Palm trees are arboreal monocots that don’t grow secondary xylem. While in dicot and coniferous trees sap flux density is high in the stem’s recent outer secondary xylem and decreases inward, in palm trees studies have shown that flux density is high in the inner stem and decreases outward. As date palms are economically important, understanding their water use is of great interest. In the past two decades a number of studies have demonstrated that sap flow can be measured with thermal dissipation probes, but few studies have focused on flux distribution in the stem.
We measured sap flow at two depths and two azimuths along with 3-D electrical resistivity tomography (ERT) in the stem of a canary date palm tree, located at the Volcani Center in Bet Dagan. We hypothesized that 3-D images from ERT, which informs mainly water content, would provide insight about the radial distribution of sap flow and water capacitance of the stem.
The tree was not irrigated for some time prior to the experiment, which began in May 2023, and continued into early 2024. We irrigated continuously for three days and then followed sap flow and ERT as the stem rehydrated and sap flow increased. ERT showed high resistivity at the center of the stem (relatively low water content), a ring of approximately 5 cm width with lower resistivity half (to 2/3) way into the stem, and higher resistivity toward the outer stem. Seasonal courses of changes in resistivity resembled those of canopy conductance.
