The response of whole-plant water-use efficiency, termed transpiration efficiency (TE), to variation in soil fertility was assessed in a tropical pioneer tree, Ficus insipida Willd. Measurements of stable isotope ratios ({delta}13C, {delta}18O, {delta}15N), elemental concentrations (C, N, P), plant growth, instantaneous leaf gas exchange, and whole-plant water use were used to analyse the mechanisms controlling TE. Plants were grown individually in 19 l pots with non-limiting soil moisture. Soil fertility was altered by mixing soil with varying proportions of rice husks, and applying a slow release fertilizer. A large variation was observed in leaf photosynthetic rate, mean relative growth rate (RGR), and TE in response to experimental treatments; these traits were well correlated with variation in leaf N concentration. Variation in TE showed a strong dependence on the ratio of intercellular to ambient CO2 mole fractions (ci/ca); both for instantaneous measurements of ci/ca (R2=0.69, P <0.0001, n=30), and integrated estimates based on C isotope discrimination (R2=0.88, P <0.0001, n=30). On the other hand, variations in the leaf-to-air humidity gradient, unproductive water loss, and respiratory C use probably played only minor roles in modulating TE in the face of variable soil fertility. The pronounced variation in TE resulted from a combination of the strong response of ci/ca to leaf N, and inherently high values of ci/ca for this tropical tree species; these two factors conspired to cause a 4-fold variation among treatments in (1ci/ca), the term that actually modifies TE. Results suggest that variation in plant N status could have important implications for the coupling between C and water exchange in tropical forest trees.
