柠条主根液流测定中ΔT_(max)与气象因子间的关系及时间步长的确定

上下两探针间温差的最大值(ΔTmax)是运用Granier方法计算树木液流速率的重要参数,确定这一参数的关键是选择合适的时间步长。运用TDP(Thermal Dissipation Probe)技术在对柠条(Caragana korshinskii)主根部液流速率(Fs)及主要环境要素进行实时、连续监测的基础上,比较分析了2008年4—10月间30d典型晴天日柠条根部液流的变化特征,结果表明,柠条根部液流测定中ΔTmax值的频率分布于夜间22:00—6:00的整个时间段,并主要集中在午夜前后的4h(占52.63%),其中ΔTmax在0:00频率最高。ΔTmax分布时刻与环境要素的相关性不明显,夜间液流的主要驱动力是体内与根系间的水势差而非大气因子。午间柠条主根液流最为旺盛,上下两探针间最小温差(ΔTmin)的频率分布峰值主要集中于10:00—16:00时段,其中以14:00的分布频率最高。ΔTmin频率的分布在13:00有一低谷,而此刻又是主要环境变量太阳辐射最高值(PYmax)与潜在蒸发散最大值(ET0max)分布频率最高的时刻,反映了午间气孔行为对液流的调控。太阳辐射是启动午间气孔行为的主要环境因子。Granier公式计算液流速率时适宜的时间步长为1周。

The relationships between △T_(max) and meteorological factors and determining a suitable time step in calculating taproot sap flow of Caragana korshinskii

The △T_(max), defined as the maximum of temperature difference between the two probes which implant adjacently in the upper and lower sap wood of the tree stem or root, is an important parameter for measuring sap flow by the Granier technique. The key for computing △T_(max) is determining a time step. The sap flow in the taproot of Caragana korshinskii from April to October in 2008 was measured continually and the environmental factors were recorded simultaneously in this study, and data from 30 sunny days were chosen for analysis. The results showed that △T_(max) was observed most frequently in the period from 22:00 to 6:00, and was higher in the 4 hours prior to and after midnight(with a frequency of 52.63%). The highest value occured at 0:00. The time of occurrence of △T_(max) was not significantly correlated with environmental factors and the driving force for sap flow was the water potential difference between leaf and root rather than the environmental factors. The △T_(min), defined as the minimum of temperature difference between the two probes, occurred most frequently between 10:00 and 16:00, with the highest value at 14:00. There was a local minimum in △T_(min) at 14:00 when the solar radiation and the potential evapotranspiration (PY_(max) and ET_(0max), respectively)achieved their peak values. This was attributed to stomata regulation on sap flow at noon. Solar radiation was a key environmental factor triggering stomata regulation at noon. The suitable time step in F_s' computing by Granier technique is one week.