Long-term Water-carbon Coupling Mechanism Of Trees Based On Tree-ring Stable Carbon Isotopes

Water and carbon cycle in forest ecosystem is the core of material and energy cycle.The study of coupling mechanism between water and carbon is the basis of realizing integrated management of water and carbon resources.Because of the shortcomings of traditional research methods in the research process,previous studies focused on the instantaneous water-carbon coupling relationship at leaf scale.The research about long-term water-carbon coupling of trees is still in the stage of development,which has become a difficult problem in the quantitative study of hydrology and ecology.Based on stable isotope technique and tree chronology principle,this study took Platycladus orientalis,Pinus tabulaeformis,Quercus variabilis,Robinia pseudoacacia and Populus simonii as the research objects,estimated annual photosynthesis-stomatal conductance coupling value(iWUE)by measuring stable carbon isotope(?13C)of tree rings,grasped its variation characteristics and response to climate change.Taking Platycladus orientalis as the research object,the carbon sequestration and transpiration at different time scales were estimated by iWUE,and the method was validated by the measured values.At the same time,the response,prediction and promotion strategies of tree photosynthesis-stomatal conductance coupling under climate change were analyzed.The main results are as follows(1)Tree ring cellulose ?13C of high density,older,steep and sunny stand was higher than that of low density,younger,slow and shady slope stand.The order of the average value of cellulose ?13C in different types of stands was Platycladus orientalis>Pinus tabulaeformis>Quercus variabilis>Robinia pseudoacacia.The annual mean iWUE of the four tree species showed a gradual upward trend,and the iWUE of coniferous tree species was significantly larger than that of broad-leaved tree species.There is a weak negative correlation between iWUE and annual precipitation,no significant correlation between iWUE and annual average RH,a very significant positive correlation between iWUE and annual average temperature,and a very significant positive correlation between iWUE and atmospheric CO2 concentration.(2)With the deepening of degradation degree,the ring cellulose ?13C gradually increased.The iWUE of normal growing and degraded poplar both showed a very significant upward trend,and the iWUE of degraded Populus simonii was higher than that of normal growing Populus simonii.There were significant diffeerences in ?13C,?,Ci and iWUE of ring cellulose of Populus simonii with different degraded degrees.Precipitation and RH were not the main factors affecting the degradation of Populus simonii:iWUE of Populus simonii with normal growth,mild degradation and severe degradation increased with the increase of annual temperature,and there was a significant positive correlation between iWUE and annual average temperature.With the increase of environmental resistance,however,this coupling relationship might appear "decoupling" phenomenon with the increase of environmental resistance,and lead to degradation or death.(3)The long-term annual carbon sequestration and transpiration of Platycladus orientalis were estimated by the water-carbon coupling model.From 1880 to 2016,the annual carbon sequestration of Platycladus orientalis showed a very significant quadratic growth trend,and the slope increased continuously.The annual transpiration showed a very significant quadratic growth trend,and the slope gradually decreased.The long-term water-use efficiency of Platycladus orientalis increased significantly.Under the background of climate change and significant increase of atmospheric CO2,the water-carbon coupling relationship of Platycladus orientalis changed and water-use efficiency increased gradually.(4)From 2010 to 2100,the predicted iWUE of Platycladus orientalis,Pinus tabulaeformis,Robinia pseudoacacia and Quercus variabilis increased gradually.The average iWUE ranked as Platycladus orientalis>Pinus tabulaeformis>Quercus variabilis>Robinia pseudoacacia.The age of normal growing Platycladus orientalis,Pinus tabulaeformis,Robinia pseudoacacia and Quercus variabilis were 220,200,134 and 150 years respectively when they reached the highest value of iWUE.The highest value of iWUE ranked as Platycladus orientalis>Pinus tabulaeformis>Quercus variabilis>Robinia pseudoacacia.iWUE has conservativeness and variability.