Significant historical developments in thermal methods for measuring sap flow in trees

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A measurement of the ascent of xylem sap in a tree trunk has implications in the fields of hydrology and physiology. Taken over a sufficiently long time period, the quantity of sap flow upward through the stem must equal transpiration at the leaves. The occurrence of transpiration indicates that stomata are open, a necessary condition for gas exchange during photosynthesis. Various thermometric methods have been tried to estimate xylem sap flow. The earliest attempts used simple relationships between the first onset of an induced heat pulse measured at one or more points downstream in the stem from the heat induction site. This was followed by a compensation technique in which heat flow in both the upstream and downstream directions was detected to allow for conduction, as well as advection of heat from one point to another. Lastly, stem anatomy was considered mathematically and various sap velocity (heat pulse velocity) and sap flux methods (thermal heat balance) were proposed to place quantitative thermometric methods on a sound physical and theoretical base. A close examination of modern thermometric methods of measuring xylem sap flow will reveal that there is no one set of theory and instrumentation that is applicable to all sizes or species of trees. Trees have three major xylem sap conducting systems that present quite different physical systems from the standpoint of the theory of the conduction and advection of heat in the sap stream within them. Furthermore, an instrumentation technique that may work satisfactorily on large tree stems (more than 5–6 cm in diameter) will generally not work on smaller stems because of unknown heat losses. Thermal heat balance (THB) methods are inherently more quantitative than those relying on heat pulse velocities (HPV). However, THB techniques require more complex instrumentation than HPV methods. And, perhaps more importantly, THB methods require considerable power, an important consideration in field situations away from electrical mains. In general a combination of THB and HPV methods should be the most useful. The quantity of sap flow determined with a THB method can be used to calibrate simultaneous velocity measurements made with HPV apparatus.