【作者】 安婷婷；

【导师】 张涛；

【作者基本信息】 沈阳农业大学 ， 大气科学， 2020， 硕士

【摘要】 蒸散(Evapotranspiration,ET)是全球水循环的重要组成部分,也是土壤-植被-大气系统之间物质和能量交换的关键环节,时刻影响着全球的气候变化。青藏高原具有特殊的地理位置及气候特点,对全球气候变化响应敏感。因此,阐明青藏高原地区蒸散的变化特征及控制机制有助于我们深刻理解和认识全球气候变化背景下高寒脆弱生态系统的发展变化趋势,对全球变化进程的预测亦有指导意义。此外,蒸散很容易受到放牧等外界干扰的影响,而青藏高原作为我国的天然牧场,放牧为当地牧民最主要的土地利用方式。因此,研究蒸散对放牧的响应特征具有现实意义。不仅可以帮助我们更加准确地评估全球变化对高寒草甸的影响,还可以为科学合理地指导放牧提供依据。本研究基于藏北高原草地生态系统研究站(那曲站)的涡度相关观测数据,借助线性扰动分析,深入剖析高寒草甸生态系统蒸散的环境及生物学控制机制。利用禁牧/放牧平行涡度相关系统及人工观测数据,探讨放牧对高寒草甸生态系统蒸散及其调控机制的影响,从而较为客观的评价气候变化背景下青藏高原高寒草甸生态系统蒸散的变异特征及其对人为扰动的响应。具体研究结论如下:(一)藏北高寒草甸生态系统蒸散变异的驱动机制分析(1)高寒草甸生态系统蒸散的变异(δET)来源可分为受气象因子驱动和生物因子驱动两部分。对于生长季干旱的年份,δET主要受生物因子驱动。而对于生长季湿润的年份,δET主要受气象因子驱动。(2)蒸散的发生主要受能量条件和物质条件(水分供给)两方面制约。在干旱年,能量条件充足,净辐射(Net Radiation,Rn)与空气温度(Air Temperature,Ta)对δET的贡献量很低。此时水分供给为蒸散的主要限制因素,生物因子冠层气孔导度(Canopy Stomatal Conductance,gs)和叶面积指数(Leaf Area Index,LAI)与水分供应密切相关,因此它们对δET的贡献量很高。此外,气象因子饱和水汽压(Vapor pressure deficit,VPD)决定了蒸腾拉力的大小,其对δET的贡献量也相对较高。而在湿润年,水分供给充分,植被长势良好,生物因子gs和LAI的贡献量明显降低。此时长时间阴雨导致能量供给相对不足,气象因子Rn与Ta对δET的贡献量相对干旱年有明显的提升。(二)放牧影响高寒草甸蒸散变异的驱动机制(1)在生长季,放牧明显提高了蒸散。放牧地蒸散年总量比禁牧地高10%~11.5%。(2)放牧改变了群落结构。放牧使植被总盖度(Vegetation Cover,Vc)、物种丰富度(Species Richness,Sr)和多样性指数(Diversity Index,H)降低。因此,放牧通过直接改变地上生物量减少了植被蒸腾作用(Transpiration,T)。但物种丰富度和多样性指数的降低导致了植被逐渐退化和土壤理化性质的改变,更高的地表暴露度导致蒸发量(Evaporation,E)升高。高寒草甸生态系统中,蒸发是蒸散的主要组分,因此放牧在一定程度上促进了蒸散。(3)放牧改变了高寒草甸生态系统的微气象环境。相较于禁牧地,放牧使土壤含水量(Soil Water Content,SWC)和VPD分别提升了14.63%和4.36%。在禁牧地,蒸散主要由Rn、SWC、Ts和降水量(Precipitation,PPT)控制;而在放牧地,蒸散主要由Rn、Ta、SWC、VPD和PPT控制。无论在禁牧地还是放牧地,Rn都是蒸散的主控因子,决策系数为64%~74%。但放牧改变了蒸散的限制因子,使蒸散的限制因子由SWC转变为VPD,从而削弱了水分对蒸散的限制作用。更多还原

【Abstract】 Evapotranspiration(ET)is an important component of water cycle.As the key process through which the water and energy exchanges in the soil-vegetation-atmosphere system,ET has a great impact on the climate change.Meanwhile,Tibetan Plateau is located in a unique geographical region with special climate,and the vegetation is very sensitive to climate change.Therefore,investigating the variations in ET and its underlying mechanisms on Tibetan Plateau is conducive to understand the evolution of alpine ecosystem,and it is also helpful to predict the global climate change in the future.In addition,ET is vulnerable to external disturbances such as grazing.As a natural pasture in China,grazing is the main land use pattern for local herdsmen.Studying the responses of ET to grazing could help us assess the impacts of global change more accurately and provide a scientific basis for guiding grazing.Therefore,this study employed the continuous eddy covariance data and the linear perturbation analysis to investigate the effects of meteorological and biological factors on ET in an alpine meadow ecosystem on the Tibetan Plateau.By comparing the eddy covariance data and manipulating observation data between grazing and control plots,the effects of grazing on ET and its underlying mechanisms were studied using path analysis.The main conclusions are as follows:1.The control mechanisms of evapotranspiration variations in the alpine meadow(1)The sources of ET variations(δET)were divided into meteorological and biological drivers.δET was mainly driven by biological factors in drought growing seasons and by meteorological factors in wet growing seasons.(2)ET was mainly limited by energy and water availability.Energy was adequate in drought growing seasons,net radiation(Rn)and air temperature(Ta)contributed less to δET.Moisture limited ET when the drought happened.While the canopy stomatal conductance(gs)and leaf area index(LAI)were closely related to moisture,they contributed more to δET.Moreover,vapor pressure deficit(VPD)represented the transpiration pull,which also contributed more to δET.In the wet growing seasons,the water was relatively adequate and vegetation grew well,then the contributions of gs and LAI reduced obviously.The contributions of Rn and Ta on δET in wet growing seasons were greater than those in the drought growing seasons due to the inadequate energy induced by increasing rainy days in the wet growing seasons.2.Grazing altered the control mechanisms of ET in the alpine meadow ecosystem(1)In the growing seasons,the ET in grazing meadow was higher than that in fencing meadow.ET in grazing meadow was 10%~11.5% higher than that in fencing meadow.(2)Grazing altered the community structure.Grazing decreased vegetation cover(Vc),species richness(Sr)and diversity index(H).Thus,grazing decreased transpiration(T)directly by decreasing the vegetation biomass.But the low Sr and H resulted in the vegetation degradation and changes of soil physicochemical properties,which led to enhanced evaporation(E)due to more exposure of bare soil.In the alpine meadow,the E dominated ET,hence,the grazing elevated the ET through its promoting effects on E.(3)Grazing altered the micrometeorological conditions in the alpine meadow ecosystem.Grazing enhanced soil water content(SWC)and VPD by 14.63% and 4.36%,respectively.In the fencing meadow,ET was mainly controlled by Rn,SWC,Ts and precipitation(PPT);in the grazing meadow,ET was mainly controlled by Rn,air temperature(Ta),SWC,VPD and PPT.The radiation dominated ET with the decision coefficient of 64~74% in both grazing and fencing meadow.Grazing shifted the limiting factor of ET from the SWC to VPD,which weakened the limiting effects of the water conditions on ET.更多还原