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Photosynthetic and gas exchange characteristics of dominant woody plants on a moisture gradient in an African savanna


Guy Franklin Midgley, fax +27 21 7976903, e-mail:


We determined key photosynthetic gas exchange parameters, and their temperature dependence, in dominant woody plants at four savanna sites on a moisture gradient in Botswana, southern Africa. Leaf stable carbon and nitrogen (N) isotope and morphological measures were made concurrently. Sampling of these predominantly non-N-fixing species took place during an exceptional rainfall season, representing near-optimum conditions for primary production at these sites. The mean specific leaf area and leaf size were positively related to mean annual rainfall (MAR); species with larger leaves of lower density were more abundant in wetter sites. Almost all species at all sites showed high net light-saturated photosynthetic rates (Amax≫10 μmol CO2 m−2 s−1) due both to high CO2 carboxylation (Vc,max) and RubP-regeneration capacity (Jmax). These high rates were associated with high values of leaf [N]. Across all sites, the temperature response of Amax showed no clear optimum, and a gradual drop from 25°C to 35°C, without notable temperature limitation at leaf temperatures in excess of 35°C. Dark respiration rate (Rday) across all species and sites increased exponentially with increasing leaf temperature. Species sampled at selected sites revealed a negative relationship between leaf δ13C (stable carbon isotope ratio) and MAR, suggesting higher leaf-level water-use efficiency at drier sites when integrated over the life of the leaf. At wetter sites, specific leaf [N] was lower and photosynthetic nitrogen-use efficiency increased, a pattern reflected at the ecosystem level by less 15N enrichment of leaves at these sites. Taken together, the results suggest a switch from water-use to nitrogen-use efficiency constraints with increasing moisture availability. These constraints impact leaf form and function significantly, and may emerge at the ecosystem level in aspects of water and N cycling.