Changes in ecohydrological function due to the loss and replacement of eastern hemlock in a New England forest

The disturbance currently occurring across the northeastern United States from the invasive pest hemlock woolly adelgid (Adelges tsugae) provides a unique opportunity to study the impact of community composition on ecosystem function. The spread of hemlock woolly adelgid has resulted in the replacement of eastern hemlock (Tsuga canadensis), an evergreen climax species, by deciduous seral species, mainly black birch (Betula lenta). Ecosystem water cycling has a large potential to be impacted by replacement of hemlock, and was the focus of this research.; Physiological characteristics of individual tree species were found to influence fluxes of water between vegetation and the atmosphere. In Chapter 2, I show the responses to step changes in environmental forces that drive transpiration are faster in black birch compared to eastern hemlock largely due to differences in capacitance. The average time constant in black birch was 8.6 minutes compared to 15.2 minutes in hemlock. Chapter 3 describes a study that shows nighttime transpiration present in paper birch ( Betula papyrifera) but absent in neighboring trees of other species. Over 10% of total daily sap flux was due to transpiration at night in paper birch. Whole-tree water use was scaled to the stand and ecosystem level in Chapters 4 and 5 to estimate the effects of replacement of eastern hemlock by black birch on ecosystem water use and streamflow. During the peak growing season, daily transpiration rates were 1.6 times greater in black birch compared to hemlock. Cumulative transpiration in black birch exceeded hemlock transpiration by 77 mm from June until October. Due to enhanced transpiration, model simulations suggest a significant decrease in flow in August with species replacement.; The results of this research suggest that shifts in physiology as stands transition from being hemlock dominated to being black birch dominated will alter the hydrologic function of the ecosystem. We expect increases in transpiration during the peak growing season months and decreases in streamflow late in the growing season with species replacement. The results highlight the significance of tree physiology and the seasonality of leaf cover in regulating ecosystem water use and streamflow.