To fully understand the changes in the evapotranspiration components in forest ecosystem and their contribution to evapotranspiration at daily scale, we used the hypothesis theory of isotopic steady state and non-steady state combined with the water isotope analyzer system to quantitatively split and compare the evapotranspiration components of Platycladus orientalis ecosystem during the growing season. Results showed that the 18O of water from different sources during the four mea-surement days (August 5, 8, 10, 11, 2016) all showed surface soil water and oxygen isotope composition (δS) > branch water and oxygen isotope composition (δX) > atmospheric water vapor oxygen isotopes composition (δV), with obvious differences due to the isotope fractionation. Oxygen isotopes composition of soil evaporated water vapor (δE) was between -26.89‰～-59.68‰ at the daily scale, showing a pattern of first rising and then decreasing. The oxygen isotopic composition of evapotranspiration water vapor in forest ecosystem (δET) was between -15.99‰～-10.04‰. The oxygen isotopic composition of transpired water vapor under steady state(δT-ISS) was between -12.10‰～-9.51‰. The oxygen isotopic composition of transpired water vapor under non-steady state (δT-NSS) was between -13.02‰～-7.23‰. δET and δT-NSS had the same changing trend throughout the day at the daily scale, while the trend of δET, δT-ISS and δT-NSS was approximately the same during 11:00-17:00. In general, the contribution rate of plant transpiration to total evapotranspiration showed that FT-ISS was between 79.1%-98.7%, and FT-NSS was between 88.7%-93.7%. Our results suggested that water consumption through soil evaporation was far less than that of vegetation transpiration in the study area, and that vegetation transpiration dominated forest evapotranspiration.
为了全面认识森林生态系统蒸散各组分及其对蒸散的贡献率在日尺度上的变化规律,本研究利用同位素稳态和非稳态假设理论结合水同位素分析仪系统,对生长季侧柏林生态系统蒸散各组分进行了定量拆分和比较。结果表明: 4个测定日(2016年8月5、8、10、11日)不同来源水体的18O都呈现表层土壤水氧同位素组成(δS)＞枝条水氧同位素组成(δX)＞大气水汽氧同位素组成(δV),说明三者可能因同位素分馏效应表现出明显的差异。土壤蒸发水汽氧同位素组成(δE)在日尺度上为-26.89‰～-59.68‰,整体上呈现出先上升后下降的变化趋势;森林生态系统蒸散水汽氧同位素组成(δET)为-15.99‰～-10.04‰,稳态(ISS)下植物蒸腾水汽氧同位素组成(δT-ISS)为-12.10‰～-9.51‰,而非稳态(NSS)下植物蒸腾水汽氧同位素组成(δT-NSS)为-13.02‰～-7.23‰,在日时间尺度上δET与δT-NSS全天的变化趋势一致,在11:00—17:00 δET、δT-ISS与δT-NSS三者的变化趋势近似一致。总体上,植物蒸腾量对蒸散量的贡献率表现为FT-ISS 79.1%～98.7%,而FT-NSS 88.7%～93.7%。这表明研究区土壤蒸发耗水远小于植被蒸腾耗水,植被蒸腾在林地蒸散中起主导作用。.
Keywords: non-steady-state assumption; plant transpiration; quantitative partitioning; soil evaporation; stable isotope.