Hemlock Declines Rapidly with Hemlock Woolly Adelgid Infestation: Impacts on the Carbon Cycle of Southern Appalachian Forests

The recent infestation of southern Appalachian eastern hemlock stands by hemlock woolly adelgid (HWA) is expected to have dramatic and lasting effects on forest structure and function. We studied the short-term changes to the carbon cycle in a mixed stand of hemlock and hardwoods, where hemlock was declining due to either girdling or HWA infestation. We expected that hemlock would decline more rapidly from girdling than from HWA infestation. Unexpectedly, in response to both girdling and HWA infestation, hemlock basal area increment (BAI) reduced substantially compared to reference hardwoods in 3 years. This decline was concurrent with moderate increases in the BAI of co-occurring hardwoods. Although the girdling treatment resulted in an initial pulse of hemlock needle inputs, cumulative litter inputs and O horizon mass did not differ between treatments over the study period. Following girdling and HWA infestation, very fine root biomass declined by 20-40% in 2 years, which suggests hemlock root mortality in the girdling treatment, and a reduction in hemlock root production in the HWA treatment. Soil CO₂ efflux (E soil) declined by approximately 20% in 1 year after both girdling and HWA infestation, even after accounting for the intra-annual variability of soil temperature and moisture. The reduction in E soil and the concurrent declines in BAI and standing very fine root biomass suggest rapid declines in hemlock productivity from HWA infestation. The accelerated inputs of detritus resulting from hemlock mortality are likely to influence carbon and nutrient fluxes, and dictate future patterns of species regeneration in these forest ecosystems.