Water use by eastern hemlock (Tsuga canadensis) and black birch (Betula lenta): implications of effects of the hemlock woolly adelgid

Eastern hemlock (Tsuga canadensis (L.) Carr.) is a coniferous evergreen species found across the northeastern United States that is currently threatened by the exotic pest hemlock woolly adelgid (HWA; Adelges tsugae Annand). As HWA kills eastern hemlock trees, black birch (Betula lenta L.) has been found to be a dominant replacement species in the region. Seasonal changes in water use by eastern hemlock and black birch were investigated utilizing whole-tree transpiration measurement techniques. Annual evapotranspiration in an eastern hemlock and deciduous stand was also estimated. During the peak growing season, daily rates of transpiration were 1.6 times greater in black birch. Cumulative transpiration in black birch exceeded hemlock transpiration by 77 mm from June until October. During the dormant season, evapotranspiration rates were higher in the hemlock stand; however, estimated annual evapotranspiration was 327 mm in eastern hemlock compared with 417 mm in the deciduous stand. Our results suggest that a transition from a hemlock-dominated to a black birch-dominated stand will alter the annual water balance with the greatest impact occurring during the peak growing season. Late in the growing season, flow may be unsustainable in streams that normally have light or moderate flow because ofincreased water use by black birch.