中文摘要：

【目的】在土壤水分条件坡位差异明显的半干旱地区,深入了解树干液流速率的坡位差异及其主导环境因子,探讨从样地到坡面尺度的上推方法,以准确估算坡面和流域尺度的森林蒸腾耗水。【方法】2015年6—9月,在宁夏六盘山叠叠沟小流域选择1个西北坡向的典型坡面,设立上、中、下3个坡位的华北落叶松林样地,利用热扩散探针法监测树干液流速率,并同步监测气象因子及土壤水势的变化。【结果】研究期间,0～60 cm土壤水势（Ψ0-60）和日均液流速率均存在显著的坡位差异,土壤水势（MPa）为下坡（-0.210）〉中坡（-0.410）〉上坡（-0.511）,日均液流速率（mL·cm^-2min^-1）为下坡（0.045）〉上坡（0.036）〉中坡（0.034）。日均液流速率与日均气温（Ta）、日最高气温（Tmax）、日最低气温（Tmin）、日潜在蒸散（PET）、日均饱和水汽压差（VPD）、日均太阳辐射强度（Rs）、Ψ0-60呈极显著正相关,与日均空气相对湿度（RH）、日降水量（P）呈极显著负相关。但相关系数大小存在显著的坡位差异,日均液流速率与气象因子的相关系数为下坡〉上坡〉中坡,与Ψ0-60的相关系数为上坡〉中坡〉下坡。为了排除其他因子的影响,基于边界线分析评价树干液流速率对单个环境因子的响应程度,结果表明存在显著的坡位差异。从上坡到下坡,液流速率对Ta、Tmax、RH、VPD及Ψ0-60的响应程度逐渐减小,而对Rs的响应程度逐渐增强。利用逐步回归分析建立日均液流速率与环境因子的多元线性模型,发现不同坡位入选环境因子存在明显差别：在土壤水势较高的下坡,对液流速率贡献最大的前2个主导环境因子依次为Rs和Ψ0-60,在土壤水势较低的上坡依次为Ψ0-60和VPD,在介于二者之间的中坡则是VPD和Ψ0-60。【结论】树干液流速率的坡位差异是由土壤水分条件及受地形影响的气象因子共同决定?

英文摘要：

【Objective】It is key to clearly recognize variation in sap flow velocity in trees in various slope positions and its main environmental factors,so that to improve the calculation accuracy for forest transpiration by means of up-scaling from a plot/position to a whole slope or a watershed,especially in semi-arid areas featuring with a strong variation of soil moisture in slopes. 【Method】Three plots of Larix principis-rupprechtii were set up respectively at the upper,middle and lower positions on the northwest-facing slope in Diediegou watershed of Liupan Mountains,and the sap flow velocity in trees,as well as the soil water potential and meteorological factors were monitored simultaneously throughout the growing season（ Jun. to Sep.） in 2015. 【Result 】The result showed that there were significant differences in the soil waterpotential of 60 cm layer（ Ψ0-60,MPa） among slope positions,in an order of lower position（-0. 210） middle position（-0. 410） upper position（-0. 511）,and in the daily average sap flow velocity（ mL·cm^-2min^-1） in an order of lower position（ 0. 045） upper position（ 0. 036） middle position（ 0. 034）. Throughout the study period,the daily average sap flow velocity was significantly and positively correlated with the average daily air temperature（ Ta）,daily maximum temperature（ Tmax）,daily minimum temperature（ Tmin）,potential evapotranspiration（ PET）,average daily saturated vapor pressure deficit（ VPD）,average daily solar radiation density（ Rs） and Ψ0-60,but significantly and negatively with the daily average air relative humidity（ RH） and precipitation（ P）. Moreover,the correlation coefficients with each other were significantly different among slope positions. The order was lower upper middle for the meteorological factors,whereas upper middle lower for the Ψ0-60. According to the boundary line analysis,significant differences were also found in the degree of sap flow velocity variation in response to each single fac