Response Mechanism Of Tree Diameter At Breast Height Variation To Hydro-thermal Factors In Larch

Tree diameter at breast height(DBH)is the most common index adopted for monitoring tree growth and DBH research at high time resolution conditions can promote the understanding on ecological and physiological changes of trees.Healthy growth of trees requires appropriate hydro-thermal conditions.If the relevance between stem diameter variation and soil-plant-atmosphere continuum(SPAC)hydro-thermal factors is confirmed,it can become easier to obtain the data of stem diameter variation indexes to decide whether tree growth environment and self hydro-thermal conditions can satisfy the physiological needs of trees.This can support forest network long-term fixed-spot observation and forest sustainable operation and management.This research focused on Larch Larix gmelinii Kuzen.widely distributed in Northeast China to analyze the response mechanism of tree DBH to hydro-thermal factors at different SPAC levels and different time windows.With the help of fixed observation instrument and data collectors,experimental data was obtained from portable devices at different time windows.By obtaining continuous observation data of stem diameter variation of Larch and matched data of SPAC hydro-thermal factors from 2015 to 2018,and applying statistical approaches including descriptive statistics,correlation analysis and significance test,curve fitting method and significance test,structural equation modeling(SEM)and mediating effect testing,the research has analyzed the response mechanism of stem diameter variation indexes to hydro-thermal factors of trees and the environment.Stem diameter variation indexes used in this research mainly included tree water deficit-induced stem shrinkage(TWD),maximum daily shrinkage(MDS),daily stem diameter increment(DSDI),daily stem diameter variation(DSDV),and stem diameter variation rate(SDVR).(1)The responses of TWD based on the zero-growth assumption or the linear growth assumption to vapor pressure deficit(VPD)were not significant under the observation time window of 7 d.For observation time windows of 0.5 h,1 d and 3 d,all TWD responses to VPD based on zero-growth assumption were better than those based on linear growth assumption.For time windows of 3 d and 7 d,all TWD values obtained based on both assumptions made poor responses to soil water potential,the dominant factor of stem diameter variation.For time windows of 0.5 h and 1 d,all TWD responses to soil water potential based on zero-growth assumption were better than those based on linear growth assumption.Under zero-growth assumption,advanced TWD calculation could promote the research accuracy to a certain point.TWD fell behind the variation of environmental factors and its response to environmental hydro-thermal factors showed a physiological delay.Such delay showed asynchronism during the research of different driver factors so the time-lag effect of TWD to dominant factors should be considered,and observation at a fixed time window would bring down both the precision and the accuracy of the research.(2)Under different time windows,SDVR difference among sample trees was small.When MDS reached relatively high values under different time windows,its difference among sample trees became significant.But such individual difference on DBH variations can be alleviated by adopting different diameter variation indexes under different time windows.SDVR is a more preferable choice for reducing the individual difference on DBH variations under different time windows.When MDS is adopted,such individual difference at peak MDS values should be noticed.(3)Under the time windows of 1 d,3 d and 7 d,the mean values,variable coefficients and variation features of 5 diameter variation indexes were all different.TWD varied the most,the variation of MDS was slightly smaller than that of TWD and SDVR varied the least.Correlation between TWD and MDS became stronger when time windows increased and such significant negative correlation stayed the same for all time windows.DSDI and DSDV showed similar variation features and their correlation was the most significant for all time windows.Different stem diameter variation indexes focus on different aspects of stem diameter variation.But there is correlation among them under certain conditions.Comprehensive analyze of different stem diameter variation indexes should consider such correlation and the combined use of TWD and MDS under long time windows may cause influence to the results.The combined use of DSDI and DSDV may also affect the precision and accuracy of the results.(4)The mean values and variable coefficients of TWD,DSDI,DSDV and SDVR showed great difference under different temperature conditions.Those of MDS were alike under the same conditions.Changes of five stem diameter variation indexes under appropriate temperature were more significant than those under low temperature.The correlation among stem diameter variation indexes was also weaker under low temperature and such temperature conditions can help to avoid various interactions among these indexes.If taking no account of different variation features of stem diameter variation under different temperature,MDS is a more preferable index for interpreting the variation of stem diameter and minimizing the influence of temperature conditions.(5)Sap flow density(Qs)correlated significantly with all stem diameter variation indexes on appropriate temperature sunny days.All stem diameter variation indexes can help explain some characteristics of Qs during appropriate temperature sunny days.TWD was correlated with daily changes in Qs and reached significant levels under all air temperature and cloudiness conditions.Only TWD responded significantly to changes in Qs on cloudy days.Among all the indexes,TWD showed the highest correlation with Qs under all conditions.MDS,DSDI and DSDV were significantly correlated with Qs on appropriate temperature sunny days,and SDVR was significantly correlated with Qs on sunny days.In sunny and cloudy days,MDS,DSDI,DSDV and SDVR could be adopted to partly analyze Qs changes under appropriate temperature conditions,but their significance became different under other conditions.Since TWD eliminates the influence of seasonal growth on stem diameter variation and retains the reversible shrinkage.caused by water loss,TWD can better explain Qs changes when compared to other stem diameter variation indexes under all air temperature and cloudiness conditions.Although TWD worked better on Qs changes than other indexes,they can still be adopted in sunny and cloudy days under appropriate temperature conditions depending on the purpose and emphasis of the research.(6)Under 1 d time window,SPAC hydro-thermal factors(precipitation,stem surface runoff,net radiation,Qs,soil water content and soil heat flux)under appropriate temperature conditions can explain 83%stem diameter variation.Under 1d time window,SPAC hydro-thermal factors(VPD,air temperature and soil heat flux)under low temperature conditions can explain 18%stem diameter variation.Under 3 d time window,SPAC hydro-thermal factors(precipitation,stem surface runoff,VPD,Qs,soil temperature and soil heat flux)can explain 86%stem diameter variation.Under 7 d time window,SPAC hydro-thermal factors(precipitation,VPD,wind speed,Qs,stem temperature and soil water content)can explain 76%stem diameter variation.Over-high time resolution can cause bad response of stem diameter variation to SPAC hydro-thermal factors and hydro-thermal factor changes cannot explain stem diameter variation clearly.Classification of different temperature conditions can promote accuracy of the research.TWD can better explain the response mechanism of tree growth to SPAC hydro-thermal factors but attention should be paid not to over interpret under long time windows.MDS is a more traditional and most commonly adopted stem diameter variation index,which can work better under long time windows and interpret the response of tree growth to SPAC hydro-thermal factor changes.(7)In the 1 d time window,the modified SEM under cold condition only involved two dimensions,and the distinction of mediating effect did not exist.The modified SEM did not have indirect effect under 1 d time window appropriate temperature condition,3 d time window and 7 d time window.There was no mediating effect between stem diameter variation and the SEM of SPAC hydro-thermal factors.SPAC hydro-thermal factor at atmosphere dimension has direct effect on stem diameter variation,but it cannot exert indirect effect to stem diameter variation through tree hydro-thermal factor and soil hydro-thermal factor.During the research of stem diameter variation responding to SPAC hydro-thermal factors,although individual changes and interactions of hydro-thermal factors at different levels could affect some factors of the SPAC system,such interaction did not cause intervention among hydro-thermal factors at different levels during the response process.Therefore,multiple hydro-thermal factors can be introduced to the research without worrying about various complex indirect response mechanisms due to interactions among factors.