Infestation of eastern hemlock (Tsuga canadensis (L.) Carr.) with hemlock woolly adelgid (HWA, Adelges tsugae) has caused widespread mortality of this key canopy species throughout much of the southern Appalachian Mountains in the past decade. Because eastern hemlock is heavily concentrated in riparian habitats, maintains a dense canopy, and has an evergreen leaf habit, its loss is expected to have a major impact on forest processes, including transpiration (Et). Our goal was to estimate changes in stand-level Et since HWA infestation, and predict future effects of forest regeneration on forest Et in declining eastern hemlock stands where hemlock represented 50–60% of forest basal area. We used a combination of community surveys, sap flux measurements, and empirical models relating sap flux-scaled leaf-level transpiration (EL) to climate to estimate the change in Et after hemlock mortality and forecast how forest Et will change in the future in response to eastern hemlock loss.
From 2004 to 2011, eastern hemlock mortality reduced annual forest Et by 22% and reduced winter Et by 74%. As hemlock mortality increased, growth of deciduous tree species—especially sweet birch (Betula lenta L.), red maple (Acer rubrum L.), yellow poplar (Liriodendron tulipifera L.), and the evergreen understory shrub rosebay rhododendron (Rhododendron maximum L.)— also increased, and these species will probably dominate post-hemlock riparian forests. All of these species have higher daytime EL rates than hemlock, and replacement of hemlock with species that have less conservative transpiration rates will result in rapid recovery of annual stand Et. Further, we predict that annual stand Et will eventually surpass Et levels observed before hemlock was infested with HWA. This long-term increase in forest Et may eventually reduce stream discharge, especially during the growing season. However, the dominance of deciduous species in the canopy will result in a permanent reduction in winter Et and possible increase in winter stream discharge. The effects of hemlock die-off and replacement with deciduous species will have a significant impact on the hydrologic flux of forest transpiration, especially in winter. These results highlight the impact that invasive species can have on landscape-level ecosystem fluxes.