The effect of elevated CO2 levels on the growth of two Acacia species.
Climate change, induced by increases in the concentration of greenhouse gases in the atmosphere, can affect the growth and community structure of ecosystems in two ways. Firstly directly through changes in atmospheric concentration of CO2, and secondly indirectly through changes in temperature and rainfall. The aim of the present investigation was to test the effect of elevated CO2 and altitude-related temperature differences on the growth of two species of Acacia that form important components of the vegetation of KwaZulu-Natal. Plants of Acacia sieberana and Acacia nilotica were grown in chambers at elevated (700 pll-1) and ambient (350 IJW1) CO2 with and without rhizobial inoculation. Both treatments (elevated CO2 and the presence of rhizobial inoculation) stimulated growth and branching. A. nilotica was the most responsive to both elevated CO2 level and inoculation. Inoculated plants showed greater increases in mass and height than uninoculated plants. While elevated CO2 had a significant effect on plant mass, height and leaf area accumulation, other factors, such as species type and rhizobial inoculation had a somewhat greater influence on the short term mass accumulation under elevated CO2 , Significant differences existed between the average percentage leaf nitrogen for the two species (P < 0.001), and for inoculated and uninoculated plants (P < 0.005). There were no significant differences in photosynthetic rates (A) at any internal CO2 concentration (Cj) between plants grown in elevated CO2 compared to those grown under ambient conditions. When photosynthesis was plotted against C, (A/CJ, the initial slopes of the graphs for both A. sieberana and A. nilotica were shallower for plants grown in elevated CO2 , compared to plants grown in ambient conditions , indicating a decreased Rubisco concentration at low C, and greater nitrogen use efficiency. At higher C; A. sieberana continued to have lower A in plants grown at elevated CO2 levels suggesting an inability to regenerate RuBP or the possible accumulation of soluble carbohydrates. A. nilotica grown in elevated CO2 had a slightly increased Pj regeneration capacity at higher CO2 concentrations. While the A/Cj results demonstrate that CO2 ·has a minor effect on photosynthesis, growth responses indicated otherwise. This is a result often reported and indicates the importance of measuring as many parameters as is possible to determine actual plant responses to elevated CO2 levels. In the field experiment, the effect of temperature was studied by transplanting twenty plants of each species at three different elevations in the Drakensberg at Cathedral Peak. Plant height, mass, condition and finally survivorship were measured . All of these attributes decreased as elevation increased. Plants growing at the highest elevation all died back prior to winter while those growing at lower elevations grew throughout the experimental period. Results suggest that elevation and hence temperature are important factors controlling Acacia distribution. If the greenhouse gas induced increases in temperature occur as predicted, and the estimated latitudinal migration rates of 30-100km per decade are required for species to remain within their current climatic envelopes, it is expected that the structure and appearance of vegetation in the Drakensberg will change markedly with global warming . The presence or absence of Rhizobia in the soil will further complicate this. Those plants that have access to the elevated nitrogen levels as a result of these root nodule bacteria will have a distinct advantage over competitors growing without them.