Double buffering of transpiration: Soil and stem water storage regulate transpiration water ages in boreal forests

Transpiration dominates terrestrial water fluxes and plays a critical role in ecosystem productivity and climate regulation, yet the travel time of water within vegetation and the contribution of internal plant water storage to transpiration remain poorly constrained. While decades of tracer studies have revealed that streamflow is sustained by older subsurface storage and responds dynamically to wetness conditions, comparable insights for transpiration are lacking. Here we present the first integrated field-based assessment of transpiration water age in boreal forests, continuously tracking transit times from the onset to the end of the growing season. Using isotope sampling of xylem, soil water, and precipitation, combined with hydrometric measurements at boreal sites dominated by Picea mariana and Pinus banksiana, we quantified mean travel times and the contribution of new versus old water to transpiration. Our results reveal that transpiration is sustained primarily by water older than one week, with newer precipitation contributing only 20–40% to transpiration fluxes. Growing-season travel times were faster than spring-only estimates but consistent with peak-summer sap-flow measurements. These findings demonstrate a "double-buffering" effect: soil water storage dampens and delays isotopic signals from new precipitation, while stem water storage further attenuates the response, particularly in drier periods. This dual buffering mechanism regulates transpiration age dynamics in response to changing wetness conditions, with storage contributions varying throughout the growing season. Our study provides critical empirical constraints on vegetation water use and transit times, essential for improving ecohydrological models and predicting ecosystem responses to water availability under changing climates.