| As reported by the IPCC,the frequency and intensity of global drought events will increase in the 21 st century,global land surface temperature will be increase and precipitation will decrease in the context of global climate change.As an important part of the global carbon-water cycle,ecosystem plants absorb carbon dioxide from the atmosphere through photosynthesis and lose water through transpiration,and these plants are important drivers of the energy and water cycles between the atmosphere and terrestrial ecosystems.Ecosystem water use efficiency(WUE)is an important indicator of the coupling of carbon and water cycles,and the WUE is often employed to characterize ecosystem carbon and water cycle indicators.The WUE is defined as the ratio of carbon absorption to water loss.The WUE links biological processes(i.e.,plant leaf photosynthesis and transpiration)to physical processes on the land surface(i.e.,evaporation).Central Asia is one of the largest arid regions in the world and includes Kazakhstan(KAZ),Turkmenistan(TKM),Uzbekistan(UZB),Kyrgyzstan(KGZ),Tajikistan(TJK)and the Xinjiang Uygur Autonomous Region of China(CHN).In recent years,climate change and frequent human activities have caused tremendous changes in Central Asia.A recent study showed that increasing temperature and decreasing precipitation led to a severe drought in Central Asia,and these changes will potentially impact vegetation growth and ecosystems in Central Asia.However,this changes also provides an opportunity to study the response mechanisms of ecosystems to drought.This study applied multi-source remote sensing data,using time series analysis,correlation analysis,regression fitting and other data processing methods.By analyzing the spatial and temporal changes in the water use efficiency of the ecosystems in Central Asia and Xinjiang,China,and the changes in water use efficiency of the main vegetation types.To evaluate the response of water use efficiency to drought and the stability of ecosystem resistance and resilience before and after a drought event,in order to find out the response mechanism and institutional changes of the arid ecosystem to climate change,so provides a theoretical reference to the scientific management of the arid ecosystem.Conclusion:1.We carried out spatial and temporal variation analyses on Central Asia focusing on the GPP,ET and WUE.The WUE has a increased trend from 2000 to 2014;the WUE spatial variation distribution was notable,and the area with a significant WUE increase was larger than that with a significant WUE decrease;the WUE increased significantly in northern Kazakhstan,the Ili Valley of Xinjiang and the high mountainous area of Kyrgyzstan,and the WUE decreased significantly in the southern Xinjiang oasis and the Uzbekistan irrigation area.We found that Kyrgyzstan had the largest proportion of WUE increase(71.7%),higher than those of the other countries,and the largest proportion of WUE decrease occurred in Uzbekistan(7%);the main land cover type with WUE changes were the farmlands and grasslands in the study area;the WUE changes of the medium-density population areas were the most notable,and the WUE increased most notably in the low to medium human impact factor range,while the WUE decreased most notably in the range of higher human impact factors.We recombined the trend results of the GPP,ET,and WUE to analyse the WUE changes for the different classifications.Our results demonstrated that the increase in WUE was mainly due to the increase in GPP and decrease in ET.The increase in rainfall and the farmlands converted into forestlands by government policies contributed to the increase in GPP.The decrease in ET was due to the physiological resistance of vegetation under drought stress;the shifts in crop type and unreasonable water use practices also led to the decline in WUE in the irrigated agricultural areas.2.The order of different vegetation types WUE was open shrubland >grassland >closed shrubland > forest >cropland,of which open shrubland WUE were greater than other vegetation types(P<0.05).All vegetation types WUE showed an upward trend from 2000 to 2014.There is a difference in the spatial distribution of vegetation WUE.The WUE of different vegetation varies much with elevation: it first remains relatively constant as the elevation varies from 0 to 1200 m and then decreases dramatically.The areas of high WUE in farmland,forest,closed shrubland,open shrubland,grassland appear at altitudes of 750,750,750,1250,and 500 m,respectively.WUE exhibits large variability with latitude.The latitude of WUE peak is 36 oN(farmland),32 oN(closed shrubland),36-38 oN(open shrubland),50 oN(forest),32 oN(grassland).3.The response of water-use efficiency to drought showed large differences in various regions and biomes.Severe arid ecosystems had the highest water-use efficiency,followed by drought and slight drought ecosystems.The lowest water-use efficiency was observed for humid ecosystems;the correlations between annual water-use efficiency and temperature vegetation dryness index in drought stress period and post drought stress period showed different patterns over Central Asia vegetated areas,both of which had large spatial heterogeneity,water-use efficiency for arid ecosystems negatively correlated with drought;when drought stress posted,water-use efficiency and TVDI showed a negatively correlation over 63.57% of vegetated lands in drought stress period.In post drought stress period,water-use efficiency positively responded to temperature vegetation dryness index in the Central Asia accounting for approximately 66.7% of vegetated lands,water-use efficiency showed obvious negative response to drought.During drought stress period,the higher correlation between water-use efficiency and previous year temperature vegetation dryness index compared to the current year temperature vegetation dryness index,obvious lag effects of drought on ecosystem water-use efficiency,however,there was not apparently lag effects after drought stress.In addition,the arid ecosystem water-use efficiency showed a sensitive response to abrupt changes in the dry-wet environment.In post drought period,water-use efficiency approximately increased 30.03%,49.57%,18.39%,54.71% and 49.28% of closed-shrublands,cropland,forest,grassland,open-shrublands respectively compared with drought stress period.A rapidly increase in ecosystem WUE was occurred when the shift from dry to wet year.This indicates that the strategy of disturbance of arid ecosystems in arid regions is relatively stable in resilience.The conclusions from our study can provide a reference of the response strategies of arid ecosystems to drought.4.The extreme wetness SPEI threshold is 0.91,and the extreme drought SPEI threshold is-0.67 in Central Asia.2002 and 2003 are the extreme humid periods in nearly 113 years,and 2008 is the driest period in nearly 15 years.In the first year of continuous humid periods,the resistance increases with the increase of the degree of wetness,ecosystem has strong resistance;in the second year of continuous humid periods,the resistance decreases with the increase of wetness,and the resistance is weak.During the drought years,the resistance increased with the increase of the degree of drought in most areas of the study area,but the resistance decreased under extreme drought conditions.After the humid event,the resilience showed an increasing trend with the increase of the wetness.After the drought event,the resilience increased with the increase of the drought.The area of shrublands increased,and the area of grassland and forest decreased after the drought.Drought events reduce the area of forests,shrublands,croplands,and snow and ice,and increase the area of grasslands,deserts,and water bodies.In wet and dry years,the transition between shrublands and grassland is the reason of change in ecosystem resistance and resilience. |
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