Species differences in carbon isotope ratios, specific leaf area and nitrogen concentrations in leaves of Eucalyptus growing in a common garden compared with along an aridity gradient

Leaves produced in 2004 of 422 species of Eucalyptus whose natural habitat is southern Australia were sampled at the Currency Creek Arboretum in South Australia where the annual (mainly winter) rainfall is about 400 mm. Tree height, leaf area, leaf dry weight, leaf nitrogen (N) concentration and leaf carbon isotope ratio (δ¹³C) were measured and the specific leaf area (SLA) calculated. Among the 422 species, the SLA varied from 1.5 to 8.8 m² kg⁻¹ and N concentration varied from 0.6 to 2.1%, much greater than in 64 species collected along an aridity transect from southwestern Western Australia to central Australia in 2003. Also, the range of leaf δ¹³C values was similar in the common garden to that across the aridity transect. For the 45 species present in both studies, the SLA and leaf N concentration in the common garden were similar to those measured in leaves along the aridity transect, indicating that these characteristics are inherent in the species and vary little with environment. The variation in leaf δ¹³C in the common garden was just as great as along the transect, but the values measured in the one location were poorly correlated with those along the transect. This was not expected, as the variation in δ¹³C at one common site in South Australia was anticipated to be less than along the aridity gradient where annual rainfall varied from 250 to 1200 mm. Path analysis on the 45 species common to both studies indicated that rainfall did not have a direct effect on δ¹³C, but the differences in δ¹³C resulted from indirect effects of rainfall on SLA and N concentration. δ¹³C was negatively correlated with SLA but positively correlated with N. Thus, both effects may compensate for each other so that no significant relationship between δ¹³C and rainfall was observable. However, there is a large degree of variation of δ¹³C at any level of rainfall. The origin and ecological implications of this observation are discussed.