The water use efficiencies were found to vary from 5.97 × 10−3 to 12.3 × 10−3 m3 m−3. Significant differences were observed between clonal-mean water use efficiencies averaged over six sampling periods. The average δ13C of the mature and young foliage was found to be significantly correlated with WUE. However, the correlation was weak, suggesting that the relationship between δ13C and WUE is more complex in trees than suggested in the literature on crop plants. It is suggested that differences between sample trees in carbon allocation and leaf-to-air vapour pressure deficits may account for the poor correlation between δ13C and WUE in the four E. grandis clones studied. 相似文献