Regulation Of Leaf Functional Traits On Carbon Metabolism-temperature Response At Canopy Scale In A Subtropical Evergreen Broad-leaved Forest

Global climate change has an important impact on the structure and function of terrestrial forest ecosystems.The current ambient temperature has approached or exceeded the threshold of plant growth temperature in tropical and subtropical regions.To cope with climate change,plants may decouple the correlation between leaf temperature and ambient temperature to promote the adaptation of plant metabolic rate to high temperature and change plant functional traits.At present,the decoupling mechanism between leaf temperature and ambient temperature of subtropical plants is not clear.The lack of measured data of plant metabolism-temperature response characteristics have caused great uncertainty in accurately predicting the impact of future environmental changes on the carbon cycle of subtropical forest ecosystems.In addition,most of plant functional traits and carbon metabolism-temperature response characteristics of the research mainly focuses on the growing season sun leaf at upper canopy position,and ignore the function of leaf traits with different growth period and canopy position change of the environment to adapt to,leading to plant functional traits has obvious seasonal variation and crown.This study measured leaf temperature and carbon metabolism-temperature response curves and the leaf functional traits of 75 woody species in winter（evergreen tree species）and the summer growing season（evergreen and deciduous species）in a subtropical evergreen broad-leaved forest in Yangjifeng National Nature Reserve,Jiangxi Province.We analyzed whether different seasonal leaf temperatures and environmental temperature decoupling and its influencing factors,quantified leaf functional traits and carbon metabolism-temperature response characteristics.（1）The reference leaf temperature（T_r）,leaf temperature（T_l）,and non-transpiration leaf temperature（T_n）of evergreen species in summer were significantly higher than those of deciduous species（P<0.05）.Shading significantly decreased T_r,T_l,and T_n of lower shade leaves compared with upper shade leaves（P<0.05）,and T_r of both sunny and shade leaves was significantly higher than that of T_n and T_l（P<0.05）,while T_l was significantly lower than T_n（P<0.05）.There were no significant differences in T_r,T_l,T_n,and three temperatures at different canopy positions in winter（P<0.05）.T_r,T_l and T_n all showed a single peak trend with time,but the peak appeared at 13:00 in summer and14:00 in winter.At the species level,T_n,T_l,and T_r were significantly positively correlated（P<0.05）.Shading significantly reduced the contribution of transpiration to lower canopy leaf temperature regulation but increased the contribution of traits（P<0.05）.In addition,the contribution of transpiration to leaf temperature regulation was significantly higher than that of traits in different seasons and canopy positions（P<0.05）.（2）Shading significantly reduced the photosynthetic rate(A_Opt,area)at optimum temperature and(A_25,area)at 25℃（P<0.05）,but did not change the optimum photosynthetic temperature(T_Opt)of the lower shade leaves（P>0.05）compared with the upper shade leaves（P>0.05）.The photosynthetic range(T₉₀)and Q₁₀ of shade leaves were significantly higher than that of sun leaves in summer,but opposite in winter（P<0.05）.The T_Opt,A_Opt,area and A_25,area of leaves in winter were significantly lower than those in summer（P<0.05）,but T₉₀ and Q₁₀ were higher than those in summer（P<0.05）.There was a significant allometric relationship between leaf T_Opt and A_Opt,area,A_25,area,with common slopes of 1.49（95%CI=1.29-1.72,P=0.59）and 1.63（95%CI=1.42-1.87,P=0.36）.The allometric relationship between A_Opt,area and A_25,area is>1,and the common slope is 1.09（95%CI=1.05-1.13,P=0.17）.However,T_Opt and Q₁₀ showed a significant allometric relationship only in the lower canopy leaves.（3）Canopy position and season had significant effects on V_Cmax,J_max and V_Cmax/Jmax of leaves（P<0.05）,but no significant effects on leaf habit.Compared with sun leaves,shading significantly reduced the maximum electron carboxylation rate(V_Cmax),maximum electron transfer rate(J_max)and V_Cmax/J_max of shade leaves（P<0.05）.In summer,V_Cmax at 20℃has no significant difference with that at 25℃,but it is significantly higher than other temperatures,while J_max at different temperatures has no significant difference.In winter,V_Cmax of sun leaves at 10℃was significantly higher than that at 20℃and 25℃,but J_max of sun leaves at different temperatures had no significant difference.V_Cmax of shade leaves had no significant difference at a different temperature,but J_max was the highest at 5℃and the lowest at 20℃.In addition,V_Cmax/J_max decreased with the increase of temperature and was significantly higher in winter than in summer.V_Cmax and J_max of both sunny and shade leaves had a common slope and common intercept at different temperatures,and all slopes were not significantly different from 1.0(P_1.0>0.05).There was a common slope of 0.79（95%CI=0.75,0.83,P=0.89）between V_Cmax and J_max of evergreen and deciduous species at different canopy positions,but the intercept of upper deciduous species（0.60）was higher than that of lower evergreen species（0.53）.（4）The PC1 axis and PC2 axis explained 46.7%and 28.8%of leaf temperature parameters-LES variation,respectively.The contribution of transpiration to leaf temperature regulation in winter was significantly positively correlated with specific leaf weight（LMA）in the upper layer,and significantly negatively correlated with nitrogen（N）and phosphorus（P）contents.The contribution of transpiration to leaf temperature regulation in summer was significantly negatively correlated with LMA,significantly positively correlated with N and P contents,A_mass,and mass-based respiration rate(R_mass),while the contribution of traits to leaf temperature regulation was opposite（P<0.05）.The PC1 axis and PC2 axis explained 37.8%and 17.5%of the variation of LES leaf traits in response to carbon metabolism temperature,respectively.Only in the upper layer of winter,N content was significantly positively correlated with Q₁₀ in the lower layer of winter,and N and P content were significantly positively correlated with A_25,area and A_Opt,area in summer（P<0.05）.PC1 axis and PC2 axis explained 49.9%and 22.2%of the variation of photosynthesis-CO₂ response parameters-leaf traits,respectively:P content was significantly negatively correlated with V_Cmax,25,V_Cmax,25,A_mass,and positively correlated with N content in the upper layer in summer.J_max,25 was significantly positively correlated with N content in summer,and A_mass was also significantly positively correlated with N content in upper summer（P<0.05）,while V_Cmax,25/J_max,25was positively correlated with LMA in lower summer,and negatively correlated with N and P content.In addition,transpiration contribution in summer was significantly positively correlated with photosynthetic-light response parameters(G_mass,LCP,LSP)and leaf drip tip（LDT）,while trait contribution was opposite（P<0.05）.V_Cmax,25 and J_max,25 were significantly positively correlated with photosynthesis-light response parameters(G_mass,LCP,LSP)（P<0.05）.In conclusion,in the subtropical region,the decoupling between leaf temperature and environment temperature of woody plants only existed in the shade leaves in summer and the shade leaves in winter.Sun leaves and shade leaves were more affected by transpiration cooling.The V_Cmax and J_maxdecreased at high temperature,indicating that the photosynthetic rate was limited by biochemistry at high temperature.In addition,leaf functional traits were covarying with photosynthesis-CO₂ response traits,but not with photosynthesis-temperature response traits.In future studies,more data are needed to empirically and theoretically test the coupling tradeoff between nutrients and water resources on multiple organs and the spatio-temporal variation,so as to provide theoretical basis for predicting the adaptation of subtropical forest ecosystems to global change.In future studies,more data are needed to empirically and theoretically test the coupling tradeoff between nutrients and water resources on multiple organs and the spatio-temporal variation,to provide the theoretical basis for predicting the adaptation of subtropical forest ecosystems to global change.