Impacts of eastern hemlock mortality on ecosystem function

Eastern hemlock (Tsuga canadensis) mortality mediated by hemlock woolly adelgid (HWA, Adelges tsugae) infestation is affecting terrestrial and aquatic ecosystem processes. Despite various biological and chemical control strategies of HWA, eastern hemlock is declining throughout much of its range. Tree decline is especially rapid in the southern Appalachians. Intensive measurements on experimental plots and monitoring at the Coweeta Hydrologic Laboratory (CWT) have been ongoing since 2003 with the objectives of determining:

1. The spatial distribution of eastern hemlock in southern Appalachian ecosystems
2. The rates of spread of HWA and decline of HWA-infested trees
3. The impacts of HWA-induced hemlock mortality on water, carbon, nutrient cycling and vegetation dynamics.

In the southern Appalachians, the highest hemlock densities occur within 50 m of streams, where it contributes as much as 50% to the total basal area. At CWT, HWA was first detected in 2003. By 2005, 100% of the trees in monitoring plots were infested; and by 2011, most hemlock trees had died. Increased growth and dominance of the evergreen understory shrub rosebay rhododendron (Rhododendron maximum) may be a major determinant of future responses in southern Appalachian ecosystems; however, our results from sampling the regeneration layer suggest that hemlock will be replaced by a mix of Acer, Betula, Fagus, and Quercus genera where establishment is not limited by rhododendron. Using a combination of sapflow measurements, modeling, and observed community dynamics we estimated that from 2004 to 2010, eastern hemlock die-off along riparian corridors reduced annual forest transpiration (E_t) by 26% and reduced winter E_t by 70%. As hemlock mortality increased, growth of mature deciduous tree species and rhododendron increased, which all had two- to four-fold higher daytime leaf-level transpiration rates than hemlock. Thus, we predict annual stand E_t to recover rapidly (~10 years) and eventually surpass levels observed before HWA infestation. Seasonal patterns of E_t will be permanently altered, however, due to the deciduous habit of species replacing hemlock. Riparian hemlock stands (with infested hemlock trees) had significantly lower spring soil temperatures than riparian hardwood stands (neighboring hardwoods with no hemlock) but did not differ in soil water content. Differences in litterfall chemistry and forest floor and soil nutrient pools suggest that these two ecosystems cycle nutrients differently. Changes in the vegetation composition following hemlock mortality may increase the rates of nutrient cycling as hemlock-dominated riparian forests are replaced by hardwoods. High elevation stands containing dead or dying hemlocks had significantly greater foliar P concentrations and fluxes of P from the forest floor than stands without hemlock at similar elevations, whereas at low and mid-elevations there were no consistent differences between stands. Higher foliar N and P as well as increased growth of hardwood species in high elevation stands suggest that hemlock decline has liberated P and thereby stimulated N uptake by healthy vegetation within this mixed forest. Increased canopy openness, light to the stream, and range in water temperature were already evident by 2010 as hemlock lost foliage mass. Contributions of hemlock to litterfall, in-stream wood and benthic organic matter indicate that the loss of hemlock will likely modify the trophic dynamics and physical structure of streams.