppress grass growth in wetter regions. Relationships differ, however, between African and North American sites representing tropical and temperate bioclimates, respectively. The results of this meta-analysis advance our understanding of tree-grass interactions in savannas and contribute a valuable data set to the developing theory behind the SGH.

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Aims: The purpose of this study was to evaluate the in vitro antioxidant activities of Datura innoxia Mill. leaves and seeds harvested in Mali. Place and Duration of Study: Collection of plant materials were done at Kolondieba (Mali) in june 2016. Evaluation of polyphenols and flavonoids contents, determination of antioxidant activities were done at Laboratory of Plant and Food Biochemistry and Biotechnology of University of Sciences, Techniques and Technologies of Bamako (Mali) between august 2016 and March 2017. Methodology: The leaves and seeds were collected in Kolondieba, Mali. The phytochemical screening based on the standard methods of tube reactions has been performed with ethanol extracts. The quantitative estimation of total polyphenols was made by the Folin-Ciocalteu method and that of flavonoids by the use of aluminum trichloride. The in vitro antioxidant activities of the ethanol extracts was determined by three methods: the 1,1- diphenyl - 2- picrylhydrazyl (DPPH) test, the ferric reducing ability power (FRAP) test and the Phosphomolybdate test (PPM). Results: The phytochemical screening revealed that leaves and seeds of this plant contains alkaloids, polyphenols, flavonoids, coumarins, tannins, triterpenes and saponin. Phenolic contents of ethanol extracts are 30.97 ± 0.33 mg equivalent gallic acid / g in leaves and 14.02 ± 0.15 mg equivalent gallic acid / g in seeds; those of flavonoids are 15.13 ± 0.2 mg equivalent of quercetin / g in the leaves and 4.93 ± 0.41 mg equivalent of quercetin / g in the seeds. The three tests showed that the leaves have a higher level of antiradical activity in vitro than seeds. Conclusion: The results of this work showed that Datura innoxia Mill. has a good antioxidant activity which would justify its use as a potential source of natural antioxidants.