Hemlock Woolly Adelgid induced mortality of eastern hemlock influences timing and magnitude of streamflow from headwater catchments in the southern Appalachians

Eastern hemlock (Tsuga canadensis (L.) Carr.), an important evergreen canopy species in the southern Appalachian Mountains, is currently experiencing widespread mortality from hemlock woolly adelgid (HWA; Adelges tsugae Annand). We used a paired watershed approach to estimate the impact of hemlock mortality on the timing and magnitude of stream discharge for a watershed that has lost ~6% basal area to hemlock woolly adelgid induced mortality since 2003. Stream discharge records from two closely–located watersheds similar in elevation and aspect, but differing in hemlock density, were used to estimate the impact of hemlock mortality on watershed yield, timing of stream discharge, and stream responses to storms. We hypothesized that hemlock mortality would result in transient increases in annual yield (ca. 10%) but changes in the magnitude and timing of discharge would be greater in the dormant season (Oct–Mar) and persistent through time. We found that although the affected watershed showed significant increases in yield during some months after HWA infestation and subsequent mortality, annual yield did not increase. In fact, annual yield was significantly lower than expected four out of seven years after infestation, perhaps due to rapid growth response of understory species with less conservative rates of leaf-level transpiration. Although annual yield did not increase as expected, discharge frequency distributions shifted toward higher discharge events during the dormant season in the affected watershed despite no change in the precipitation frequency distribution. Additionally, peak flow increased 16.4 ± 10.8% and quick flow increased 35.7 ± 29.6% for precipitation events greater than five cm during the dormant season. Discharge frequency distributions, peakflow and quickflow did not change during the growing season. Although hemlock represented a relatively small proportion of basal area in the infested watershed and annual yield did not increase as expected, the evergreen hemlock canopy appeared to serve an important function in moderating stream discharge during periods when deciduous species were physiologically inactive. Hence, eastern hemlock loss may enhance the risk of high flows from large storm events during the dormant season.