ORIGINAL ARTICLE Tree distribution on a steep environmental gradient in an arid savanna Edmund C. February 1 *, Steven I. Higgins 2  , Rosemary Newton 1 à and Adam G. West 1 § INTRODUCTION Savannas are a tropical vegetation type in which trees and grasses co-dominate. This vegetation type covers 12% of the global land surface and some 65% of the land surface of sub- Saharan Africa (Huntley & Walker, 1982). This extensive geographic distribution means that savannas occur over a broad climatic gradient. Throughout this broad range in climate, savanna systems are typically composed of a continuous grass layer and a discontinuous layer of shrubs 1 Department of Botany, University of Cape Town, Private Bag, Rondebosch 7701, 2 Centre for Water in the Environment, Department of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Johannesburg, South Africa *Correspondence: Edmund February, Department of Botany, University of Cape Town, Private Bag, Rondebosch, 7701, South Africa. E-mail: efeb@botzoo.uct.ac.za  Present address: Lehrstuhl fu ¨r Vegetationso ¨kologie, Technische Universita ¨t Mu ¨nchen, D-85350 Freising-Weihenstephan, Germany. àPresent address: Seed Conservation Department, Royal Botanical Gardens, Kew, Wakehurst Place, Ardingly, Haywards Heath RH17 6TN, UK. §Present address: Department of Biology, University of Utah, 257S 1400E, Salt Lake City, UT 84112, USA. ABSTRACT Aim The structure of savannas ranges from scrub to woodland over broad geographical gradients. Here we examine the hypothesis that water availability is a major determinant of these structural differences by ascertaining the relationship between water availability and tree growth across a steep moisture gradient. Location The study site is a sub-tropical savanna, with a mean annual precipitation of 400 mm, located in the Phugwane river basin, Kruger National Park, Limpopo Province, South Africa. Methods We determined plant moisture stress using xylem pressure potentials, stem growth using dendrometer bands, and estimated the water sources available to plants using stable hydrogen and oxygen isotope ratios. The primary objective was to understand tree growth relative to available water along an environmental gradient. Results We found that Philenoptera violacea trees growing close to the stream have lower water stress (least negative xylem pressure potentials) and higher cumulative growth than those growing away from the stream. The stem growth of P. violacea was characterized by steady incremental growth and could not be related to antecedent rainfall. Colophospermum mopane trees experienced higher water stress, yet trees growing adjacent to the stream achieved the highest cumulative growth rates over the study period. The growth of C. mopane could be clearly linked to antecedent rainfall, and most growth was achieved during short growth pulses that followed rainfall events. d 18 O values become progressively more enriched in the heavier isotope with distance from the stream, suggesting that access to groundwater decreases with distance from the stream side. The isotopic data suggest that P. violacea has access to groundwater, but that C. mopane does not. Main conclusions Our results show that water stress increases with distance from the stream side as a result of reduced access to groundwater. Trees without access to deep water adopt an opportunistic growth strategy. Keywords Arid savanna, Colophospermum mopane, dendrometer bands, Philenoptera violacea, stable oxygen isotope ratios, tree growth rates, water, xylem pressure potentials. Journal of Biogeography (J. Biogeogr.) (2007) 34, 270–278 270 www.blackwellpublishing.com/jbi ª 2006 The Authors doi:10.1111/j.1365-2699.2006.01583.x Journal compilation ª 2006 Blackwell Publishing Ltd