The responses of predawn leaf water potential (φwp), leaf conductance to water vapour diffusion (g), CO2 assimilation rate (A) and carbon isotope competition (δ13C) to a soil drying cycle were assessed in Pinus pinaster, a drought‐avoiding species with high stomatal sensitivity to drought, and Quercus petraea, a drought‐tolerant species with lower stomatal sensitivity to drought, under present (350 μmol−1) and elevated (700 μmol−1) atmospheric CO2 concentrations ([CO2]). In P. pinaster, decreasing A in response to drought was associated with increasing plant intrinsic water use efficiency (A/g) and with decreasing calculated intercellular [CO2] (C1), suggesting a stomatal limitation of A. In contrast, in Q. petraea, A/g declined and C1 increased during the drying cycle, which suggests a non‐stomatal origin for the decrease in A. In P. pinaster, a negative relationship was observed between the gas exchange‐derived values of Ci/Ca and δ13C, which conforms to the classical two‐step carbon isotope discrimination model. In Q. petraea, the relationship between C1/Ca and δ13C was positive. Possible causes of this discrepancy are discussed. Lower g values were observed under elevated [CO2] than under present [CO2] in Q. petraea, whereas g was unaffected in P. pinaster. A stimulation of A by elevated [CO2] was found in P. pinaster but not in Q. petraea. In both species, A/g was markedly higher under elevated than under present [CO2]. Whether the differences in the g response to elevated [CO2] found here can be generalized to other drought‐avoiding and non‐avoiding species remains to be assessed.

SVMMARYGrowth, COj assimilation rate {A), leaf conductance {g), transpiration efficiency (W = ratio biomass production/plant water use) and carbon isotope discrimination (A) were assessed in maritime pine (Pinus pinaster Ait.) and pedunculate oak (Quercus robur L.) grown on a sand-peat mixture with three levels of fertilization : FlOO, optimal complete fertilization; F25, 25 "o of the optima! fertilizer supply; FO, no fertilization. Leaf phosphorus (P) and potassium (K) concentrations were affected little by the diminishing nutrient availability. Reduced fertilization decreased plant nitrogen (N) concentration in both species but leaf N concentration was less affected in oak than in pine. In pine W was markedly reduced in response to reduced leaf or whole plant N concentration, which was consistent with the sharp decrease also observed for plant intrinsic water-use efficiency (ratio A/g) both at the instantaneous (gas exchange data) and time-integrated (A/g derived from A measurements) levels. In this species, lowered W in the N deficient conditions was primarily associated with enhanced values of g. The existence of such a stomatal response pattern, confirmed by the increase in plant transpiration between FlOO and F25, has not been reported before. In oak, both A and g were decreased in F25 and FO as compared with FlOO. W was not affected -and instantaneous as well as time-integrated A/g values were oni}' slightly decreased -in relation to decreasing plant N concentration. For FlOO, no difference in W was noticed between pine and oak though the A values were 2-6°^ lower in oak. We speculate that this discrepancy was linked with higher plant-carbon losses through processes like respiration, fine-root mortality or root exudation in oak. The isotopic approach proved useful for assessing the effects of nutritional status on W^ but has to be used with caution w hen comparing different species.Key words: Pinuspinaster (maritime pine), Quercus robur [pedunculate o-dk), nitrogen deficiency, gas exchange and water-use efficiency, carbon isotope discrimination.

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.

IntroductionFaidherbia albida (Del.) A. Chev. (syn. Acacia albida: Mimosoideae) is a leguminous tree species widely distributed in Africa. It is present in Sudanese and Sahelian zones that are affected by a long dry season. The species is distributed over territories with an annual rainfall ranging from 50 to 1500 mm (Fagg & Barnes 1990). In eastern, central and southern Africa, it occurs naturally along riverbanks on alluvial soils. The best known peculiarity of F. albida is its reverse phenology (Wickens 1969). Trees are in leaf, growing and fruiting during the dry season, whereas leaves are shed after the first rains and growth resumes only at the end of the wet season. This phenology is advantageous for agroforestry, because competition with associated crops growing during the wet season is minimized. Faidherbia albida trees are rather vigor- 3. Radial trunk growth ceased before the end of the dry season and could have been affected by the moderate drought stress. However, leafiness remained constant during the dry season. Leaf shedding occurred after the first rains and was probably independent of drought. 4. Faidherbia albida displayed large transpiration rates under favourable conditions but the ratio of sapflow to Penman evapotranspiration and the soil-to-leaf specific hydraulic conductance decreased severely towards the end of the dry season. 5. Roots of F. albida were distributed through the weathered rock, down to a depth of 7 m, and vanished in the vicinity of a permanent water-table. The isotopic composition of oxygen in the xylem sap (δ 18 O) remained very close to the values recorded in the water-table during the course of the year. Phreatophytism thus explained the maintenance of growth and transpiration during the dry season. Nevertheless, during early rains, δ 18 O of sap switched towards the composition of the superficial soil layers, indicating facultative phreatophytism. 6. Reverse phenology, low density and depth of water uptake of F. albida indicated a low competition with annual crops for water; the fraction of annual rainfall used by the trees was estimated to remain below 5%.