The concentration of atmospheric carbon dioxide (CO2) in the atmosphere has increased by 35% since pre-industrial levels. Projections for the next 100 years indicate an increase to levels between 490 and 1260 parts per million by volume (ppm) of CO2, equating to a 75 % to 350 % increase in concentration since the year 1750. Associated with this increase in [CO2] will be a 1.4 to 5.8?? C increase in lower atmospheric temperature. While past research has attempted to address the effects of such climatic changes on individual plant responses, predictions of plant responses at the ecosystem level are still highly uncertain. Difficulties lie in the enormous variation of plant responses to climate change variables among and within species, and between and within environmental conditions. Past research assumed that plants using either the C3 or C4 metabolic pathways would respond differently but predictably to climate-change variables based on their metabolic pathway. Recent evidence has suggested however, that the added interactions of external environmental variables and species-specific sensitivities to climate change make it difficult to predict plant and ecosystem responses to climate change. To investigate the mechanisms behind responses of Australian grasses to climate change, 2 pot experiments was conducted using growth cabinets to compare the effect of elevated CO2 and water-limitation on the invasive C4 grassland plant, Eragrostis curvula (E. curvula), native Australian C3 grassland plant, Austrodanthonia racemosa (A. racemosa), and wheat species, Triticum aestivum (T. aestivum). The experiment was run at ambient levels of CO2 maintained at 390 ppm compared to elevated levels of 740 ppm. Imposed restrictions to water supply consisted of gradually drying the soil down to 30 % available soil water (ASW) followed by re-wetting to 50 % ASW. Well-watered conditions for the experiment consisted of gradually drying the soil down to 50 % ASW, followed by rewetting to 95 % ASW. Plants were grown in mixtures and monocultures, consisting of 9 plants equally spaced in a grid design. The three significant findings of the thesis were that: 1) the metabolic pathway (C3 versus C4) was not always an accurate predictor of biomass accumulation under elevated CO2 in the plants studied. Previous research suggested that CO2-stimulation of photosynthesis in C3 plants would lead to greater increases in biomass under elevated CO2 compared to C4 plants, though both C3 and C4 plants could benefit from any reduction in stomatal conductance under dry conditions at elevated CO2. The results from the experiments in this thesis showed a strongly significant biomass response to elevated CO2 in both dry and wet conditions for C4 grass E. curvula. The C3 grass A. racemosa in dry conditions, did not. It was speculated that without the CO2-induced water conservation effect, the C3 grass experienced photosynthetic down-regulation and this precluded a positive biomass response under elevated CO2. 2) the magnitude and direction of biomass response to elevated CO2 was dependant on factors such as resource-availability and the phenotypic variability of the plants species. 3) critical analysis of results from this thesis, combined with past research on plant responses under elevated CO2 showed a tendency for researchers to repeatedly test plants from the Poaceae family, or close relatives of the Poaceae family. As a result, when past data were corrected for this lack of independence, there was no relationship between the evolution of the C3 and C4 metabolic pathway and biomass response to elevated CO2. Instead, other factors (such as growth rate, plant height, leaf number, etc) were presented as being more important in determining biomass response. These observations were supported by results found in this thesis.

The concentration of atmospheric carbon dioxide (CO2) in the atmosphere has increased by 35% since pre-industrial levels. Projections for the next 100 years indicate an increase to levels between 490 and 1260 parts per million by volume (ppm) of CO2, equating to a 75 % to 350 % increase in concentration since the year 1750. Associated with this increase in [CO2] will be a 1.4 to 5.8?? C increase in lower atmospheric temperature. While past research has attempted to address the effects of such climatic changes on individual plant responses, predictions of plant responses at the ecosystem level are still highly uncertain. Difficulties lie in the enormous variation of plant responses to climate change variables among and within species, and between and within environmental conditions. Past research assumed that plants using either the C3 or C4 metabolic pathways would respond differently but predictably to climate-change variables based on their metabolic pathway. Recent evidence has suggested however, that the added interactions of external environmental variables and species-specific sensitivities to climate change make it difficult to predict plant and ecosystem responses to climate change. To investigate the mechanisms behind responses of Australian grasses to climate change, 2 pot experiments was conducted using growth cabinets to compare the effect of elevated CO2 and water-limitation on the invasive C4 grassland plant, Eragrostis curvula (E. curvula), native Australian C3 grassland plant, Austrodanthonia racemosa (A. racemosa), and wheat species, Triticum aestivum (T. aestivum). The experiment was run at ambient levels of CO2 maintained at 390 ppm compared to elevated levels of 740 ppm. Imposed restrictions to water supply consisted of gradually drying the soil down to 30 % available soil water (ASW) followed by re-wetting to 50 % ASW. Well-watered conditions for the experiment consisted of gradually drying the soil down to 50 % ASW, followed by rewetting to 95 % ASW. Plants were grown in mixtures and monocultures, consisting of 9 plants equally spaced in a grid design. The three significant findings of the thesis were that: 1) the metabolic pathway (C3 versus C4) was not always an accurate predictor of biomass accumulation under elevated CO2 in the plants studied. Previous research suggested that CO2-stimulation of photosynthesis in C3 plants would lead to greater increases in biomass under elevated CO2 compared to C4 plants, though both C3 and C4 plants could benefit from any reduction in stomatal conductance under dry conditions at elevated CO2. The results from the experiments in this thesis showed a strongly significant biomass response to elevated CO2 in both dry and wet conditions for C4 grass E. curvula. The C3 grass A. racemosa in dry conditions, did not. It was speculated that without the CO2-induced water conservation effect, the C3 grass experienced photosynthetic down-regulation and this precluded a positive biomass response under elevated CO2. 2) the magnitude and direction of biomass response to elevated CO2 was dependant on factors such as resource-availability and the phenotypic variability of the plants species. 3) critical analysis of results from this thesis, combined with past research on plant responses under elevated CO2 showed a tendency for researchers to repeatedly test plants from the Poaceae family, or close relatives of the Poaceae family. As a result, when past data were corrected for this lack of independence, there was no relationship between the evolution of the C3 and C4 metabolic pathway and biomass response to elevated CO2. Instead, other factors (such as growth rate, plant height, leaf number, etc) were presented as being more important in determining biomass response. These observations were supported by results found in this thesis.