Seedlings of several species of gymnosperm trees angiosperm trees and angiosperm lianas were grown under tropical field conditions in the Republic of Panama; physiological processes controlling plant C and water fluxes were assessed across this functionally diverse range of species. Relative growth rate r was primarily controlled by the ratio of leaf area to plant mass of which specific leaf area was a key component. Instantaneous photosynthesis when expressed on a leaf-mass basis explained 69% of variation in r (P < 0.0001 n = 94). Mean r of angiosperms was significantly higher than that of the gymnosperms; within angiosperms mean r of lianas was higher than that of trees. Whole-plant nitrogen use efficiency was also significantly higher in angiosperm than in gymnosperm species and was primarily controlled by the rate of photosynthesis for a given amount of leaf nitrogen. Whole-plant water use efficiency TE(c) varied significantly among species and was primarily controlled by c(i)/c(a) the ratio of intercellular to ambient CO(2) partial pressures during photosynthesis. Instantaneous measurements of c(i)/c(a) explained 51% of variation in TE(c) (P < 0.0001 n = 94). Whole-plant (13)C discrimination also varied significantly as a function of c(i)/c(a) (R(2) = 0.57 P < 0.0001 n = 94) and was accordingly a good predictor of TE(c). The (18)O enrichment of stem dry matter was primarily controlled by the predicted (18)O enrichment of evaporative sites within leaves (R(2) = 0.61 P < 0.0001 n = 94) with some residual variation explained by mean transpiration rate. Measurements of carbon and oxygen stable isotope ratios could provide a useful means of parameterizing physiological models of tropical forest trees.