Transpiration efficiency over an annual cycle, leaf gas exchange and wood carbon isotope ratio of three tropical tree species

Variation in transpiration efficiency (TE) and its relationship with the stable carbon isotope ratio of wood was investigated in the saplings of three tropical tree species. Five individuals each of Platymiscium pinnatum (Jacq.) Dugand, Swietenia macrophylla King and Tectona grandis Linn. f. were grown individually in large (760 1) pots over 16 months in the Republic of Panama. Cumulative transpiration was determined by repeatedly weighing the pots with a pallet truck scale. Dry matter production was determined by destructive harvest. The TE, expressed as experiment-long dry matter production divided by cumulative water use, averaged 4.1, 4.3 and 2.9 g dry matter kg(-1) water for P. pinnatum, S. macrophylla and T. grandis, respectively. The TE of T. grandis was significantly lower than that of the other two species. Instantaneous measurements of the ratio of intercellular to ambient CO2 partial pressures (c(i)/c(a)), taken near the end of the experiment, explained 66% of variation in TE. Stomatal conductance was lower in S. macrophylla than in T. grandis, whereas P. pinnatum had similar stomatal conductance to T. grandis, but with a higher photosynthetic rate. Thus, c(i)/c(a) and TE appeared to vary in response to both stomatal conductance and photosynthetic capacity. Stem-wood delta C-13 varied over a relatively narrow range of just 2.2 parts per thousand, but still explained 28% of variation in TE. The results suggest that leaf-level processes largely determined variation among the three tropical tree species in whole-plant water-use efficiency integrated over a full annual cycle.