Physiological and growth responses of two African species, Acacia karroo and Themeda triandra, to combined increases in CO2 and UV‐B radiation

Abstract

The interactive effects of increased carbon dioxide (CO2) concentration and ultraviolet‐B (UV‐B, 280–320 nm) radiation on Acacia karroo Hayne, a C3 tree, and Themeda triandra Forsk., a C4 grass, were investigated. We tested the hypothesis that A. karroo would show greater CO2‐induced growth stimulation than T. triandra, which would partially explain current encroachment of A. karroo into C4 grasslands, but that increased UV‐B could mitigate this advantage. Seedlings were grown in open‐top chambers in a greenhouse in ambient (360 μmol mol‐1) and elevated (650 μmol mol‐1) CO2, combined with ambient (1.56 to 8.66 kJ m‐2 day‐1) or increased (2.22 to 11.93 kJ m‐2 day‐1) biologically effective (weighted) UV‐B irradiances. After 30 weeks, elevated CO2 had no effect on biomass of A. karroo, despite increased net CO2 assimilation rates. Interaction between UV‐B and CO2 on stomatal conductance was found, with conductances decreasing only where elevated CO2 and UV‐B were supplied separately. Increases in water use efficiencies, foliar starch concentrations, root nodule numbers and total nodule mass were measured in elevated CO2. Elevated UV‐B caused only an increase in foliar carbon concentrations. In T. triandra, net CO2 assimilation rates were unaffected in elevated CO2, but stomatal conductances and foliar nitrogen concentrations decreased, and water use efficiencies increased. Biomass of all vegetative fractions, particularly leaf sheaths, was increased in elevated CO2. and was accompanied by increased leaf blade lengths and individual leaf and leaf sheath masses. However, tiller numbers were reduced in elevated CO2. Significantly moderating effects of elevated UV‐B were apparent only in individual masses of leaf blades and sheaths, and in total sheath and shoot biomass. The direct CO2‐induced growth responses of the species therefore do not support the hypothesis of CO2‐driven woody encroachment of C4 grasslands. Rather, differential changes in resource use efficiency between grass and woody species, or morphological responses of grass species, could alter the competitive balance. Increased UV‐B radiation is unlikely to substantially alter the CO2 response of these species.