Pioneer and late stage tropical rainforest tree species (French Guiana) growing under common conditions differ in leaf gas exchange regulation, carbon isotope discrimination and leaf water potential.

Leaf gas exchange rates, predawn Ψ and daily minimum Ψ leaf water potentials were measured during a wet-to-dry season transition in pioneer (Jacaranda copaia, Goupia glabra andCarapa guianensis) and late stage rainforest tree species (Dicorynia guianensis andEperua falcata) growing in common conditions in artificial stands in French Guiana. Carbon isotope discrimination (Δ) was assessed by measuring the stable carbon isotope composition of the cellulose fraction of wood cores. The Δ values were 2.7‰ higher in the pioneer species than in the late stage species. The calculated time integratedC values derived from the Δ values averaged 281 μmol mol in the pioneers and 240 μmol mol in the late stage species. The corresponding time-integrated values of intrinsinc water-use efficiency [ratio CO assimilation rate (A)/leaf conductance (g)] ranged from 37 to 47 mmol mol in the pioneers and the values were 64 and 74 mmol mol for the two late stage species. The high Δ values were associated-at least inJ. copaia-with high maximumg values and with high plant intrinsinc specific hydraulic conductance [C≔g/(Ψ-Ψ], which could reflect a high competitive ability for water and nutrient uptake in the absence of soil drought in the pioneers. A further clear discriminating trait of the pioneer species was the very sensitive stomatal response to drought in the soil, which might be associated with a high vulnerability to cavitation in these species. From a methodological point of view, the results show the relevance of Δ for distinguishing ecophysiological functional types among rainforest trees.