Vertical Variation And Driving Mechanism Of Leaf Photosynthetic Traits In Subtropical Forest Canopy

Forest ecosystems are the interface between the atmosphere and the land surface and have a positive effect on global change issues such as CO₂ increase,climate warming,atmospheric nitrogen deposition,and biodiversity reduction.The subtropical forests in the East Asian monsoon region are affected by the East Asian monsoon and have better moisture and thermal conditions,which provide better conditions for forest growth and carbon fixation.It is important to understand the biochemical process of the subtropical forest system in China for the variation of carbon source and sink and accurately estimate the carbon budget of the terrestrial ecosystem.The canopy is the place for the photosynthesis of the forest.However,the transmission and distribution of energy changes within the canopy,and the photosynthetic traits of leaves vary along the vertical gradient of the canopy.At present,there are few studies on the variation of leaf photosynthetic traits along the vertical gradient of the canopy,and the response mechanism of leaves on the canopy gradient to environmental factors is still unclear.Therefore,it is important to understand the photosynthetic plasticity of leaves,which can accurately simulate plant productivity at a canopy scale.In this study,slash pine（Pinus elliottii Engelmann）and schima（Schima superba Gardn.&Champ）were taken as the research objects.Photosynthetic traits such as the CO₂ response of leaves,stomatal conductance（g_s）,net photosynthetic rate（A_n）and specific leaf weight（LMA）,and environmental factors such as photoelectron flux density（PPFD）,leaf temperature(T_leaf),saturated water vapor pressure deficit（VPD）was observed at the top,middle and bottom layers of the canopy in the middle of every month in 2019.At the same time,the leaf nitrogen content per unit area(N_area)and the leaf chlorophyll content per unit area(Chl_area)were measured.The temporal and spatial variation of environmental factors and leaf photosynthetic traits along the vertical gradient of subtropical forest canopy were analyzed,and the regulation mechanism of environmental factors to leaf photosynthetic traits was discussed.In addition,the temporal and spatial variation of water use efficiency（WUE_T）,nitrogen use efficiency（NUE）,and light use efficiency（LUE）along the vertical gradient of the canopy were analyzed,and the relationship between environmental factors with leaf resource utilization efficiency was established.The relationship between Chl_areaand N_area with the maximum carboxylation rate(V_cmax)in subtropical evergreen coniferous forests and broad forests were analyzed,and the relationship between V_cmax with Chl_area on different tree species and different vertical gradients of the canopy was compared.The main research conclusions are as follows:（1）There are significant effects of seasonal on environmental factors and leaf photosynthetic traits in the canopy of slash pine and schima.V_cmax,J_max,and A_n increase with the canopy height.N_area and Chl_area have similar spatial distribution patterns.PPFD is the main limiting factor for photosynthesis in the subtropical forest canopy,which affects the A_nby regulating the V_cmax.（2）Our study found that WUE_T and LUE were negatively correlated with PPFD,T_leaf,and VPD in the canopy of slash pine and schima.NUE and PPFD were significantly positively correlated at the top layer of the slash pine canopy,which was weakened in the bottom and middle layers.The relationship between NUE and PPFD was significantly positively correlated along the canopy vertical gradient of schima.（3）Our results showed that there was a significant positive correlation between V_cmaxand Chl_area in the canopy of subtropical evergreen coniferous tree species（slash pine）,while there is no significant correlation in the canopy of evergreen broad-leaved forest（schima）.This is related to the investment strategy of N in the photosynthetic component of the two tree species.In addition,the dynamic variation of N allocation in light-harvesting pigment components regulates the relationship between V_cmax and Chl_area.