Variations in Leaf Photosynthesis and Its Limitations at Different Canopy Positions in Mature Camphor Trees

Urban forests play important roles in carbon sequestration and climate change mitigation. However, their adaptive mechanisms and limitations on photosynthesis throughout the canopy are poorly understood. This study takes the most widely distributed 50-year-old camphor plantations (Cinnamomum camphora) in Shanghai as the research objects. We investigated the variations in leaf morphology and photosynthetic physiology and biochemistry at six different canopy positions during a summer and an autumn period. We discovered that on account of leaf nitrogen loss and water deficit, light-saturated photosynthesis (Amax) declined in upper sunlit leaves despite being exposed to high sunlight in the same fashion as stomatal and mesophyll conductance (gsw, gm), photochemical quenching coefficient and actual photochemical efficiency of PSII (ΦPSII, qP), and maximum rate of electron transport and carboxylation (Jmax, Vcmax) during the growing season. Although seasonal change had little effect on Amax, the relative importance of limitations varied temporally. Mesophyll and biochemical limitation were the major contributors to the decline in the Amax in upper sunlit leaves between summer and autumn, respectively. Our study highlights the constraints of carbon fixation capacity in dense stands of mature camphor trees and offers technical support for the accurate prediction of canopy photosynthesis and the enhancement of carbon sequestration management in urban forests.