Continuous In-Situ Water Stable Isotopes Reveal Rapid Changes in Root Water Uptake by Fagus sylvatica During Severe Drought.

Adaptation of root water uptake (RWU) is critical for drought resilience in temperate forest trees, yet information on water sources and uptake depths dynamics is scarce. Continuous in-situ stable isotope measurements in soil and xylem water of Fagus sylvatica during the severe drought 2022 revealed daily changes in RWU depth and water ages. Xylem water comprised mainly recent precipitation in early summer, but winter and spring precipitation contributed up to 70% during drought, with longer transit times (206 ± 60 days) compared to summer precipitation (62 ± 11 days). Concurrently, trees shifted RWU to deeper soil layers while also responding to individual precipitation events by absorbing fresh precipitation from topsoil layers within 2-4 days, demonstrating the significance of individual precipitation events for tree water dynamic. F. sylvatica used > 80% of a fresh precipitation event before drought, but < 20% during recovery, indicating potential drought legacies on precipitation use. Unravelling these rapid dynamics in RWU and water ages offers novel insights into the importance of single and seasonal precipitation events for forest water fluxes.