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. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 2001.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/5315 |
| Date | January 2001 |
| Creators | Lotz, Michelle Karen. |
| Contributors | Beckett, Richard P., Van Staden, Johannes., Yeaton, R. I. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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