Study On The Water Utilization Mechanism Of Three Trees In Daxing’an Mountains Based On Stable Isotope Technology

As the most important limiting factor in the ecological process and function of forest ecosystem,water plays an extremely critical role in plant growth and development.Global climate change intensifies the variability of rainfall pattern,leading to an increase in the frequency and intensity of seasonal drought.Water shortage will directly change the distribution of plants and their water use pattern and pattern,which will have a serious impact on the growth adaptability and distribution pattern of plants.Understanding the plant water use patterns and patterns in forest ecosystems is key to accurately understanding the plant water use mechanisms in response to climate change.As the most sensitive region under the global climate change,the Daxing’an Mountains is increasingly subjected by water stress.However,the understanding of the water cycle process of the typical forest ecosystem and the water utilization mechanism of the greater Daxing’an Mountains is not perfect.Therefore,this study took Betula platyphylla forest,Larix gmelinii forest,and Pinus sylvestris var.mongolica forest as the research objects,and based on the measured data of soil structure,environmental factors,hydrogen and oxygen stable isotopes in various water bodies of the water cycle,as well as the carbon stable isotopes and physiological and ecological characteristics of the leaves of its constructive species in the three typical forest stands during the growing season The analytical methods such as variance decomposition model and random forest model,systematically elucidated the stable isotope characteristics of hydrogen and oxygen in atmospheric rainfall and the source of water vapor in the region,analyzed the transformation relationship between water bodies in the three stands,and quantitatively clarified the water source,intrinsic water use efficiency,water use patterns and patterns of the three tree species during the growing season,as well as the main influencing factors,To preliminarily reveal the water cycle process and water use mechanism of tree species in typical forest ecosystems in the Daxing’an Mountains,providing basic data and scientific reference for forest management and management in the context of future climate change.The main results are as follows:（1）The atmospheric water line of research area is:δD=6.70δ¹⁸O-14.09;rainfallδD is between-194.34‰and-44.90‰,δ¹⁸O is between-26.02‰and-4.81‰;Temperature effect is significant;In May and August,water vapor mainly comes from the input of near-source air mass and air mass inside the Asian continent.In September,water vapor cloud mainly comes from the Arctic Ocean water vapor transported by the northern monsoon and the western monsoon.（2）The difference betweenδD,trunk stem flow andδD were not significant,while the rainfallδD andδD and trunk stem flowδD in the Birch forest,Larch forest and Pinus forest.The difference between rainfallδD and three stands penetrating rainδD and trunk stem flow△δD will change with the rainfall,when the rainfall is 13.5mm,the difference is△δD>0;when the rainfall is<13.5mm,the difference is△δD<0,indicating that the penetrating rainδD and trunk stem flowδD are the combined effects of rainfall,evaporation,canopy characteristics,temperature and humidity.（3）Three stand soil water mainly comes from the supply of atmospheric rainfall,and in the process of water migration by evaporation and isotopic fractionation,three stand soil water fractionation degree of Pinus forest>larch forest>birch forest,three stand layer soil moistureδD andδ¹⁸O gradually decrease with the increase of soil depth.In light rain（0mm<rainfall<10mm）,rainfall infiltration is characterized by top-down piston infiltration.In moderate rain（10mm<rainfall<25mm）and heavy rain（≥25mm rainfall）,rainfall quickly penetrates into the deep soil in the form of preferential flow to recharge the deeper soil.（4）The photosynthetic characteristics of the three species in the growing season have obvious differences between months and species.The net photosynthetic rates and transpiration rates of the three tree leaves during the growing season ranged from7.59±1.36～2.17±0.42μmol CO₂m^-2s^-1and 2.49±0.20～0.31±0.11mmol H₂Om^-2s^-1,the larch net photosynthetic rates and transpiration rates were significantly higher than birch and pine,but birch and pine not significantly different.There were inter species differences between leaf anatomy and stomatal properties in the three tree species,with specific leaf weight,leaf thickness,epidermal thickness and stomatal density had significant effects on the net photosynthetic rate and transpiration rate of the respective leaves.Three tree species fluid circulation has obvious dynamic change characteristics and different months and tree species,the average fluid circulation for camphor pine（21.46±3.36kg/d）>birch（15.90±5.28kg/d）>utilize modern larch（4.36±1.34kg/d）,forest air temperature and humidity,photosynthesis effective radiation,net photosynthesis rate and transpiration rate have a significant impact on liquid circulation.There are obvious differences in the utilization levels of soil water among the three tree species in different months of the growing season.In May and September,larch and birch mainly use soil water in the upper layer（0-10 cm）,while in June and August,the two species changed to mainly use soil water in the lower layer（20-40 cm）.In May and August,pine mainly uses the soil water in the lower layer（20-40cm）,while in September,it mainly uses the soil water in the upper layer（0-10cm）.There are significant differences in the utilization rate of soil water among the three tree species in the growing season,and the main influencing factor of the difference is the tree species attribute.The water sources of the three tree species are the same with different levels of rainfall.After moderate rain and heavy rain,the three tree species mainly use the lower layer（20-40cm）of soil water,and after light rain,the three tree species mainly use the upper layer（0-10cm）of soil water,indicating that the water use patterns of the three tree species will change due to different levels of rainfall.（5）The internal water use efficiency（iWUE）of three tree species varies significantly in different months of the growing season,and there are significant differences among tree species.The average iWUE of the three tree species during the study period was larch(3.84±0.13mmol·mol^-1)>birch(3.39±0.22mmol·mol^-1)>pine(3.04±0.08mmol·mol^-1);The environmental factors,photosynthetic parameters and leaf structure of the stands where the three tree species are located will have a significant impact on the iWUE of the leaves of the three tree species.The environmental factors play a leading role in the change of their iWUE,and photosynthetic parameters play a secondary role,leaf structure is relatively small.During the growing season,the iWUE among different canopies of the three tree species showed a significant difference among different canopies.This indicates that it is necessary to distinguish the crown position when studying the iWUE of typical tree species in the Daxing’an Mountains.（6）After parameter calibration and validation,the NASH value of the BP neural network model is greater than 0.9,and the RMSE and MAE values are both less than 0.05,which can meet the accuracy requirements for model use.The model was used to predict and simulate the leaf iWUE of typical tree species in the Daxing’an Mountains under three future climate change scenarios,and the results showed that the leaf iWUE of typical tree species showed a gradual upward trend under the three climate change scenarios.The growth rates of iWUE in leaves of birch,larch and pine were S1（12.2%,6.02%,4.02%）>S2（4.78%,3.21%,1.89%）>S3（2.28%,1.26%,0.62%）,respectively.Under the background of future atmospheric CO₂concentration increase,temperature increase,and precipitation decrease,the iWUE of leaves of larch is higher than that of birch and pine.In conclusion,the three species absorbed and utilized soil water in different periods of the growth season and at different rainfall levels,indicating that the three species showed positive adaptability to the changes of water and environment changes.Due to global climate change in the Daxing’an Mountains,forest ecosystems will cause plants to adjust their potential water sources and growth conditions due to changes in soil moisture.At the same time,due to the physiological and ecological differences of the three trees,the water consumption of larch was significantly lower than that of birch and pine,but iWUE was significantly higher than that of birch and pine.Finally,there are some differences in the water utilization mechanism of the three tree species.Due to this difference,the water loss of soil in the future,and this potential threat may be particularly serious for birch and pine.From the perspective of water use,larch has greater ecological adaptability than white birch and pine,which may be more susceptible to water stress.