Total transpiration of plants, mainly trees, can be nowadays estimated or even monitored in time by in-situ sap flow measurements. The sap flow (Qs), represents the passage of water containing inorganic ions through the system of very thin, plant structural pipes called tracheids and vessels within the conductive sapwood area (xylem) of the stems and branches. The Qs is calculated as a product of sap velocity which is also known as sap flux density (v) and sapwood (xylem) area (A) The v is typically estimated with thermal methods that apply heat transfer. The popularity of thermal methods is due to their simplicity, cost effectiveness and easy adaptation to logger based monitoring. The thermal methods such as thermal dissipation probe (TDP) and heat pulse (HP) methods are the most frequently used ones (probably because of relatively low cost) although they have some limitations [13, 14]. The newly developed heat field deformation (HFD) method  seems to overcome most of these limitations but its commercial implementation which is distributed by ICT Instruments is expensive. The sapwood area is the area between bark and non-permeable central part of stems called heartwood. The most common method of assessing Ax is coring of the stem across the sapwood to determine its depth. Other more rigid method requires cutting off branches or even whole trees. Many species show clear visual difference between sapwood and heartwood. Otherwise the staining of the conductive xylem can be applied.
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