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Abstract

Stable carbon and nitrogen isotope ratios (delta13C and delta15N) in leaves of Eucalyptus and Acacia species were investigated together with specific leaf area (SLA), leaf nitrogen (N) and leaf phosphorous (P) concentration along a north-south transect through Western Australia covering winter- and summer-dominant rainfall between 100 and 1200 mm annually. We investigated 62 Eucalyptus and 78 woody Fabaceae species, mainly of the genus Acacia. Delta13C increased with latitude without distinct separation of rainfall regime (-29.5‰ + 1.3‰ at tropical latitude (15°S) and -25.7‰ + 1.1‰ at winter-rains (31°S)). Delta15N increased with southern latitudes (0.5‰ + 1.6‰ at 15°S; 5.8‰ + 3.3‰ at 24 to 29°S; 4.6‰ + 3.5‰ at 31°S). The variation in delta13C and delta15N was mainly due to speciation of Eucalyptus and Acacia into very local populations. There was not a single species that covered the range of conditions. The variation in leaf traits was larger between than within species. Nitrogen concentrations were 11.9 + 1.05 mg g-1 in Eucalyptus, but reached 18.7 + 4.1 mg g-1 in Acacia, even though delta15N gave no clear indication for N2-fixation. Comparing freshly burnt and un-burnt sites, delta15N values appear to be mainly determined by fire. In a multiple regression, latitude (as surrogate for seasonality), rainfall, leaf nitrogen concentration, specific leaf area and nitrogen fixation were significant and explained 69% of the variation of delta13C, but only 36% of the variation of delta15N. Higher nitrogen and phosphorus concentration could give Acacia an advantage over Eucalyptus in arid regions of undefined rainfall.