PIONEER AND LATE-STAGE TROPICAL RAIN-FOREST 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 (psi(wp)) and daily minimum (psi(wm)) leaf water potentials were measured during a wet-to-dry season transi tion in pioneer (Jacaranda copaia, Goupia glabra and Carapa guianensis ) and late stage rainforest tree species (Dicorynia guianensis and Epe rua falcata) growing in common conditions in artificial stands in Fren ch Guiana. Carbon isotope discrimination (Delta) was assessed by measu ring the stable carbon isotope composition of the cellulose fraction o f wood cores; The Delta Values were 2.7 parts per thousand higher in t he pioneer species than in the late stage species. The calculated time integrated C-i values derived from the Delta values averaged 281 mu m ol mol(-1) in the pioneers and 240 mu mol mol(-1) in the late stage sp ecies. The corresponding time-integrated values of intrinsinc water-us e efficiency [ratio CO2 assimilation rate (A)/leaf conductance (g)] ra nged from 37 to 47 mmol mol(-1) in the pioneers and the values were 64 and 74 mmol mol(-1) for the two late stage species. The high Delta va lues were associated - at least in J. copaia - with high maximum g val ues and with high plant intrinsinc specific hydraulic conductance [C=g /(Psi(wp) - Psi(wm)], which could reflect a high competitive ability f or water and nutrient uptake in the absence of soil drought in the pio neers. A further clear discriminating trait of the pioneer species was the very sensitive stomatal response to drought in the soil, which mi ght be associated with a high vulnerability to cavitation in these spe cies. From a methodological point of view, the results show the releva nce of Delta for distinguishing ecophysiological functional types amon g rainforest trees.