Water use by mature and sapling western juniper (Juniperus occidentalis) Trees

A common argument for western juniper (Juniperus occidentalis) control in Oregon is the amount of water saved due to reduction in the number of trees. Yet a good understanding of water use by mature trees and sapling regrowth following mature juniper removal has not been documented. Such information is important to better assess how much water can be saved by juniper control. We used sap flow sensors to monitor water use by mature juniper in a control area and saplings in an area where juniper control occurred in 2005. Sap flow data collected between May 2017 and Sep 2019 showed the period of highest water use was in the summer, although this varied each year. In July 2017 (wettest yr of the study), mature trees used 144 L/d, approximately twice as much as in the average of July 2018 and 2019 (dry yr). During the period of maximum water uptake, mature trees used between 45 and 69 times more water than saplings depending on precipitation and, consequently, soil water availability. In summary, 1) juniper water use varies greatly with precipitation and 2) because of the large difference between mature and sapling trees, juniper control results in considerable water savings, even after a 14-yr period of juniper regrowth.