Study On Carbon And Water Flux Of Coniferous And Broad-Leaved Mixed Forest In Jinyun Mountain,Chongqing

Jinyun Mountain is located in the Chongqing area of the Three Gorges Reservoir area of the Yangtze River,with humid climate,abundant water resources,rich vegetation species and sound ecosystem.It is an important water conservation forest area in the middle and upper reaches of the Yangtze River in China.The long-term observation and study of carbon and water flux in Jinyun Mountain Nature Reserve,and the exploration of the change law of carbon and water flux in this area and its response to environmental factors will help to deeply understand the internal mechanism of carbon and water cycle in Jinyun Mountain forest ecosystem,and provide data support and theoretical guidance for the protection and management of forest ecosystem at the tail end of the Three Gorges Reservoir area.Based on the 5-year carbon and water flux observation data of the coniferous and broad-leaved mixed forest ecosystem in Jinyun Mountain,this study evaluated the applicability of the eddy covariance observation method in Jinyun Mountain from different angles,analyzed the change characteristics of carbon and water flux in different time scales of the coniferous and broad-leaved mixed forest in Jinyun Mountain,and explored the reasons for the differences in the change characteristics of carbon and water flux of the coniferous and broad-leaved mixed forest in combination with the foggy weather characteristics in this area.The boosted regression trees model（BRT）was used to quantitatively analyze the non-linear effects of environmental factors on the carbon and water fluxes of the coniferous and broad-leaved mixed forest ecosystem in Jinyun Mountain under different scenarios.The results show that:（1）The medium and high quality data of energy flux,carbon flux and water flux account for about 70%;The 5-year energy closure rate was between 0.73 and 0.86;The direction of the peak contribution area obtained from the flux footprint simulation was consistent with the main wind direction observed by meteorology;The carbon and water fluxes of the two-layer eddy covariance system were in good agreement;The eddy covariance method had good correlation with the experimental results.In conclusion,the eddy covariance method had good applicability in this region and highly representative of the carbon and water flux of the whole stand.（2）The annual average carbon sequestration of coniferous and broadleaved mixed forest was at a high level,with the annual average carbon sequestration of 659.89 g C·m^-2·y^-1 and the annual average evapotranspiration of 1142.55mm.the environmental carrying capacity threshold carbon sequestration of the ecosystem was 685.02 g C·m^-2·y^-1.Beyond the sustainable environmental carrying capacity threshold,the carbon sequestration capacity of coniferous and broad-leaved mixed forest will decrease.（3）The change characteristics of carbon and water fluxes at different time scales were different.The diurnal variation of net ecosystem exchange（NEE）was characterized by a"V"trend.The diurnal minimum of NEE occurs between 12:00 and 14:00,the diurnal dynamic curve of evapotranspiration（ET）was a single peak curve,and the diurnal maximum of ET occurs between 12:00 and 14:30;The ecosystem was in the process of carbon sink in all seasons,with the strongest carbon sink capacity in summer and the weakest in winter;The characteristics of ET in different seasons are similar to NEE,with the strongest ET in summer and the weakest in winter;In addition,the carbon sink capacity and evaporation capacity in foggy days are stronger than those in non-foggy days.（4）Under the background of global climate change,the ecosystem in Jinyun Mountain area showed a trend of cooling and rainfall polarization year by year.The flux exchange of coniferous and broad-leaved mixed forest was mainly affected by net radiation（Rn）,air temperature（Tair）,soil temperature（Tsoil）,relative humidity（RH）and vapor pressure deficit（VPD）.Among them,NEE was mainly affected by net radiation,saturated vapor pressure deficit and soil temperature,and ET was mainly affected by net radiation,air temperature and soil temperature.（5）The main influencing factors of carbon and water fluxes in coniferous and broad-leaved mixed forests were different under different scenarios.The main influence factors of NEE and ET in daytime were net radiation,followed by air temperature,soil temperature and vapor pressure deficit,while the soil temperature at night was the main environmental factor of flux change;In the daytime,the relationship between NEE and net radiation showed a double Valley trend,indicating that there were negative and positive tree species with different minimum light reception in the ecosystem.NEE in foggy days was mainly affected by air temperature and soil temperature,ET was mainly affected by relative humidity and air temperature;In non-foggy days,NEE was mainly affected by saturated vapor pressure deficit and soil temperature,and ET was mainly affected by relative humidity and soil temperature;In addition,the effect of fog on radiation made the light saturation point of vegetation move backward,which increased the difference of carbon sequestration capacity between negative and positive tree species;In the growing season,NEE was mainly affected by vapor pressure deficit difference and soil temperature,while ET was mainly affected by soil water content and air temperature;NEE in non-growing season was mainly affected by vapor pressure deficit and air temperature,and ET was mainly affected by soil temperature and vapor pressure deficit.