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

摘要

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 (A(max)much greater than10 mumol CO2 m(-2) s(-1)) due both to high CO2 carboxylation (V-c,V-max) and RubP-regeneration capacity (J(max)). These high rates were associated with high values of leaf [N]. Across all sites, the temperature response of A(max) showed no clear optimum, and a gradual drop from 25degreesC to 35degreesC, without notable temperature limitation at leaf temperatures in excess of 35degreesC. Dark respiration rate (R-day) across all species and sites increased exponentially with increasing leaf temperature. Species sampled at selected sites revealed a negative relationship between leaf delta(13)C (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 N-15 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.