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.

Carbon isotope discrimination (Δ) and nitrogen isotope ratios, N-concentrations and specific leaf area of 50 tree species were investigated along a continental-scale transect through northern Australia over which annual rainfall varied from 1800 mm to 216 mm rainfall. Average specific leaf area (SLA, m2 kg-1) of leaves ranged from 10.7 ± 1.7 (av. ± s.d.) in N2 fixing deciduous trees to 0.8 ± 0.4 in spinescent sclerophylls shrubs. SLA generally decreased with increasing aridity. N2 fixing species had higher leaf N concentration (average N-concentration 20.1 ± 3.7 mgN g-1) than non- N2 fixing (10.8 ± 3.3) or spinescent species (7.05 ± 1.8). Community-averaged Δ was approximately constant at rainfalls above 475 mm (average Δ = 19.4 ± 1.2‰). Where rainfall was less than 475 mm, Δ decreased from 19‰ to 17‰ at 220 mm. Δ was positively correlated with SLA. Δ of deciduous N2 fixing species and spinescent species were 1‰ and 2.4‰ lower than in evergreen sclerophyllous species. Δ in the N2 fixing Allocasuarina was 1.2‰ lower than in non- N2 fixing sclerophyllous species. The δ15N-values indicated N2 fixation only at high rainfall. Burning of the field layer in a Eucalyptus forest had no effect on all measured tree parameters including δ15N, but δ15N increased under grazing conditions to >5‰. The constant value of community averaged Δ between 1800 and 450 mm may be the result of replacement of functional types and species. The decline in Δ in the more arid section may be a function of both low species diversity, and a highly aseasonal and unpredictable rainfall regime.