Knowledge of variations in morphophysiological leaf traits with forest height is essential for quantifying carbon and water fluxes from forest ecosystems. Here, we examined changes in leaf traits with forest height in diverse tree species and their role in environmental acclimation in a tropical rain forest in Borneo that does not experience dry spells. Height-related changes in leaf physiological and morphological traits [e.g., maximum photosynthetic rate (Amax), stomatal conductance (gs), dark respiration rate (Rd), carbon isotope ratio (δ(13)C), nitrogen (N) content, and leaf mass per area (LMA)] from understory to emergent trees were investigated in 104 species in 29 families. We found that many leaf area-based physiological traits (e.g., A(max-area), Rd, gs), N, δ(13)C, and LMA increased linearly with tree height, while leaf mass-based physiological traits (e.g., A(max-mass)) only increased slightly. These patterns differed from other biomes such as temperate and tropical dry forests, where trees usually show decreased photosynthetic capacity (e.g., A(max-area), A(max-mass)) with height. Increases in photosynthetic capacity, LMA, and δ(13)C are favored under bright and dry upper canopy conditions with higher photosynthetic productivity and drought tolerance, whereas lower R d and LMA may improve shade tolerance in lower canopy trees. 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 were associated with tree height and the degree of leaf drought stress, even in diverse tropical rain forest trees.

Abstract:We developed allometric relationships between tree size parameters (stem diameter at breast height (dbh), at ground surface (D0) and tree height) and leaf, stem, small-root (diameter <5 mm) and total root biomass in various tropical secondary-forest trees in Sarawak, Malaysia. In total, 136 individuals from 23 species were harvested to measure above-ground parts. Root systems of 77 individuals of 16 species were also excavated. The coefficients of correlation for the obtained allometric relationships between tree diameter and plant-part biomass showed high values, ranging from 0.83 to 0.99. In addition, there were few interspecific differences in relationships for all biomass parts, except for leaves. We also found relatively high coefficients of allometric relationships between tree height and plant-part biomass ranging from 0.83 to 0.94. Comparison of above- and below-ground biomass equations for various tropical rainforests implies that our allometric equations differ largely from the equations for tropical primary forests. Thus, choosing both above- and below-ground allometric equations for biomass estimation in tropical secondary forests of South-East Asia requires careful consideration of their suitability.

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Tropical canopy tree species can be classified into two types by their heterobaric and homobaric leaves. We studied the relation between both leaf types and their water use, together with the morphological characteristics of leaves and xylem, in 23 canopy species in a tropical rain forest. The maximum rates of photosynthesis and transpiration were significantly higher in heterobaric leaf species, which also underwent larger diurnal variations of leaf water potential compared to homobaric leaf species. The vessel diameter was significantly larger and the stomatal pore index (SPI) was significantly higher in heterobaric than that in homobaric leaf species. There was a significant positive correlation between the vessel diameter, SPI, and maximum transpiration rates in all the studied species of both leaf types. However, there was no significant difference in other properties, such as leaf water-use efficiency, leaf mass per area, leaf nitrogen content, and leaf δ 13 C between heterobaric and homobaric leaf species. Our results indicate that leaf and xylem morphological differences between heterobaric and homobaric leaf species are closely related to leaf water-use characteristics, even in the same habitat: heterobaric leaf species achieved a high carbon gain with large water use under strong light conditions, whereas homobaric leaf species can maintain a high leaf water potential even at midday as a result of low water use in the canopy environment.

In this study, we demonstrate changes in leaf morphological and physiological traits with tree height from dark understory to bright canopy conditions in various tree species in the Cambodian tropical dry evergreen forest. The vegetation mainly consisted of Dipterocarpaceae and Myristicaceae and the canopy trees usually reached 30-40 m in height. We investigated 25 individuals of 18 tree species ranging from 0.8 to 33 m in height. We measured the leaf photosynthetic rate, stomatal conductance and respiration rate for 3 to 5 leaves per sampling position in the early dry season. All leaves were then divided into two parts: one for measuring dry weight, nitrogen content and δ 13 C; the other for observation of leaf morphology. The leaf morphological traits, such as leaf mass per area (LMA), cuticle thickness, palisade layer thickness, leaf hardness and stomatal density increased linearly with tree height. The leaf nitrogen content per unit leaf area (N area ) peaked at 10 m from the ground, though the nitrogen content per unit dry leaf mass (N mass ) decreased linearly with tree height. Higher LMA, cuticle thickness and hard leaves in canopy condition may contribute to high drought tolerance and physical strength. The leaf-area-based photosynthetic rate (A max-area ) peaked at an intermediate tree height of approximately 10 m, and then decreased toward the upper canopy. In contrast, the leafmass-based photosynthetic rate (A max-mass ) decreased linearly with tree height. Reduction of leaf nitrogen content and stomatal conductance mainly limit photosynthetic capacities with tree height. Overall, many leaf morphological traits could be summarized in a simple and significant relation with tree height, though increasing tree height, which is related to the micro-climatic gradient, leads to both nitrogen and stomatal constraints of leaf photosynthetic capacities, even when considering many different tree species.