Steppe Kathy, Lemeur Raoul, Dierick Diego
Department of Applied Ecology and Environmental Biology, Laboratory of Plant Ecology, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium.
Funct Plant Biol. 2005 Aug;32(7):599-609. doi: 10.1071/FP04242.
Results from measurement of sap flow by heat balance sensors on the stem of a young oak tree (Quercus robur L.) revealed that thermal disequilibrium (i.e. heat storage) within the heated stem segment can introduce considerable errors in the measured sap-flow rates. The magnitude, sign and significance of these errors depend on the sap-flow rate and on the relationship between stem temperature and air temperature. Sap-flow rates were found to be more prone to errors caused by heat storage effects under low flow conditions than at higher rates of sap flow. Furthermore, daytime fluctuations of air temperature and stem temperature inside the heat balance sensor were either in phase when a low, or in opposite phase when a high sap-flow rate was passing through the stem of the young tree. To investigate this relationship, we developed an experimental set-up with cut stem segments through which tap water could be pressed. This set-up allowed the effects of air temperature and sap-flow rates on stem temperatures within heat balance sensors to be clearly separated. Good mathematical relationships were obtained and were successfully used to assess the relative importance of air temperature and sap-flow rate with respect to the fluctuations in stem temperature of the young oak tree. Based on the established relationships, a novel approach was put forward to correct for errors introduced into sap-flow calculations caused by heat storage effects if no measured data on stem temperature are available.
通过热平衡传感器对一棵年轻栎树（欧洲栎）树干的液流进行测量的结果表明，加热树干段内的热不平衡（即蓄热）会在测量的液流速率中引入相当大的误差。这些误差的大小、符号和显著性取决于液流速率以及树干温度与气温之间的关系。研究发现，在低流量条件下，液流速率比在较高液流速率时更容易受到蓄热效应引起的误差影响。此外，当低液流速率通过幼树树干时，热平衡传感器内的气温和树干温度的日间波动是同相的，而当高液流速率通过时则是反相的。为了研究这种关系，我们开发了一种实验装置，使用切割的树干段，通过该装置可以压入自来水。这种装置能够清晰地分离气温和液流速率对热平衡传感器内树干温度的影响。获得了良好的数学关系，并成功地用于评估气温和液流速率对年轻栎树树干温度波动的相对重要性。基于已建立的关系，提出了一种新方法，在没有树干温度测量数据的情况下，校正由蓄热效应引入液流计算中的误差。
