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Questions: Do large trees improve the nutrient content and the structure of the grass layer in savannas? Does the magnitude of this improvement differ with locality (soil nutrients) and season (water availability)? Are grass structure and species composition beneath tree canopies influenced by soil fertility and season? Location: South Africa. Methods: We compared grass leaf nutrient contents and grass sward structure beneath and outside tree canopy areas in three savannas of different soil fertility during the dry and the wet seasons. Results: Grass nitrogen contents were twice as high during the wet season as compared to the dry season, being more strongly elevated underneath tree canopies during the wet season. Grasses had significantly less stem material and provided less dead material underneath trees on the high soil fertility site. Grass species composition differed significantly beneath and outside tree canopies, with more nutritious grass species found sub-canopy. Grass species richness was significantly lower beneath than outside of trees at the site of high soil fertility. Conclusions: Trees improve overall quality of savanna grasses by enhancing grass growth and nutrient uptake during the wet season, and by delaying grass wilting in the dry season. The positive effect of trees on the grass layer might attract grazing herbivores in otherwise nutrient-poor savannas. Hence, single standing large trees should be maintained to sustain high grass quality and, consequently, grazer populations in savanna habitats.

Introduction: Forests form a major component of the carbon (C) reserves in the world's ecosystems. However, little is known on how management influences C stocks of woody vegetation, particularly in dry areas. We developed regression models for two dominant tree species to predict C stocks and quantified the potential of community managed forests as C sinks. Methods: Plots were randomly selected from community-managed natural forest, herbivore exclosures, and from communal grazing land. Tree and shrub biomass were estimated using a regression model on the most dominant woody species while herbaceous biomass was determined using destructive sampling.Results: The simplest model, based on only one single predictor variable, showed a good fit to the data for both species (Juniperus procera and Acacia abyssinica). Diameter at breast height (r 2 > 0.95) was a more reliable predictor than height (r 2 > 0.54), crown diameter (r 2 > 0.68) (p < 0.001). The C content of the total biomass for the managed natural forest and the exclosure were estimated as, 58.11 and 22.29 Mg ha −1 , respectively, while that for the grazing land was 7.76 Mg ha −1 , and the mean carbon content between the three land uses were significantly different (p < 0.05). Conclusions: We conclude that forests managed by the community have a high potential for C sequestration and storage and their conservation should be promoted.

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Large single-standing trees are rapidly declining in savannahs, ecosystems supporting a high diversity of large herbivorous mammals. Savannah trees are important as they support both a unique flora and fauna. The herbaceous layer in particular responds to the structural and functional properties of a tree. As shrubland expands stem thickening occurs and large trees are replaced by smaller trees. Here we examine whether small trees are as effective in providing advantages for grasses growing beneath their crowns as large trees are. The role of herbivory in this positive tree-grass interaction is also investigated. We assessed soil and grass nutrient content, structural properties, and herbaceous species composition beneath trees of three size classes and under two grazing regimes in a South African savannah. We found that grass leaf content (N and P) beneath the crowns of particularly large (ca. 3.5 m) and very large trees (ca. 9 m) was as much as 40% greater than the same grass species not growing under a tree canopy, whereas nutrient contents of grasses did not differ beneath small trees (\2.3 m). Moderate herbivory enhanced these effects slightly. Grass species composition differed beneath and beyond the tree canopy but not between tree size classes. As large trees significantly improve the grass nutrient quality for grazers in contrast to smaller trees, the decline of the former should be halted. The presence of trees further increases grass species diversity and patchiness by favouring shade-tolerant species. Both grazing wildlife and livestock will benefit from the presence of large trees because of their structural and functional importance for savannahs.

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Herbivores generally have strong structural and compositional effects on vegetation, which in turn determines the plant forage species available. We investigated how selected large mammalian herbivore assemblages use and alter herbaceous vegetation structure and composition in a southern African savanna in and adjacent to the Kruger National Park, South Africa. We compared mixed and mono-specific herbivore assemblages of varying density and investigated similarities in vegetation patterns under wildlife and livestock herbivory. Grass species composition differed significantly, standing biomass and grass height were almost twice as high at sites of low density compared to high density mixed wildlife species. Selection of various grass species by herbivores was positively correlated with greenness, nutrient content and palatability. Nutrient-rich Urochloa mosambicensis Hack. and Panicum maximum Jacq. grasses were preferred forage species, which significantly differed in abundance across sites of varying grazing pressure. Green grasses growing beneath trees were grazed more frequently than dry grasses growing in the open. Our results indicate that grazing herbivores appear to base their grass species preferences on nutrient content cues and that a characteristic grass species abundance and herb layer structure can be matched with mammalian herbivory types.

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