PHOTOSYNTHESIS RATES OF SELECTED TREE SPECIES IN LOWLAND DIPTEROCARP RAIN-FOREST OF SABAH, MALAYSIA

Diurnal courses of net photosynthesis, transpiration and water potenti al of leaves of ten woody species from the natural lowland dipterocarp forests in Sabah (North Borneo, Malaysia) and one exotic tree species were studied in the field. The indigenous species represent different ecological niches and successional stages in the various layers of th e dipterocarp forest, such as pioneers, trees of the understorey or ma in canopy and emergents. Diurnal changes in CO2 exchange and transpira tion reflected primarily differences in irradiance. The diurnal course s of water potential mainly tracked the rate of transpiratory water lo ss. Light-dependency describes most of the diurnal variations of leave s' gas exchange. Light response curves of net photosynthesis of the in vestigated species of the Dipterocapaceae were almost equal (light sat urated assimilation rate, A(max): 5.0-7.2 mu mol CO2 m(-2) s(-1)), whi le those of the other species exhibited remarkable differences (A(max) : 5.5-14.2 mu mol CO2 m(-2) s(-1)). Leaf area, chlorophyll content and specific leaf dry weight as the reference parameters for assimilation gave a general ranking of the A(max), which is highest for the pionee ring species, less for the understorey trees and lowest for emergents. Light compensation points and light saturation of net photosynthesis were attained mainly between 6 and 9 mu mol photons m(-2) s(-1) and be tween 230 and 534 mu mol photons m(-2) s(-1), respectively, but were h igher for pioneering species. Photosynthetic performance may be a diag nostic feature of the successional and ecological status of species, i .e. to characterize pioneering species from understorey species or fro m emergents of the dipterocarp forest.