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Nutrient and light controls on grass productivity in an African woodland savanna

Lydia P. Ries, Department of Environmental Sciences, University of Virginia, lr9n@cms.mail.virginia.edu (Presenting)
Herman H. Shugart, Department of Environmental Sciences, University of Virginia, hhs@virginia.edu

Despite the ubiquity of sub-tropical savannas throughout the earth, limitations of savanna productivity are understudied relative to other terrestrial systems. The limitations of savanna productivity by precipitation have been documented but the role of nutrients and light is not as clear. In particular, there has been little attention on the role of phosphorus in savanna productivity and structure. This study examined the role of increased nitrogen and phosphorus in grass productivity in a woodland savanna in Botswana. We added aqueous forms of nitrogen and phosphorus to randomly selected 1-m2 plots inhabited primarily by grasses. During the following growing season we measured foliar nutrient concentrations and photosynthetic response at various light levels to estimate the productivity response. We observed an increase in foliar nutrient concentrations (mg P/g and mg N/g) for all grasses with increased levels of soil N and P. We also observed a significant increase in net carbon assimilation and Amax for these grasses with the largest increase in those grasses to which both N and P were added. These results suggest that the aboveground productivity of these woodland savanna grasses is limited by both nitrogen and phosphorus. Additionally, under constant C02 availability, photosynthesis is limited by nutrients for light levels greater than 1000 μmol m-2 s-1. These findings are helpful in addressing issues such as desertification and bush encroachment in the Kalahari region. Additionally, these data could be used in leaf- and canopy-scale modeling to predict changes in savanna productivity under a changing climate.

Presentation Type:  Poster

Abstract ID: 96

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