Soekisman Tjitrosemito

Leaf gas exchange of trees in old-growth and young secondary forest stands in Sulawesi, Indonesia

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 (gsmax) in the secondary forest (SF) species was 2.1 times higher than in the old-growth forest (OGF) species. Species with a high gsmax reduced gs sharply when vapour pressure deficit of the air increased, whereas species with a low gsmax were much less sensitive to air humidity. For area-based photosynthetic capacity (Amax-area), the SF species had a 2.3 times higher average than the OGF species. For both, gsmax and Amax-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 gsmax and Amax-area. The strong correlation between Amax-area and foliar P (r2=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 Amax; in the SF species strong correlations were found between leaf size and Amax-area and gsmax. 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.

The hydraulic performance of tropical rainforest trees in their perhumid environment Is there evidence for drought vulnerability

In a species-rich perhumid tropical rainforest of Central Sulawesi, Indonesia, we studied the hydraulic properties of eight representative tree species and searched for evidence that analyzing hydraulic patterns at the species level provides valuable information to understand the role of abiotic drivers and structural parameters in controlling plant water consumption.We investigated the relationship between xylem hydraulic properties and tree size with the aim to determine possible hydraulic plasticity in response to the vertical variation in environmental conditions in an otherwise constantly humid forest.We found leaf-specific (LSC) and sapwood-area specific (kS) hydraulic conductivity of twigs to significantly increase with tree height across species. The marked increase of LSC and kS with tree height was closely coupled with an increase in mean vessel diameters and this trend was consistent for both, stem and twig xylem. Rates of xylem sap flow (J) of all tree species were strongly positively related to atmospheric vapor pressure deficit (VPD) and radiation (R), when evaluated on a daily basis. However, J started to level off at VPD values of about 0.4 kPa in trees of all size classes. We therefore concluded that the stomatal response of tropical moist forest trees is very sensitive to changes in the atmospheric evaporative demand.