Carbon and nitrogen isotope discrimination and nitrogen nutrition of trees along a rainfall gradient in northern 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.