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ItemEffect of carbon dioxide concentration on Eucalyptus ovata foliage and its utilisation by the common ringtail possum, (Pseudocheirus peregrinus)(University of Melbourne, 1998)Elevated C02 stimulates photosynthesis and alters the allocation of resources in leaves, which in turn influences plant-herbivore interactions. The alterations to leaf chemistry may render leaves less nutritious for herbivores. To test this hypothesis, Eucalyptus ovata trees were grown from seed for 1 year in glasshouses, where the concentration of C02 was ambient (~350 ?mol.mol-1) or elevated (~700 ?mol.mol-1), and their chemical compositions compared. As has been found previously with other eucalypts, E. ovata leaves from trees grown at elevated C02 contained 30% less nitrogen. Despite an increase in the concentration of total phenolics in leaves from plants grown at elevated C02 compared to ambient, an individual compound (i.e. macrocarpal G) that is important in mediating feeding by herbivores did not increase. Leaves also had a greater dry matter content, were thicker and less tough when grown at ambient compared to elevated C02. The chemical composition of leaves collected from the field also was investigated. All E. ovata saplings tested were non-cyanogenic. Foliage from the glasshouse was predicted to be palatable to ringtail possums by comparison with foliage of known palatability using principal component analysis of near infrared spectra. For the first time a mammalian herbivore, the common ringtail possum (Pseudocheirus peregrinus), was fed a diet of leaves grown at both ambient and elevated C02. There was no difference in intake of E. ovata foliage between the two C02 treatments, despite differences in foliage composition. Average values for foliage intake were 36.8 and 39.0 kg-0.75.d-1 for ambient and elevated C02, respectively. Thus unlike insects, marsupial folivores did not increase consumption to compensate for a diet that was lower in nutritional quality. There was a trend for measures of animal performance to be lower when fed leaves grown at elevated C02 compared to ambient; however, differences were not statistically significant. Gross energy intakes were similar between animals fed the foliage either from ambient or elevated C02, although digestible energy intakes (DEI) were lower at elevated C02. Average values for DEI were 276.5 and 257.0 kJ.g-1 for ambient and elevated C02, respectively. It is concluded that twice ambient C02 altered the chemical composition of E. ovata leaves although the common ringtail possums managed these changes, at least in the short term, due to metabolic and digestive adaptations to a diet of eucalypt leaves.