Journal of Tropical Ecology (1992) 8:499-504. With 2 figures SHORT COMMUNICATION Midday depression of net CO 2 exchange of leaves of an emergent rain forest tree in French Guiana JACQUES ROY and JEAN-LOUIS SALAGER Centre d'Ecologie Fonclionnelle el Evolutive, CJVRS, BP 5051, 34033 Montpellier Cedex, France KEY WORDS: French Guiana, midday depression, microclimate, photosynthesis, Qualea rosea, tropical rain forest, stomata. Photosynthesis of tropical rain forest plants has primarily been studied in relation to variation in the light environment. Two aspects have been emphasized: acclimation to the contrasting light intensity of understorey, gap or canopy environment (Bjorkman el al. 1972, Doley el al. 1988, Kwesiga et al. 1986, Langenheim el al. 1984, Pearcy 1987, Thompson et al. 1988, Walters & Field 1987) and the response of understorey plants to lightflecks (Chazdon & Pearcy 1986, Pearcy 1988, Pearcy et al. 1985). Photosynthetic response to other environmental factors has been studied less, perhaps because it is thought that other factors impose only a minor limitation on photosynthesis. Langenheim et al. (1984) and Pearcy (1987) found a decrease in photosynthetic rate with increasing leaf-air vapour pressure deficit, (VPD); Chiariello et al. (1987), and Pearcy (1987) reported a weak response to temperature; and Thompson et al. (1988) have investigated the reduction of photosynthetic rate at low nutrient levels. Few studies have determined if these factors limit photosynthesis in natural conditions. Pearcy (1987) did not find evidence that temperature or VPD significantly limited CO 2 uptake. Doley el al. (1988) found that during the first two days following a rainfall event, gas exchange rates in the upper crown of an emergent tree closely followed the daily course of irradiance, while during the subsequent days, they declined markedly following the morning peak. Whitehead et al. (1981) in two planted forest tree species, and Mooney et al. (1983) in an understorey shrub, have both shown a midday decrease of stomatal conductance which was attributed to an increase in VPD, although the consequences for 499