SIGNIFICANT HISTORICAL DEVELOPMENTS IN THERMAL METHODS FOR MEASURING SAP FLOW IN TREES

A measurement of the ascent of xylem sap in a tree trunk. has implicat ions in the fields of hydrology and physiology. Taken over a sufficien tly long time period, the quantity of sap flow upward through the stem must equal transpiration at the leaves. The occurrence of transpirati on indicates that stomata are open, a necessary condition for gas exch ange during photosynthesis. Various thermometric methods have been tri ed to estimate xylem sap flow. The earliest attempts used simple relat ionships 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. La stly, stem anatomy was considered mathematically and various sap veloc ity (heat pulse Velocity) and sap flux methods (thermal heat balance) were proposed to place quantitative thermometric methods on a sound ph ysical and theoretical base. A close examination of modern thermometri c 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 t he theory of the conduction and advection of heat in the sap stream wi thin them. Furthermore, an instrumentation technique that may work sat isfactorily on large tree stems (more than 5-6 cm in diameter) will ge nerally not work on smaller stems because of unknown heat losses. Ther mal heat balance (THB) methods are inherently more quantitative than t hose relying on heat pulse velocities (HPV). However, THB techniques r equire more complex instrumentation than HPV methods. And, perhaps mor e importantly, THB methods require considerable power, an important co nsideration in held situations away from electrical mains. In general a combination of THB and HPV methods should be the most useful. The qu antity of sap flow determined with a THE method can be used to calibra te simultaneous velocity measurements made with HPV apparatus.