It has been suggested that plant species from the warmer ecosystems will show different and potentially larger photosynthesis and productivity responses to elevated CO2 (eCO2, ambient + 150 ppm) compared to those from the cold temperate ecosystems, on the basis of higher average annual temperature and greater water deficits in the former ecosystems. Based on these expectations, it has further been predicted that the warm water-limited ecosystems may have a greater potential to sequester the extra C that has been assimilated under eCO2. However, empirical evidences testing these expectations are scarce. The overall aim of this thesis was to investigate the effects of eCO2 on photosynthesis and productivity responses of the evergreen C3 herbaceous species from the understory of a periodically water-limited warm-temperate Eucalyptus woodland. In a three-year field study conducted at the Eucalyptus free-air CO2 enrichment experiment (EucFACE), I investigated how eCO2-induced enhancement of photosynthetic rates (Anet) in herbaceous species varied with seasonal water availability. During the second and third year of CO2 fertilisation at EucFACE, I measured the seasonal photosynthetic acclimation responses to eCO2 in two dominant species- a C3 forb and a C3 grass, and measured responses of peak above-ground biomass to eCO2 for total forbs and grasses. In a glasshouse experiment, I tested whether the species or functional groups growing under similar water inputs and nutrient availability differed in their photosynthetic or biomass allocation and growth responses to eCO2 for two C3 forbs and two C3 grasses. also evidence of photosynthetic acclimation under eCO2 in the dominant C3 herbaceous species, especially during the peak growing season of spring. Also, there was no proportional stimulation of peak above-ground biomass in the understory grasses and forbs, which may have been a result of lack of a ‘water-savings effect’ of eCO2 and/or higher soil nutrient limitation. C3 grasses and C3 forbs differed in their photosynthetic and biomass allocation responses to eCO2. Differences in leaf N content, N allocation and changes in above-ground biomass allocation likely affected the CO2 responsiveness in these functional groups. In particular, there was an ability to maintain greater leaf area index, N allocation to photosynthesis and avoid down-regulation under eCO2 by the grasses but not by the forbs. Findings from the current study suggest that interactions between seasonal water-availability eCO2 will be critical in determining relative Anet enhancement response in herbaceous species from a water-limited ecosystem. However, the enhancement response may not be mediated via a ‘water-savings effect’ of eCO2, which contrasts with the earlier findings from cold temperate ecosystems. Furthermore, evidence of photosynthetic capacity down-regulation in the dominant species and lack of relative increase in biomass under eCO2, suggest a limited capacity of the understory herbaceous species from a grassy woodland to respond to eCO2 and ultimately act as an aboveground C sink in future
