Do trees use stemflow water? A manipulative experiment on Singleleaf piñon and Utah juniper in Great Basin woodlands

It has been postulated that stemflow, precipitation that flows from plant crowns down along branches and stems to soils, benefits plants that generate it because it increases plant-available soil water near the base of the plant; however, little direct evidence supports this postulation. Were plants' crowns to preferentially route water to their roots, woody plants with large canopies could benefit. For example, piñon and juniper tree encroachment into sagebrush steppe ecosystems could be facilitated by intercepted precipitation routed to tree roots as stemflow, hypothetically reducing water available for shrubs and grasses. We tested whether Great Basin piñon and juniper trees use and benefit from stemflow. In a drier-than-average and wetter-than-average water year, isotopically labeled water was applied to tree stems to simulate stemflow. Both species took up stemflow, with label signals peaking and receding over 2-4 days. Despite this uptake, no alleviation of water stress was detected in the drier year. The stemflow uptake resulted in some water stress alleviation in the wetter year, specifically for piñons, which took up water from deeper in the soil profile than did junipers. Mixing model analyses suggested that stemflow was a small fraction of the water in stems (⁓0-2%), but an order-of-magnitude larger fraction of the stemflow was transpired in those few days after addition. These findings represent a novel demonstration of the rapid uptake and use of stemflow that infiltrates the rhizosphere, but they also prompt questions about the remaining stemflow's fate and why alleviation of water stress was so minor.