Growth form and seasonal variation in leaf gas exchange of Colophospermum mopane savanna trees in northwest Botswana
Elmar M. Veenendaal (1, 2, 3), Khanyisa B. Mantlana (1, 4), Norman W. Pammenter (5), Piet Weber (1), Phillipa Huntsman-Mapila (1) and Jon Lloyd (6)
1. Harry Oppenheimer Okavango Research Centre, University of Botswana, Private Bag 285, Maun, Botswana / 2. Nature Conservation and Plant Ecology Group, Department of Environmental Sciences, Wageningen University, Droevendaalsesteeg
3a, NL 6708 PB Wageningen, The Netherlands / 3. Corresponding author () / 4. Global Change Research Group, South African National Biodiversity Institute, Kirstenbosch Research Center, Cape Town, South
Africa / 5. School of Life and Environmental Sciences, University of Natal, Durban, South Africa / 6. Earth and Biosphere Institute, School of Geography, University of Leeds, U.K. / Received August 1, 2006; accepted April 18, 2007; published online January 2, 2008
Summary
We investigated differences in physiological and morphological traits between the tall and short forms of mopane (Colophospermum mopane (Kirk ex Benth.) Kirk ex J. Léonard) trees growing near Maun, Botswana on a Kalahari sandveld overlying an impermeable calcrete
duricrust. We sought to determine if differences between the two physiognomic types are attributable to the way they exploit
available soil water. The tall form, which was located on deeper soil than the short form (5.5 versus 1.6 m), had a lower
leaf:fine root biomass ratio (1:20 versus 1:6), but a similar leaf area index (0.9–1.0). Leaf nitrogen concentrations varied
between 18 and 27 mg g–1 and were about 20% higher in the tall form than in the short form. Maximum net assimilation rates (Asat) occurred during the rainy seasons (March–April 2000 and January–February 2001) and were similar in the tall and short forms
(15–22 μmol m–2 s–1) before declining to less than 10 μmol m–2 s–1 at the end of the rainy season in late April. As the dry season progressed, Asat, soil water content, predawn leaf water potential (Ψpd) and leaf nitrogen concentration declined rapidly. Before leaf abscission, Ψpd was more negative in the short form (–3.4 MPa) than in the tall form (–2.7 MPa) despite the greater availability of soil
water beneath the short form trees. This difference appeared attributable to differences in root depth and density between
the physiognomic types. Stomatal regulation of water use and carbon assimilation differed between years, with the tall form
having a consistently more conservative water-use strategy as the dry season progressed than the short form.
Keywords:
edaphic factors, fine roots, Kalahari, water relations, woodland structure.
