Annual rainfall does not directly determine the carbon isotope ratio of leaves of Eucalyptus species

Leaf carbon isotope discrimination (δ¹³C) was widely considered to directly reflect the rainfall environment in which the leaf developed, but recent observations have queried this. The relationship between δ¹³C and rainfall was explored in Eucalyptus species growing along a rainfall gradient in Australia. The leaves of 43 species of Eucalyptus and the closely related Corymbia species produced in 2003 were sampled in September 2004 at 50 sites and grouped into 15 locations along a rainfall gradient in southwest Western Australia. At 24 sites, the same species and same trees were sampled as in a study in September 2003 when leaves produced in 2002 were sampled. The rainfall in 2004 was on average 190 mm (range 135-270 mm) higher at all locations than in 2003. In the leaves sampled in 2004, the mean carbon isotope discrimination (δ¹³C) across the 15 locations decreased 2.9[per thousand] per 1000 mm of rainfall, the specific leaf area (SLA) increased by 2.9 m² kg⁻¹ per 1000 mm of rainfall and the nitrogen (N) content decreased by 1.56 g m⁻² per 1000 mm of rainfall. In contrast, a comparison between the leaves produced in the drier 2002 year compared with the wetter 2003 year showed that there was a strong correlation (r²= 0.85) between the SLA values between years and a trend for higher values with increasing SLA, but the values of δ¹³C were on average only 0.38[per thousand] lower (more negative) at all locations in the wetter year, equivalent to a decrease of 2.0[per thousand] per 1000 mm of rainfall. The results suggest that while there may be constitutive differences in leaf morphology, SLA and N content per unit area, increasing rainfall or cloudiness associated with higher rainfall increases SLA and decreases N content per unit area. We conclude that rainfall does not directly influence δ¹³C, but induces leaf morphological and physiological changes that affect the resultant δ¹³C.