Diverse Responses Of Different Definitions Of Ecosystem Water Use Efficiency To Multiannual Precipitation Fluctuations

Climate change is one of the core issues of global change research.Global climate change is likely to produce more frequent extreme precipitation events and alter the frequency and duration of drought,which may further affect the structure and function of terrestrial ecosystem.Terrestrial plants remove CO₂ from the atmosphere through photosynthesis,a process that is accompanied by the loss of water vapour from leaves.Water use efficiency?WUE?,the ratio of carbon assimilation to water loss,has been recognized as an important characteristic of ecosystem productivity,which is a key characteristic of ecosystem function that is central to the global cycles of water,carbon and energy.Precipitation is one of the important factors affecting terrestrial ecosystem.The response of terrestrial ecosystems to changes in precipitation and water availability is one of the greatest challenges associated with climate change.Understanding the response mechanism of ecosystem WUE to water supply changes could help us understand ecosystem adjustment and feedback in the context of global climate change.Due to the differences in research purposes and data acquisition,previous studies have discrepance calculation methods for the ecosystem WUE,and the carbon sequestration and water consumption have different expressions.The ecosystem WUE,based on different algorithms,has different connotations due to different carbon and water exchange processes with different ecological processes.Meanwhile,the different definitions of ecosystem WUE are closely related to each other for the same core process.At present,the researches on ecosystem water use efficiency were lack of comparability and fragmentation,which is not conducive to the study of the coupling of ecosystem water and carbon cycle.In this paper,three ecosystem water use efficiency were defined.The carbon absorption of the ecosystem was characterized by Gross Primary Productivity?GPP?,which was fixed by vegetation photosynthesis.The water consumption of the ecosystem was characterized by precipitation?P?,evapotranspiration?ET?and vegetation transpiration?T?.WUE-P represents the ratio of GPP to ET,WUE-ET represents the ratio of GPP to ET and the ratio of GPP to T was represented by WUE-T.Here the study investigated the association and difference between three ecosystems WUE of different vegetation function types and different climatic zones,as well as differences in response to precipitation fluctuations.In the study of the impact of climate change on global vegetation,all of the three water use efficiency indicators have applications,and which one is more sensitive to climate change and more suitable for the analysis of ecosystem change characteristics is the core issue discussed emphatically in this paper.In this paper,the characteristics of three WUE are studied at site scale and global scale.Firstly,the evapotranspiration model was optimized by the actual observed from FLUXNET data.Based on an optimized PT-JPL model,ET was partitioned into three components transpiration?T?,canopy interception evaporation?EI?,and soil evaporation?ES?.The T/ET ratio ranged from 0.29 to 0.72 with obvious differences across biomes and with ratios generally lower than in previous studies.The?T+EI?/ET ratio was limited to a relatively narrow band from 0.57 to 0.86.The results of evapotranspiration segmentation cross different vegetation functional types and different climate zones were different.The T/ET values show an obvious decreasing trend with increasing annual precipitation.Secondly,the results of three WUE cross different vegetation function types and climate zones of the site scale and global grid were analyzed.While there were significant differences between the three WUE.The most significant factor associated with WUE is net primary productivity GPP,and the magnitude of GPP is significantly correlated with annual precipitation.The effects of muiltannual precipitation changes on the ecosystem WUE were analyzed,and the responses of the three WUE to precipitation fluctuations were significantly different.At the site scale,WUE-P increased significantly in dry years compared with wet years.WUE-ET did not show significant changes between dry years and wet years.WUE-T decreased in dry years and increased in wet years.At global scale,there are some differences in the influence of drought on three WUE.As drought intensified,the area with WUE-P showing an increasing trend gradually increased and with WUE-P showing a decreasing trend gradually decreases.As the degree of drought increased,the areas with an increasing trend of WUE-ET were gradually decreased and the areas with a decreasing trend were gradually increased.Similar to the spatial distribution of WUE-ET,WUE-T appeared to increase at more areas.The main reason for the different responses of three WUE to precipitation fluctuations is that there are differences in the sensitivity of GPP,ET and T to annual precipitation change,among which GPP is more sensitive to precipitation change,ET is medium,while the transpiration of vegetation T is less sensitive to precipitation.Three WUE with different definitions play different roles as evaluation indicators,WUE-P is obviously affected by precipitation,and WUE-ET is relatively stable without significant changes,while WUE-T is a suitable indicator to evaluate the effects of climate change on the water-carbon coupling.