Stable carbon and nitrogen isotope ratios of Eucalyptus and Acacia species along a seasonal rainfall gradient in Western Australia

Eucalyptus and Acacia species were surprisingly similar with respect to variations in delta (13) C, delta (15) N. Both genera respond with speciation and associated changes in leaf structure to drought. Stable carbon and nitrogen isotope ratios (delta C-13 and delta N-15) in leaves of eucalypts (Corymbia and Eucalyptus) and Acacia (and some additional Fabaceae) 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-dominated rainfall between 100 and 1,200 mm annually. We investigated 62 eucalypts and 78 woody Fabaceae species, mainly of the genus Acacia. Leaf delta C-13 values of Eucalyptus and Acacia species generally increased linearly with latitude from -29.5 +/- A 1.3 aEuro degrees in the summer-dominated rainfall zone (15A degrees S-18A degrees S) to about -25.7 +/- A 1.1 aEuro degrees in the winter-dominated rainfall zone (29A degrees S-31A degrees S). delta N-15 increased initially with southern latitudes (0.5 +/- A 1.6 aEuro degrees at 15A degrees S; 5.8 +/- A 3.3 aEuro degrees at 24-29A degrees S) but decreased again further South (4.6 +/- A 3.5 aEuro degrees at 31A degrees S). The variation in delta C-13 and delta N-15 was probably due to speciation of Eucalyptus and Acacia into very local populations. There were no species that were distributed over the whole sampling area. The variation in leaf traits was larger between species than within species. Average nitrogen concentrations were 11.9 +/- A 1.05 mg g(-1) in Eucalyptus, and were 18.7 +/- A 4.1 mg g(-1) in Acacia. Even though the average nitrogen concentration was higher in Acacia than Eucalyptus, delta N-15 gave no clear indication for N-2 fixation in Acacia. In a multiple regression, latitude (as a surrogate for rainfall seasonality), mean rainfall, leaf nitrogen concentration, specific leaf area and nitrogen fixation were significant and explained 69 % of the variation of delta C-13, but only 36 % of the variation of delta N-15. Higher nitrogen and phosphorus concentration could give Acacia an advantage over Eucalyptus in arid regions of undefined rainfall seasonality.