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In the tropics, old-growth forests are converted to other land cover types at a high rate and young secondary forest may gain in importance. Information on associated changes in leaf gas exchange and other leaf traits can be valuable for modelling biogeochemical fluxes under altered land-use patterns. We studied in situ photosynthetic parameters and stomatal conductance for water vapour in eight abundant tree species of young secondary forest and eight tree species of natural old-growth forest in Central Sulawesi, Indonesia. In sun leaves, the average maximal stomatal conductance (g smax) in the secondary forest (SF) species was 2.1 times higher than in the old-growth forest (OGF) species. Species with a high g smax reduced g s sharply when vapour pressure deficit of the air increased, whereas species with a low g smax were much less sensitive to air humidity. For area-based photosynthetic capacity (A max-area), the SF species had a 2.3 times higher average than the OGF species. For both, g smax and A max-area the variation among species was higher in the OGF than in the SF. When all tree species (n=16) are considered, species means of specific leaf area (SLA), leaf N concentration and leaf P concentration were significantly correlated with g smax and A max-area. The strong correlation between A max-area and foliar P (r 2=0.8) is remarkable as the alluvial soils in the study region are rich in nutrients. If the eight OGF species are analysed separately, the only significant correlation was observed between SLA and mass-based A max; in the SF species strong correlations were found between leaf size and A max-area and g smax. These results show that the conversion of old-growth forest to young secondary forest in Sulawesi significantly alters tree leaf gas exchange characteristics and that chemical and structural leaf traits can be used for the prediction of these changes. The best correlations between leaf gas exchange parameters and leaf traits were obtained by different traits in the SF species, the OGF species and the entire pool of studied species.