The Effects Of Extreme Drought On Water Use Efficiency In The Zoige Alpine Wetland

Wetland ecosystems play an important role in the global carbon and water cycles by acting as important carbon sinks and water conserving areas.Located on the eastern edge of the Tibetan Plateau,the Zoige Alpine Wetland is the largest highland wetland in the world and is highly vulnerable to climate change,particularly to changes in the variability in precipitation.Climate models predict an increase in the frequency and intensity of extreme droughts as a result of global warming.Less attention has been paid to how water use efficiency and its components of alpine wetland ecosystems in response to extreme drought events in different seasonal periods.This study investigated canopy water use efficiency,ecosystem water use efficiency and their components,including gross primary productivity,ecosystem evapotranspiration,soil evaporation and vegetation canopy transpiration,in response to extreme drought events in Zoige Alpine Wetland by using a simulated extreme drought experiment during different seasonal periods in 2021-2022.Early drought,in which the early stage of plant growth occurred from mid-June to mid-July;Midterm drought,in which the peak plant growth occurred from mid-July to late-August;Late drought,in which the wilting period of plants occurred from late-August to late-September.The results show that:(1)The early,midterm and late extreme drought events in the growing season significantly reduced mean soil water content during treatment periods(P<0.05),however,there were differences among the depths in the reduction of soil water content.Extreme drought reduced soil water content by an average of 18.6%and 17.1%at 0～5 cm and 0～10 cm depths,which was greater than the 7.0%reduction at 0-20 cm depth.There were significant seasonal variation in the temporal dynamics of soil water content and soil temperature throughout the growing season,and soil water content showed a significant decreasing trend in the middle of the growing season.(2)Over two years,the early,midterm and late extreme droughts reduced evapotranspiration on average by 21.6%,3.1%and 2.5%,soil evaporation by 26.5%,22.1%and 2.9%,gross primary productivity by 30.6%,18.4%and 18.6%,and ecosystem water use efficiency by 15.6%,20.5%and 20.2%during treatment periods,respectively.Ecosystem evapotranspiration was dominated by vegetation transpiration,with soil evaporation accounting for only 6.9 to 11%of evapotranspiration,resulting in a minor difference between canopy water use efficiency and ecosystem water use efficiency.(3)Evapotranspiration and vegetation transpiration were significantly positively correlated with air temperature(P<0.01),but not significantly correlated with soil water content.Gross primary productivity was significantly positively correlated with soil water content,soil temperature and air temperature(P<0.01).Ecosystem and canopy water use efficiency were highly significantly positively correlated with soil water content at 0～5 and0～10 cm depth(P<0.01),while they were all significantly negatively correlated with air temperature(P<0.05).Linear regression analyses showed that gross primary productivity explained the response of evapotranspiration well with a coefficient of determination R~2 of0.56.However,three extreme drought treatments were different in terms of their effects on evapotranspiration and gross primary production.The early drought treatment had a negative effect on both evapotranspiration and gross primary production,while the midterm and late drought treatments had positive or almost no effects on evapotranspiration and negative effects on gross primary production.The response ratios of ecosystem and canopy water use efficiency were significantly negatively correlated with the response ratio of vegetation evapotranspiration with both coefficients of determination R~2 of 0.65.In conclusion,the early extreme drought event in the growing season had a greater negative impact on ecosystem evapotranspiration,vegetation transpiration,and gross primary productivity.Seasonal mean gross primary productivity was consistently reduced in both the midterm and late drought treatments within growing season compared with ambient condition,while evapotranspiration and vegetative transpiration did not respond consistently.Gross primary productivity had a longer recovery time after extreme drought events and was related to the recovery time of soil water content,in contrast to evapotranspiration and vegetation transpiration.Differences in gross primary productivity,evapotranspiration and vegetation transpiration responses to extreme drought resulted in seasonal differences in ecosystem and canopy water use efficiency responses.The response of gross primary productivity to extreme droughts was mainly regulated by soil water content,and the response of evapotranspiration to extreme droughts was mainly regulated by gross primary production,soil and air temperatures played important roles in regulating these responses.Vegetation transpiration was the dominant process controlling the responses of ecosystem water use efficiency and canopy water use efficiency to extreme drought events.