Analysis On Characteristics And Influencing Factors Of Vegetation’s Water Use Efficiency For Main Ecological Types In Northern Hemisphere

The intrinsic water use efficiency is one of the key indicators to evaluate the adaptability of plants to different environments,and it can provide a theoretical basis for understanding the response and adaptation mechanism of different vegetation types to global change.In this paper,we used the flux and meteorological data of 20 FLUXNET sites in Europe and North America,and analyzed the relationship between environmental factors and the net ecosystem exchange（NEE）,latent heat（LE）of typical vegetation types（include: closed shrubland,deciduous broad-leaved forest,evergreen needleleaf forest,mixed forest,permanent wetland,grassland,cropland）.Based on the above studies,the Intrinsic Water Use Efficiency（IWUE）was estimated,and the correlation between seasonal and interannual variation of intrinsic water use efficiency and environmental factors was analyzed.The main conclusions are as follows:（1）Carbon fixation and carbon sequestration of different vegetation types were different: Carbon fixation period of deciduous broad-leaved forest,mixed forest,wetland was mainly in autumn,and coniferous forest was in summer and autumn,and grassland mainly was in summer.For the total amount of carbon sequestration,the closed shrubland was largest,which was 800-1200 g m^-2,followed by the deciduous broadleaved forest,which was 600 g m^-2,and the rest was 200-400 g m^-2.While farmland ecosystem’s carbon fixation period and the amount of fixed carbon were related to the climate and its farming manner in the area.（2）The magnitude of latent heat flux was related to the latitude,the surrounding environment and vegetation types in the area.The latent heat flux of different vegetation types was the largest in summer,and most of them were 300-700 W m^-2,followed by spring and autumn,which were100-200 W m^-2;while the smallest was in winter,which were mostly about 10 W m^-2.The total amount of latent heat is 1300-1800 W m^-2 for the closed shrubland,and the latent heat flux of the remaining sites were about 500-800 W m^-2 generally.（3）The intrinsic water use efficiency of different vegetation types had obvious seasonal variation characteristics: The intrinsic water use efficiency of deciduous broad-leaved forest,mixed forest and coniferous forest reached the maximum at the end of spring or early summer,and fluctuated smoothly in the following period of time.The time of maximum water use efficiency occurrence varies with the crop species,farming manner and so on.The interannual chanage of intrinsic water use efficiency showed a declining trend over the years in closed shrubland and cropland,and the intrinsic water use efficiency of deciduous broad-leaved forest and wetland increased year by year,while the general trend of intrinsic water use efficiency in grassland and mixed forest was gentle.（4）The relationships between the net ecosystem exchange,the latent heat flux,the intrinsic water use efficiency and environmental factors in different vegetation types: The net ecosystem exchange,the latent heat flux,the intrinsic water use efficiency had a significant correlation with solar radiation,air temperature,soil temperature and vapor pressure deficit（p <0.05）for most vegetation types,while the effects of precipitation,soil moisture content,wind speed and CO₂ concentration on the the net ecosystem exchange,the latent heat flux,the intrinsic water use efficiency of different vegetation types were different.The results showed that the vegetation types with low latitude and good hydrothermal condition were less affected by the environment.