Carbon And Water Exchange Process And Mechanism Of Larix Gmelinii Ecosystem In Cold Temperate Zone

Reducing and limiting carbon emissions is the strategic demand for mitigating global climate change.Forest carbon sequestration is an economical and efficient way to achieve the goal of carbon neutralization.As the most active and widely influential factor in the forest,water directly affects the productivity of the forest ecosystem.A comprehensive and systematic study on the carbon and water exchange process and mechanism of forest ecosystem can be of great significance for the scientific evaluation of carbon sink capacity of forest ecosystem,the sustainable management of forest ecosystem and the mitigation of global climate change risk.In order to better understand the key process and mechanism of carbon and water cycle in cold temperate forest ecosystem,this study took the Larix gmelinii primitive forest ecosystem as the research object in the experimental area of National Field Scientific Observation and Research Station of the Greater Khingan Forest Ecosystem in Inner Mongolia,based on stable isotope technology,long-term positioning monitoring,field investigation and sampling,indoor sample determination and model construction were adopted.The temporal and spatial dynamics of carbon and water exchange in different phenological periods of the ecosystem during the growth season were studied,the relative contributions of various ecological processes to carbon release and water dissipation during carbon and water exchange in the ecosystem were quantified,the action mechanism of environmental factors on various ecological processes was discussed,and the relationship between carbon sequestration capacity and water consumption cost of Larix gmelinii was evaluated,and the carbon and water coupling mechanism model of leaf and canopy scale was established.The main results were as follows:（1）The CO₂concentration at different heights of Larix gmelinii ecosystem showed a single peak trend on the daily scale and the growing season scale.The maximum values appeared at 02:00(782.81μmol CO₂·mol^-1)and leaf spreading period(522.34μmol CO₂·mol^-1)respectively,and the minimum appeared at 11:00(379.72μmol CO₂·mol^-1)and leaf opening period(406.07±10.79μmol CO₂·mol^-1)respectively.The CO₂concentration and itsδ¹³C at different heights had obvious stratification phenomenon.The increase of atmospheric stability at night increased the difference of CO₂concentration and itsδ¹³C inside and outside the canopy.On the scale of growing season,there was a very significant negative correlation between CO₂concentration andδ¹³C in the ecosystem（P<0.001）.Theδ¹³C values at different heights decreased with the increase of CO₂concentration and increased with the increase of vertical height.The average explanation of environmental factors for the change of CO₂concentration at different height in different phenological periods could reach 78.1%,which has reached a very significant level（P<0.001）.The influence of temperature on CO₂concentration at each height was better than that of humidity,and the influence of meteorological factors on CO₂concentration at each height was better than that of soil factors.（2）The net carbon flux of Larix gmelinii ecosystem(Fδ¹³C_NEE)showed a single peak trend on the daily scale and the growing season scale.The maximum values appeared at10:00(156.50±105.12μmol CO₂·m^-2·s^-1·‰)and leaf spreading period(37.11±157.51μmol CO₂·m^-2·s^-1·‰)respectively.The main environmental factor determining the change of Fδ¹³C_NEEin the growing season was solar radiation（SR）（PC=0.565）.In the bud opening and leaf falling periods,the Larix gmelinii ecosystemin showed carbon source(-32.26--21.01μmol CO₂·m^-2·s^-1·‰),and in the leaf spreading and discoloring periods,the ecosystem showed canbon sink(32.31-255.36μmol CO₂·m^-2·s^-1·‰).The carbon assimilation flux of Larix gmelinii ecosystem(Fδ¹³C_A)showed a single peak trend on the daily scale and the growing season scale.The maximum values appeared at 12:00(265.13±92.26μmol CO₂·m^-2·s^-1·‰)and leaf spreading period(403.82μmol CO₂·m^-2·s^-1·‰)respectively.The main environmental factor determining the change of Fδ¹³C_Ain the growing season was SR（PC=1.163）.