A comparison of in situ leaf photosynthesis and chlorophyll fluorescence at the top canopies in rainforest mature trees

Abstract

The top canopy leaves of mature trees are exposed to high solar radiation, regardless of whether they are climax or pioneer trees. We compared leaf physiology and morphology at the top canopies of a climax Dipterocarpus cornutus tree and a pioneer Macaranga gigantea tree in a rainforest, East Kalimantan. The mass-based maximum net photosynthetic rate (Pnmax) and the nitrogen-based Pnmax for M. gigantea were 2.3-fold and 1.6-fold greater than those for D. cornutus, respectively. However, no significant difference in the area-based Pnmax between the two species was found, because of its thick lamina of D. cornutus. The dependency of net photosynthetic rate and stomatal conductance on leaf-to-air vapor pressure deficit did not differ between the two trees. Under high irradiance, effective quantum yield (ΔF/Fm′) of photosystem II (PSII), photochemical quenching (qP), non-photochemical quenchings (qN and NPQ), and electron transport rate in PSII were significantly higher in M. gigantea than in D. cornutus. Under moderate irradiance, a hysteresis in chlorophyll fluorescence parameters between the morning and afternoon was found especially in D. cornutus, i.e., the fraction of qP within the PSII centers decreased and that of excess energy increased in the afternoon. These data indicate low light-use capacity and high susceptibility for excess light energy of PSII in D. cornutus. The leaf properties were different between the mature climax and pioneer trees, even if their leaves grew under similar light environments at the top canopies.