<S1-8> Vertical changes in leaf ecophysiological traits in diverse tropical rainforest tree species in Lambir Hills National Park in Sarawak

Abstract

Information about vertical changes in morphological and physiological leaf traits with forest height are essential for quantifying carbon and water fluxes of forest ecosystems. We examined changes in leaf traits with forest height in diverse tree species and their role in environmental acclimation in a mixed dipterocarp forest in Lambir Hills National Park in Sarawak that does not experience dry spells. The mean canopy height in the forest was about 30-40 m; some emergent trees reached 50 m in height. The upper canopy layer is dominated by many dipterocarp trees, such as Dryobalanops and Shorea spp. Vertical changes in the leaf traits [e.g., maximum photosynthetic rate (Amax), stomatal conductance (gs), dark respiration rate (Rd), carbon isotope ratio (δ13C), nitrogen (N) content, and leaf mass per area (LMA)] from understory to emergent trees were investigated in 104 species in 29 families. We used an 85-m canopy crane to conduct all physiological measurements. We found that many leaf area-based gas exchange traits (e.g., Amax-area, Rd, gs), N, δ13C, and LMA increased linearly with tree height, whereas leaf mass-based physiological traits (e.g., Amax-mass) only increased slightly. These patterns differed from those of other biomes such as temperate and tropical dry forests, where trees usually show decreased photosynthetic capacity (e.g., Amax-area, Amax-mass) with height. Increases in photosynthetic capacity, LMA, and δ13C are favored under bright and dry upper canopy conditions with higher photosynthetic productivity and drought tolerance, whereas lower Rd and LMA may improve shade tolerance in lower canopy trees. The rapid recovery of leaf midday water potential to theoretical gravity potential during the night supports the idea that the majority of trees do not suffer from strong drought stress. Overall, leaf area-based photosynthetic traits 71 were associated with tree height and the degree of leaf drought stress, even in diverse tropical rain forest trees.