（3）The autotrophic respiratory flux of Larix gmelinii ecosystem(Fδ¹³C_Ra)showed a bimodal trend on a daily scale,the maximum values appeared at 02:00(66.79±43.24μmol CO₂·m^-2·s^-1·‰)and 14:00(47.91±27.73μmol CO₂·m^-2·s^-1·‰)respectively.Fδ¹³C_Rashowed a single peak trend on the scale of growing season,and the maximum value appeared in the leaf spreading period(79.09μmol CO₂·m^-2·s^-1·‰).The main environmental factor determining the change of Fδ¹³C_Rain growing season was soil 10 cm temperature（Ts10）（PC=0.944）.The heterotrophic respiration flux of Larix gmelinii ecosystem(Fδ¹³C_Rh)showed a single peak trend on the daily scale and the growing season scale.The maximum values appeared at 14:00(39.50±21.50μmol CO₂·m^-2·s^-1·‰)and leaf spreading period(69.53±4.48μmol CO₂·m^-2·s^-1·‰)respectively.The main environmental factor determining the change of Fδ¹³C_Rhin the growing season was Ts10（PC=0.819）.Autotrophic respiration was the main way of carbon release in Larix gmelinii ecosystem,and its contribution rate could reach 51.18%.In different stand types of the ecosystem,the contribution rate of autotrophic respiration of shrub-Larix gmelinii forest（52.30%）was greater than that of herb-Larix gmelinii forest（52.19%）and moss-Larix gmelinii forest（52.01%）.（4）The H₂O vapor concentration at different heights of Larix gmelinii ecosystem showed a single valley trend on a daily scale,and the minimum value appeared in the afternoon(0.49±0.27 mmol H₂O·mol^-1).And it showed a single peak trend on the scale of growing season,and the maximum appeared in the leaf spreading period(28.80±19.43mmol H₂O·mol^-1).The difference between H₂O vapor concentration and itsδ¹⁸O at different heights was small.The enhancement of solar radiation and the increase of temperature increased the difference between H₂O vapor concentration and itsδ¹⁸O at different heights.On the scale of growing season,there was a very significant positive correlation between H₂O vapor concentration andδ¹⁸O in the ecosystem（P<0.001）.Theδ¹⁸O value at different heights increased with the increase of H₂O vapor concentration and decreased with the increase of vertical height.In the bud opening and leaf spreading periods,the change of H₂O vapor concentration at each height was mainly affected by soil water content,and in the leaf discoloring and falling periods,the change of H₂O vapor concentration at each height was mainly affected by temperature.（5）The evapotranspiration flux of Larix gmelinii ecosystem(Fδ¹⁸O_ET)showed a single peak trend on the daily scale and the growing season scale.The maximum values appeared at 12:00(30.47±17.86 mmol H₂O·m^-2·s^-1·‰)and leaf spreading period(22.55±15.05 mmol H₂O·m^-2·s^-1·‰)respectively.The main environmental factor determining the change of Fδ¹⁸O_ETin the growing season was SR（PC=0.662）.The soil evaporation flux of Larix gmelinii ecosystem(Fδ¹⁸O_E)showed a single peak trend on the daily scale and the growing season scale.The maximum values appeared at 12:00(9.77±4.65 mmol H₂O·m^-2·s^-1·‰)and leaf spreading period(5.93±5.19 mmol H₂O·m^-2·s^-1·‰)respectively.The main environmental factor determining the change of Fδ¹⁸O_Ein the growing season was Ts10（PC=0.572）.The vegetation transpiration flux of Larix gmelinii ecosystem(Fδ¹⁸O_T)showed a single peak trend on the daily scale and the growing season scale.The maximum values appeared at 12:00(25.88±7.72 mmol H₂O·m^-2·s^-1·‰)and leaf spreading period(7.94±16.62 mmol H₂O·m^-2·s^-1·‰)respectively.The main environmental factor determining the change of Fδ¹⁸O_Tin the growing season was SR（PC=0.969）.Vegetation transpiration was the main way of water output of Larix gmelinii ecosystem,and its contribution rate could reach 76.05%.In different stand types of the ecosystem,the contribution rate of transpiration of shrub-Larix gmelinii forest（77.72%）was greater than that of herb-Larix gmelinii forest（76.65%）and moss-Larix gmelinii forest（73.74%）.（6）The leaf scale water use efficiency of Larix gmelinii decreased with the decrease of canopy height and light intensity.The leaf scale water use efficiency(5.46μmol CO₂·mmol H₂O^-1)was larger than the canopy scale(3.74μmol CO₂·mmol H₂O^-1)and the ecosystem scale(1.97μmol CO₂·mmol H₂O^-1),the water use efficiency（WUE）decreased with the expansion of scale.The average interpretation of environmental factors for WUE changes at different scales in different phenological periods was 87.2%,which reached a very significant level（P<0.001）.SR was the main environmental factor determining the change of WUE at leaf scale（PC=0.634）,canopy scale（PC=0.553）and ecosystem scale（PC=0.679）.The influence of meteorological factors on WUE at each scale was better than that of soil factors.（7）The canopy carbon and water coupling mechanism model of Larix gmelinii ecosystem could simulate the dynamic changes of canopy average transpiration rate,canopy average stomatal conductance and canopy average carbon assimilation rate,the fitting degree between simulated value and measured value was R²=0.876（P<0.001）.The average canopy carbon assimilation rate simulated by carbon assimilation process model and the average canopy transpiration rate simulated by transpiration process model showed a very significant positive correlation（P<0.001）,and the fitting degree was R²=0.864.The results showed that the canopy carbon and water coupling mechanism model of Larix gmelinii ecosystem based on stable isotopes had high accuracy,accurate simulation results and high application value.