Effects of Stand Density and Pruning on Canopy Photosynthetic Characteristics in Urban Cinnamomum camphora Plantations

In urban green spaces, stand density and pruning jointly modulate canopy structure, plant resource allocation, and leaf physiological traits. However, their interactive effects on the spatial heterogeneity of canopy photosynthetic characteristics remain poorly understood, which hinders the development of low-carbon, high-efficiency management strategies for urban forests. In this study, 50-year-old camphor tree (Cinnamomum camphora (L.) Presl) plantations with three density gradients were investigated to disentangle the interactive effects between stand density and pruning. The results showed that stand density dominated the resource competition regime within the canopy. In low-density stands, mean canopy leaf carbon (C), nitrogen (N), chlorophyll concentration (Chl), maximum net photosynthetic rate (Amax), stomatal conductance (gsw), mesophyll conductance (gm), actual photochemical efficiency of PSII (ΦPSII), maximum Rubisco carboxylation rate (Vcmax), and maximum electron transport rate for RuBP regeneration (Jmax), were significantly higher than those in medium- and high-density stands. Most photosynthetic traits increased logarithmically with increasing light intensity in low-density stand, whereas they followed parabolic trends in medium- and high-density stands with depressed values observed in upper sunlit leaves. Pruning triggered a physiological compensatory response in remaining leaves, significantly enhancing canopy C, N, Chl, and photosynthetic parameters such as Amax, gsw, gm, and ΦPSII. Notably, the increments in canopy-averaged N, Chl, ΦPSII and Vcmax/Jmax following pruning were substantially greater in low-density stand than in medium- and high-density stands. Pruning predominantly regulated the spatial distribution of canopy photosynthesis. As stand density increased, pruning compensated for the insufficient photosynthetic capacity in the lower canopy, and decreased photosynthetic heterogeneity across canopy positions, but it failed to achieve targeted enhancements of whole-canopy photosynthetic potential. Concurrently, increasing stand density gradually shifted photosynthetic limitation from mesophyll to stomatal, and pruning further exacerbated stomatal limitation in the upper sunlit leaves of high-density stand. Consequently, high stand density weakened the responsiveness of photosynthetic traits to N. In addition, pruning significantly reduced nitrogen use efficiencies for Amax, gsw, gm, and Jmax, due to N redundancy in the low-density stand. This study advances the mechanistic understanding of how urban forest management optimizes within-canopy photosynthetic resource allocation, and provides scientific support for enhancing the carbon sink function of urban green spaces.