Journal of Tropical Ecology

Research Article

Seasonal and inter-annual photosynthetic response of representative C4 species to soil water content and leaf nitrogen concentration across a tropical seasonal floodplain

K. B. Mantlanaa1a2a3 c1, A. Arnetha4a5, E. M. Veenendaala2a6, P. Wohlanda4a7, P. Wolskia6, O. Kollea1 and J. Lloyda7

a1 Max Planck Institute for Biogeochemistry, Postfach 100164 07701 Jena, Germany

a2 Nature Conservation and Plant Ecology Group, Department of Environmental Sciences, Wageningen University and Research Centre, Borne steeg 69, NL 6708 PD, Wageningen, The Netherlands

a3 Global Change Research Group, South African National Biodiversity Institute, Kirstenbosch Research Center; P/Bag X7, Claremont, 7735, Cape Town, South Africa

a4 Max Planck Institute for Meteorology, Hamburg, Germany

a5 Department of Physical Geography and Ecosystems Analysis, Lund University Sölvegatan 12, 223 62, Lund, Sweden

a6 Harry Oppenheimer Okavango Research Centre (HOORC), University of Botswana, Private Bag 285, Maun, Botswana

a7 Earth and Biosphere Institute, School of Geography, University of Leeds, LS2 9JT, UK

Abstract

We examined the seasonal and inter-annual variation of leaf-level photosynthetic characteristics of three C4 perennial species, Cyperus articulatus, Panicum repens and Imperata cylindrica, and their response to environmental variables, to determine comparative physiological responses of plants representing particular microhabitats within a seasonal tropical floodplain in the Okavango River Delta, Botswana. Five measurement campaigns were carried out over a period of 2 y which covered two early rainy seasons, two late rainy seasons and one dry season. For all three species, light-saturated net photosynthetic rates (Asat) and stomatal conductance (gsat) decreased with decreasing soil water content with a seasonal range for Asat of approximately 5–45 μmol m−2 s−1, and for gsat of 0.03–0.35 mol m−2 s−1. The species representing the wettest microhabitat (Cyperus) had the highest gsat at low leaf-to-air vapour pressure deficits (Dl), the highest ratio of intercellular to ambient CO2 concentration (Ci/Ca), as well as the highest degree of variation in Ci/Ca from season to season. We interpret this as being indicative of its adaptation to a moist growth environment allowing for non-conservative water use strategies as soil moisture is usually abundant. For all three species there was significant variation in photosynthetic fluxes from one year to another that was related to variation in leaf nitrogen and phosphorus. This study shows that when assessing the role of savanna stands in large-scale carbon balance models, the remarkable inter-annual variation in leaf photosynthesis reported in this study should be taken into account.

(Accepted January 10 2008)

Correspondence:

c1 Corresponding author, at Global Change Research Group address; Email: mantlana@sanbi.org