Carbon isotope discrimination by a sequence of Eucalyptus species along a subcontinental rainfall gradient in Australia

1, The C-13/C-12 discrimination (Delta) by a series of co-occurring and replacement Eucalyptus species was measured across an eightfold decrease in rainfall. As Delta is a measure of the stomatal limitation on photosynthesis, it should provide a subcontinental scale measure of water-limited plant physiological performance. 2, Leaf Delta of five of 13 species decreased with decreasing rainfall, seven exhibited no trend, and one increased. Wood Delta decreased in eight species, showed no trend in four, and increased in one species. 3, Species replacements were marked by a shift in Delta reflecting greater stomatal limitation on carbon assimilation. 4. Wood Delta was less variable than leaf Delta. 5. There was a non-linear response of the multispecies average leaf and wood Delta to decreasing total annual rainfall. This response reflected the spatial pattern of the sensitivities of Delta to decreasing rainfall of the individual species. It was not the result of a proposed emergent behaviour where the trend in the multispecies average differed from that of the individual species. 6. Patterns of Delta across the distributions of species (reflecting increasing stomatal limitation on assimilation) did not provide a simple measure of the physiological limits of the distribution of eucalypts in north-western Australia.