Photosynthesis rates of selected tree species in lowland dipterocarp rainforest of Sabah, Malaysia

Diurnal courses of net photosynthesis, transpiration and water potential 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 the dipterocarp forest, such as pioneers, trees of the understorey or main canopy and emergents. Diurnal changes in CO2 exchange and transpiration reflected primarily differences in irradiance. The diurnal courses of water potential mainly tracked the rate of transpiratory water loss. Light-dependency describes most of the diurnal variations of leaves' gas exchange. Light response curves of net photosynthesis of the investigated species of the Dipterocapaceae were almost equal (light saturated assimilation rate, A(max): 5.0-7.2 mu mol CO2 m(-2) s(-1)), while 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 pioneering 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 between 230 and 534 mu mol photons m(-2) s(-1), respectively, but were higher for pioneering species. Photosynthetic performance may be a diagnostic feature of the successional and ecological status of species, i.e. to characterize pioneering species from understorey species or from emergents of the dipterocarp forest.