Growth form and seasonal variation in leaf gas exchange of Colophospermum mopane savanna trees in northwest Botswana

We investigated differences in physiological and morphological traits between the tall and short forms of mopane (Colophospermum mopane (Kirk ex Benth.) Kirk ex J. Leonard) trees growing near Maun, Botswana on a Kalahari sandveld overlying an impermeable calcrete duricrust. We sought to determine if differences between the two physicgnomic 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 (A(sat)) occurred during the rainy seasons (March-April 2000 and January-February 200 1) and were similar in the tall and short forms (15-22 mu mol m(-2) s(-1)) before declining to less than 10 mu mol m(-2) s(-1) at the end of the rainy season in late April. As the dry season progressed, A(sat), soil water content, predawn leaf water potential (Psi(pd)) and leaf nitrogen concentration declined rapidly. Before leaf abscission, Psi(pd) 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.