Savanna ecosystems are dominated by two distinct plant life forms, grasses and trees, but the interactions between them are poorly understood. Here, we quantified the effects of isolated savanna trees on grass biomass as a function of distance from the base of the tree and tree height, across a precipitation gradient in the Kruger National Park, South Africa. Our results suggest that mean annual precipitation (MAP) mediates the nature of tree-grass interactions in these ecosystems, with the impact of trees on grass biomass shifting qualitatively between 550 and 737 mm MAP. Tree effects on grass biomass were facilitative in drier sites (MAP≤550 mm), with higher grass biomass observed beneath tree canopies than outside. In contrast, at the wettest site (MAP = 737 mm), grass biomass did not differ significantly beneath and outside tree canopies. Within this overall precipitation-driven pattern, tree height had positive effect on sub-canopy grass biomass atsome sites, but these effects were weak and not consistent across the rainfall gradient. For a more synthetic understanding of tree-grass interactions in savannas, future studies should focus on isolating the different mechanisms by which trees influence grass biomass, both positively and negatively, and elucidate how their relative strengths change over broad environmental gradients.
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Logging is one of the most pervasive threats to biodiversity in tropical forests. In this study, we concurrently examined the responses of the avian frugivore community and the community of fruiting trees to past logging in a tropical forest in northeastern India. We predicted that the abundance of, and visitation by large-bodied frugivores would be lower in logged forests, resulting in reduced seed dispersal and recruitment of biotically dispersed species (especially large-seeded species). Data were collected from two logged and unlogged sites using (a) belt transects for estimating avian frugivore abundance (b) fruiting tree watches to quantify frugivore visitation and (c) vegetation plots to characterize recruitment. Our results show that differences in the abundance of bird species between logged and unlogged sites was correlated with body mass, with larger species being scarcer and smaller species more abundant in logged areas. Correspondingly, visitation rates by large avian frugivores was lower in logged compared with unlogged sites. Finally, biotically dispersed tree species, especially large-seeded species, had lower recruitment in logged forests, potentially due to lowered visitation rates and decreased fruit removal by large bodied avian frugivores. In the long term, these differences in recruitment may result in a tree community shift towards small-seeded biotically dispersed species and abiotically dispersed species in logged areas.© 2012 Elsevier Ltd.
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Tropical forest biodiversity is declining, but the resulting effects on key ecosystem services, such as carbon storage and sequestration, remain unknown. We assessed the influence of the loss of tropical tree species on carbon storage by simulating 18 possible extinction scenarios within a well-studied 50-hectare tropical forest plot in Panama, which contains 227 tree species. Among extinction scenarios, aboveground carbon stocks varied by more than 600%, and biological insurance varied by more than 400%. These results indicate that future carbon storage in tropical forests will be influenced strongly by future species composition.
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The world is witnessing a decline in biodiversity which may be greater in magnitude than even previous mass-extinction events(1-3). This has rekindled interest in the relationships between biodiversity and the stability of community and ecosystem processes(4) that have been reported in some empirical studies(5-7), Diversity has been linked with community and ecosystem processess(8-14), but disputes remain over whether it is diversity, environmental factors or the variety of functional groups in a community that drive these patterns(15-21). Furthermore, it remains unclear whether variation in diversity resulting from species loss within communities has similar effects on stability as natural variation in diversity associated with gradients in factors that regulate diversity. We believe that, across larger ecological scales, extrinsic determinants of biodiversity such as disturbance regimes and site history may be the primary determinants of certain measures of community stability. Here we use controlled field experiments in savanna grasslands in southern India to demonstrate and explain how low-diversity plant communities can show greater compositional stability when subject to experimental perturbations characteristic of their native environments. These results are best explained by the ecological history and species characteristics of communities rather than by species diversity in itself.
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