新疆杨边材贮存水分对单株液流通量的影响

树木体内贮存水分量的大小及其参与液流循环的程度被认为是树木抵御干旱胁迫的重要机制之一.以我国北方广泛适生的新疆杨为研究对象,利用热扩散技术(TDP)分别监测了树冠基部、树杆基部处液流通量,并结合同步气象观测,分析了树杆不同高度处液流通量与大气蒸发潜力(ET0)间的关系,发现:冠基部的液流通量(Qu)是估算新疆杨单株液流通量可靠的指标,试验期间(6-9月)典型晴天日Qu日平均值为(7.61±0.65) L/d,比云天、阴天分别高0.41和2.71 L/d.新疆杨杆基部液流通量(Qd)在不同季节变化较大,在前3个月的典型晴天日,Qd小于Qu,树木处于失水过程,但在8月下旬连续多日降雨后迅速增加而反超,新疆杨能通过这种方式缓解季节间的水分亏缺,维持水分的总体平衡.在8月下旬连续多日降雨、土壤水分得到有效改善后,Qd占全天液流总量的比例由先前的31.98％-35.36％下降到6.72％-7.99％,夜间液流成为补偿与缓解日间水分亏缺的重要方式.在整个生长季内,新疆杨体内经历了水分补充(6月)—失水(7、8月份)—再补充(9月)的过程.ET0是评价液流环境驱动效应可靠的综合变量,建立基于Logistic方程的模型可通过ET0较好的估算Qu,估算新疆杨Qu的上限约为7.82L/d.分析显示当ET0超过5mm/d时,Qu、Qd均不再随ET0的增加而增加或有下降,显示了树木主动保护、抵御干旱的策略.8月下旬连续降雨使得新疆杨蒸腾量占大气蒸发潜力的比例(T/ET0)由先前的0.32增加到之后的0.47,可以看出土壤水分改善对液流量的贡献较大.

Impacts of water stored in sapwood Populus bolleana on its sap flux

Water amount stored in sapwood and its participation in sap flow cycle are considered to be one of the important mechanisms for trees' drought resistance. Based on monitoring the sap flux of Populus bolleana at crown base and stem base with thermal dissipation probe technique (TDP), combined with synchronous meteorological observation, this paper analyzed the relationship between sap flow fluxes and atmospheric evaporation potential(ET₀) at two positions. Results showed that sap flow flux at crown base (Q_u) was a reliable indicator for estimating sap flux of P. bolleana individuals. The average daily Q_u in typical sunny days from June to September was (7.61±0.65) L/d, which was higher than those in cloudy and overcast days by 0.41 and 2.71 L/d respectively. The sap flow flux in stem base (Q_d), varied greatly in different seasons, was lower than Q_u during the sunny days from June to August but reversed rapidly in late August following continuous rainfall, by which P. bolleana maintained water-balance and solved water deficit partly. After continued rainfall in late August, soil moisture improved effectively, so the percentage of nocturnal sap flux to total daily Q_d reduced from 31.98-35.36% to 6.72%-7.99%, by which water deficit could be compensated effectively. P. bolleana have experienced water supplement (in June), loss (in July and August) and re-supplement (in September) processes during the whole growing season. ET₀ was a reliable integrated variable for evaluating sap flow environment-driving effects which could be modeled well with Q_u by Logistic equation, the modeled results showed that the maximum of Q_u was 7.82 L/d. When ET₀ is more than 5 mm/d, Q_u and Q_d no longer increased, which is a kind of positive drought resisting strategy of Populus bolleana. The continuous rainfall in late August caused the ratio of transpiration to ET₀ (T/ ET₀) increased from 0.32 to 0.47, obviously, the improvement of soil moisture affected sap flux of P. bolleana significantly.