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中国旱区典型生态系统水分利用效率动态特征及其生物物理调控

Dynamics of Ecosystem Water Use Efficiency and Its Response to Biophysical Factors of Typical Ecosystems in Dryland of China

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【Author in Chinese】 白玉洁

【Supervisor】 查天山

【Author's Information】 北京林业大学, 水土保持与荒漠化防治, 2020, 博士

【Abstract in Chinese】 气候变化导致全球气温不断升高,降雨格局发生改变、干旱频率增加、干旱程度加剧,对生态系统物质、能量循环产生重大影响。水分利用效率反映了生态系统碳吸收与水分损失之间平衡关系,是评价生态系统对环境变化响应的重要指标。本研究以中国干旱半干旱区为研究区域,依托涡度相关技术,通过单站点分析和多站点整合,研究生态系统尺度上水分利用效率时、空特征及其生物物理调控机制。单站点研究基于宁夏盐池荒漠生态系统定位研究站涡度相关监测通量数据,分析了荒漠草原生态系统水分利用效率的季节变化特征和季节性干旱对水分利用效率的影响。多站点研究基于对中国干旱半干旱区沿不同干旱程度梯度分布的34个通量观测站的水分利用效率文献研究结果进行整合,分析了生态系统水分利用效率的空间分布格局及其驱动因素,比较了不同生态系统类型和气候区域之间水分利用效率的差别。主要研究结果如下:(1)毛乌素沙地荒漠草原生态系统2012—2015生长季平均水分利用效率(EWUE)和固有水分利用效率(IWUE)分别为1.15g C kg-1 H2O和1.47 g C k Pa kg-1 H2O。水分利用效率具有明显的季节变化规律,不同的水分利用效率指标的季节变化受到不同环境和生物因子的调控。在整个生长季,EWUE主要受到空气温度(Ta)、入射光合有效辐射(PARin)以及饱和水汽压差(VPD)的调控。IWUE主要受到Ta、VPD、归一化植被指数(NDVI)以及冠层导度(gc)的调控,IWUE更能够反映生物因子对生态系统碳—水耦合的调控。(2)毛乌素沙地荒漠草原生态系统水分利用效率在不同季节对环境和生物因子的敏感性存在差异。Ta在春季和秋季对EWUE都存在显著的促进作用,而夏季表现出一定的抑制作用。PARin在每个季节都对EWUE存在显著的抑制作用,在调节荒漠草原生态系统各季节水分利用效率的日值变化中起主导作用。但EWUE在不同季节对PARin的敏感性存在差异,其标准化相关系数范围为-0.49—-0.27。Ta和VPD在春季和秋季对IWUE有显著促进作用,在夏季没有影响或影响较小。gc在春季和秋季对IWUE有较小的抑制作用,在夏季显著的抑制IWUE,标准化相关系数为-0.30。(3)降雨对毛乌素沙地荒漠草原水分效率的影响可分为降雨事件等级、降雨模式以及降雨量的影响。大降雨事件比中等降雨更有可能使当日的水分利用效率升高。短时间(10—15天)降雨模式的变化对水分利用效率没有显著影响,较长时间段内(90天)降雨量相同时,频繁的中低强度降雨比零星的高强度降雨模式更能够刺激总生态系统生产力(GEP)的增长,使生态系统有更高的水分利用效率。月尺度上降雨量的降低会降低生态系统的水分利用效率,生长季中期月降雨量的异常降低对水分利用效率存在一个月的滞后影响。(4)季节性干旱对毛乌素沙地荒漠草原生态系统水分利用效率的影响分为干旱时间和干旱强度的影响。短时间的干旱胁迫对水分利用效率没有显著影响,而长期干旱会显著的降低总生态系统生产力(GEP)和提升生态系统蒸散(ET),从而降低生态系统的水分利用效率。轻度干旱胁迫(相对土壤含水量REW=0.4)时水分利用效率会轻微上升,随着干旱胁迫的程度加强水分利用效率开始降低。生态系统在受到干旱胁迫时主要通过加强土壤水分利用和改变冠层导度来提升水分利用效率。(5)中国干旱半干旱区生态系统的EWUE为1.64±0.93 g C kg-1 H2O,农田和森林比灌丛和草地生态系统有更高的EWUE,干旱区和亚湿润干旱区的EWUE高于半干旱区。EWUE随着生长季降雨量的增加而升高,降雨量不仅直接影响EWUE,还通过促进归一化植被指数(NDVI)而影响EWUE。不同的生态类型中,农田生态系统水分利用效率对降雨量(PPT)和NDVI的敏感性更高,灌丛生态系统最不敏感。不同气候区域中,半干旱区生态系统水分利用效率对PPT和NDVI的变化最敏感。总生态系统生产力和生态系统蒸散分别主导了半干旱和亚湿润干旱区的水分利用效率的空间变化。综上所述,降雨是干旱半干旱区生态系统水分利用效率的重要影响因素。未来气候变化导致降雨的时间分布发生变化,由此带来的极端降雨事件、降雨频率变化以及季节性干旱,可能会改变生态系统的组分结构以及碳—水耦合机制。而气候变化导致降雨的空间分布的改变也会沿干旱程度梯度对总生态系统生产力和蒸散产生不同的影响,导致碳—水耦合关系存在明显的跨气候梯度的差异。

