Growth in two common gardens reveals species by environment interaction in carbon isotope discrimination of Eucalyptus

One-year-old sun leaves of 60 species of Eucalyptus were collected in August 2005 at an arboretum in South Australia with a mean annual rainfall of 427 mm, and 14 of the same species were sampled at an arboretum in Western Australia with a mean annual rainfall of 216 mm. We determined the genetic and phenotypic variation in carbon isotope composition (delta13C), specific leaf area (SLA) and nitrogen content per unit area of the species at each site. There were very significant (P < 0.001) differences in delta13C among the species at both sites. The mean delta13C of the 60 species at the wetter site was -27.6 per thousand (from -25.8 per thousand in Eucalyptus youngiana to -29.9 per thousand in Eucalyptus salicola) and of the 14 species at the drier site was -25.3 per thousand (from -23.7 per thousand in Eucalyptus ravida to -27.3 per thousand in Eucalyptus ewartiana). Of the 14 species common to both sites, four species had similar values of delta13C at the two sites despite the differences in rainfall, whereas in others the values of delta13C were significantly (P < 0.001) lower (more negative) at the wet than at the dry site. The SLA and nitrogen content per unit leaf area also differed significantly among the species (P < 0.001), but there was not a common relationship between delta13C and SLA or between delta13C and nitrogen content at the two sites. The strong species by environment interaction resulted from some species demonstrating phenotypic plasticity for delta13C, while others were inherently stable across environments.