A major gap in our ecohydrological understanding of forests is how the hydrological connections between transpiration and streamflow influence the source and age distributions of these water fluxes. The first step is understanding the timing and manner of the hydrological connections between these fluxes. We combined observations of transpiration onset with isotope tracing to investigate this connectivity in a hillslope within the Canadian shield. We traced maple (Acer saccharum), beech (Fagus grandifolia), and hemlock (Tsuga canadensis) transpiration sources during snowmelt. Over the course of snowmelt, subsurface water isotopic signatures became increasingly depleted in heavier isotopes. Hemlock transpiration onset occurred during snowmelt, and hemlock xylem water then reflected subsurface flow signatures, indicating an overlap between transpiration and streamflow sources. Subsurface water (i.e. new water) contributions were large during transpiration onset (72%), while a smaller contribution came from old water (tree storage). The old water contribution increased as hemlocks continued to transpire. The isotopic evolution of broadleaves is distinct from evergreens. Maple and beech started to transpire in May after snowmelt and during leaf out. Their xylem water became increasingly positive during bud development, indicating evaporative enrichment; and then, more depleted during transpiration onset. While in evergreens most of the water contributing to the onset of transpiration is new, in broadleaves storage might have a larger contribution. In maples, sap water contributed 50% to transpiration during onset. Our investigation provides evidence of the connectivity between transpiration source water and streamflow during snowmelt and the relative importance of tree water storage to transpiration.