晋西黄土区苹果树边材液流速率的方位差异研究

目的 通过对苹果树不同方位边材液流速率的测定与比较，明确苹果树液流速率的方位特征，为通过液流测定来估算单株蒸腾提供依据。 方法 利用热扩散技术（TDP）对黄土区苹果树主要生长季4个方位边材液流速率及土壤水分、气象因子进行同步、连续监测。 结果 苹果树树干边材液流速率（Js）具有显著的方位差异（P < 0.01），其中北侧Js最高，整个生长季日平均值达189.3 cm·d-1，其次为南侧（为北侧的83%）、东侧（北侧的80%），西侧最小（北侧的63%）。各方位Js总体均表现出5—8月间递增、9—10月间递减、11月基本停止的季节动态，且均与冠层净辐射（Rn）、大汽水分亏缺（VPD）间呈较好的指数正相关关系。在典型晴天，不同方位Js的日峰值时刻均明显提前于VPD的峰值，平均提前约1.6 h（最大2.43 h），且提前的时长与VPD日平均值呈线性递增关系。当VPD高于2.0~2.2 kPa时，Js不再随VPD的增加而上升；不同方位间Js峰值时刻与VPD（P=0.97）峰值时刻间的差异不显著。 结论 苹果树边材液流速率存在着明显的方位差异（P < 0.01），其中北、南方位液流速率较高，东、西方位液流速率较低。不同方位液流传输受大气环境影响的过程具有一致性。为提高果树液流通量估算的精度，在实际测定中应考虑方位差异。

Study on Azimuthal Variation of Sap Flow Velocity of Apple Trees in Loess Plateau Area,Western Shanxi Province

Objective To determine the azimuthal character of sap flow velocity (Js) of apple trees. Method the thermal dissipation probe technology (TDP) was used to simultaneously and continuously monitor the sap flow velocity in the four locations of apple trees during the whole growing season in the loess region. The soil moisture and meteorological factors were measured simultaneously, too. Result The Js was significant different among directions (P < 0.01). The highest Js in the sapwood was observed in the north side with an average of 189.3 cm per day over the entire growing season, followed by that in the south side (83% of the north side) and the east side (80% of the north side). The Js from the west side was the smallest (63% of the north side). The Js of all sides showed seasonal dynamics, i.e. increasing in May-August, dropping in September-October, and nearly stopped in November. The Js showed an exponentially increasing relationship with the net radiation above canopy (Rn) as well as with vapor pressure deficit (VPD). On a typical sunny day, the diurnal peak time of Js in different directions appeared earlier than that of the VPD (1.6 hours in average with the maximum of 2.43 hours). The length of advancing time increased linearly with the increasing of the daily mean VPD. The time advancing reflects that the peak Js of apple was limited by VPD while over 2.0~2.2 kPa. The difference of time advancing between Js and VPD in four directions was not significant (P=0.97). Conclusion There are obvious azimuthal differences (P < 0.01) in the sap flow velocity of the apple tree. The Js in the north side and the south side are high than that of the east or the west side. However, the process of sap flow in different directions is consistent with the influence of atmospheric environment. In order to improve the accuracy of estimating sap flux of apple trees, the azimuthal difference should be taken into account in actual measurement.