【Abstract】 Climate change with continuous rise of global temperature,which leads to the change of rainfall pattern,the increase of drought frequency,and the intensification of drought,has a significant impact on the physical and energy cycle of the ecosystem.Water use efficiency(WUE)is an important index to evaluate the response of ecosystem to climate change and to reflect the interaction of carbon gain and water loss of the ecosystem.In this study,temporal and special dynamics of ecosystem WUE and its biophysical control were studied in drylands of China by means of single-site eddy-covariance(EC)flux measurements and multi-site meta-analysis of EC-related publications.Seasonal variations in ecosystem WUE and its biophysical control were studied in a desert steppe site in Yanchi,northwest China of Mu Us Desert.The magnitude,spatial patterns in WUE,and underlying drivers among ecosystems and climate zones were examined on the basis of published data from 34 EC sites.(1)Growing season mean ecosystem water use efficiency(EWUE)and inherent water use efficiency(IWUE)of desert steppe from 2012 to 2015 was 1.15 g C kg-1 H2 O and 1.47 g C k Pa kg-1 H2 O,respectively.EWUE and IWUE had obvious seasonal variation,which were regulated by different environmental and biological factors.EWUE was mostly controlled by air temperature(Ta),incident photosynthetically active radiation(PARin)and vapor pressure deficit(VPD).IWUE was mostly controlled by Ta,VPD,normalized difference vegetation index(NDVI)and canopy conductance(gc).IWUE was more sensitive to the regulation of biological factors on the carbon-water coupling.(2)Water use efficiency of desert steppe in Mu Us desert showed different sensitivity to biophysical factors among seasons.Variations in Ta had significantly positive effects on variations in EWUE in spring and autumn,but negative in summer.PARin dominantly regulated daily variation in EWUE with different sensitivity among seasons,the standardized correlation coefficient ranging from-0.49 to-0.27.Ta and VPD exerted affected daily variations in IWUE in spring and autumn,but not in summer.The gc affected the variations in IWUE mostly in summer,with a correlation coefficient of-0.30.(3)Precipitation affected WUE of desert steppe through its intensity,pattern and amount.The big rain increased WUE than the moderate rain.Frequently medium and low-intensity rainfall can stimulate the growth of gross ecosystem productivity(GEP)more than sporadic high-intensity rainfall,leading to a higher WUE.The decrease of rainfall amount on the monthly scale reduced the ecosystem WUE.The abnormal decrease of monthly rainfall in the middle of the growing season had a lagged effect of about one month on WUE.(4)Seasonal drought affected WUE of desert steppe by the duration and intensity of drought stress.Short drought stress had little effect on WUE,long time drought stress could reduce WUE by enhancing GEP and reducing evapotranspiration(ET).Mild drought stress(relative soil moisture content,REW=0.4)slightly enhanced WUE,and severe drought reduced WUE.WUE was mainly regulated by soil water content in 10 cm depth(VWC10)and gc during the drought stress.(5)Average growing-season EWUE at the 34 sites in drylands of China was 1.64 ± 0.93 g C kg-1 H2 O.EWUEs among vegetation types were significantly higher for cropland and forest,and lower for shrubland and grassland ecosystems.EWUEs among climate zones were significantly higher for arid and dry sub-humid zone,and lower for semi-arid zone.EWUE was shown to increase in direct association with increasing precipitation(PPT)and indirectly through PPT’s effect on NDVI.Among ecosystem types,EWUE for cropland was most sensitive to PPT and NDVI,whereas shrubland EWUE was the least sensitive to these factors.EWUE in the semi-arid zone was more sensitive to the PPT and NDVI,unlike the other climatic zones.GEP and ET dominated spatial variation in EWUE in semi-arid and dry subhumid zones,respectively.In summary,precipitation was a key factor that controlled the ecosystem water use efficiency.The changes in distribution,frequency,intensity and extreme rainfall events of rainfall under future climate change,may result in changes in ecosystem composition and the carbon-water coupling,leading to differential outcomes on ecosystem carbon and water cycling along an aridity gradient,thus a crossgradient differences in ecosystem WUE’s.

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