Asymptotic Height as a Predictor of Photosynthetic Characteristics in Malaysian Rain Forest Trees

Comparative studies of photosynthetic physiology in rain forest trees have focused on differences related to successional status, with the general finding that early-successional species tend to show physiological characteristics of sun plants, while late-successional species show shade-plant characteristics. The present study examines analogous evolutionary responses to vertical gradients in light availability, through an analysis of patterns of photosynthetic variation among late-successional tropical tree species that differ in adult stature. Larger statured tree species are expected to have higher values for light-saturated photosynthetic rate (Amax) as adults, due to the inevitable gradient in light availability through the canopy. However, we argue that larger statured species should also show a higher Amax, and other “sun-plant” characteristics, as saplings under relatively uniform low light conditions in the forest understory. This prediction follows if the potential for photosynthetic acclimation is finite, and if developmental processes that determine adult-phase physiology also affect the physiology of sapling leaves. We examined relationships between photosynthetic parameters and tree species’ stature using comparative data on 28 late-successional species at Pasoh Forest Reserve, West Malaysia. Species chosen for study represent four genera that each include taxa ranging in size from understory treelets to canopy-level trees, thus enabling “phylogenetically corrected” analyses and stronger inference that observed patterns reflect evolutionary convergence. Amax of understory saplings, as measured on a leaf area, mass, or nitrogen basis, was positively correlated with asymptotic height (Hmax) reached by mature trees of a given species. These relationships were similar in each of the four main study genera, thus supporting the hypothesis of an evolutionary response in photosynthetic characteristics to the vertical gradient in light availability through the canopy. Understory species also commonly exhibited higher leaf-level photosynthetic rates at low light levels than did canopy species within a given genus; however, such “crosses” in photosynthetic light response curves were only pronounced when photosynthesis was expressed on a leaf mass basis. Midcanopy leaves from adult trees displayed Amax(area) values similar to leaves from understory saplings of a given species, while Amax(mass) values for adult trees were lower than those of saplings. This pattern corresponded to lower values for specific leaf area in adult trees than in saplings, a difference that was systematically greater in larger statured species. In sum, a range of both adult tree and sapling physiological parameters, including photosynthetic capacity, light saturation point, and leaf nitrogen content, may be predicted as a function of asymptotic species height. Previous research on vertical gradients in photosynthetic characteristics of forest trees has focused on proximate mechanisms, such as light acclimation responses and nutrient reallocation within individual tree canopies. The present study documents evolved differences among species that also contribute to the overall pattern of photosynthetic variation within forest canopies. Our results suggest that much of the variation in leaf-level physiology among late-successional tropical trees is related to an evolved sun–shade trade-off that corresponds to differences in size among species.