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 fin ding that early-successional species tend to show physiological characteris tics of sun plants, while late-successional species show shade-plant charac teristics. The present study examines analogous evolutionary responses to v ertical gradients in light availability, through an analysis of patterns of photosynthetic variation among late-successional tropical tree species tha t differ in adult stature. Larger statured tree species are expected to hav e higher values for light-saturated photosynthetic rate (A(max)) as adults, due to the inevitable gradient in light availability through the canopy. H owever, we argue that larger statured species should also show a higher A(m ax), and other "sun-plant" characteristics, as saplings under relatively un iform low light conditions in the forest understory. This prediction follow s if the potential for photosynthetic acclimation is finite, and if develop mental processes that determine adult-phase physiology also affect the phys iology of sapling leaves. We examined relationships between photosynthetic parameters and tree species' stature using comparative data on 28 late-succ essional species at Pasoh Forest Reserve, West Malaysia. Species chosen for study represent four genera that each include taxa ranging in size from un derstory treelets to canopy-level trees, thus enabling "phylogenetically co rrected" analyses and stronger inference that observed patterns reflect evo lutionary convergence. A,,, of understory saplings, as measured on a leaf area, mass, or nitrogen basis, was positively correlated with asymptotic height (H-max) reached by mature trees of a given species. These relationships were similar in each o f the four main study genera, thus supporting the hypothesis of an evolutio nary response in photosynthetic characteristics to the vertical gradient in light availability through the canopy. Understory species also commonly ex hibited higher leaf-level photosynthetic rates at low light levels than did canopy species within a given genus; however, such "crosses" in photosynth etic light response curves were only pronounced when photosynthesis was exp ressed on a leaf mass basis. Midcanopy leaves from adult trees displayed A( max)(area) values similar to leaves from understory saplings of a given spe cies, while A(max)(mass) values for adult trees were lower than those of sa plings. 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 saturat ion point, and leaf nitrogen content, may be predicted as a function of asy mptotic species height. Previous research on vertical gradients in photosynthetic characteristics o f forest trees has focused on proximate mechanisms, such as light acclimati on responses and nutrient reallocation within individual tree canopies, The present study documents evolved differences among species that also contri bute to the overall pattern of photosynthetic variation within forest canop ies. Our results suggest that much of the variation in leaf-level physiolog y among late-successional tropical trees is related to an evolved sun-shade trade-off that corresponds to differences in size among species.