The effect of axial conduit widening on sap flow sensors readings

It is well known that, in woody species, the vascular structure is designed for compensating the potential progressive increase in hydraulic resistance, which would be caused by the increasing distance from roots to leaves with ontogenesis. This hydraulic compensation is essentially achieved by a simple axial anatomical change: the conduit lumina become larger basipetally (i.e., conduit widening). This implies that small plants have smaller conduits at their stem base (where sap flow is commonly measured) than tall plants. Since the dimensions of the sap flow probe are constant, our main concern is that the sensor will thermally affect a similar area of sapwood in tall and small plants but the vessel density will be systematically lower in tall plants (due to cell widening downwards). Indeed, sap flow density (Fd) is reported to be generally lower in taller trees. However, no clear explanation of this variation has been provided yet. By using our own measurements and data in the literature, we demonstrated that variation in vessel density explains the relative variation in measured Fd. If this relationship holds true, by knowing the scaling of leaf and sapwood area with tree height we could assess the capacity of trees to adjust their hydraulic architecture to compensate for the increased path length resistance during ontogeneis. �� ISHS 2013.