The structure and composition of Carolina hemlock communities in the Blue Ridge and Ridge and Valley of Virginia

Background/Question/Methods

The near-threatened Carolina hemlock (Tsuga caroliniana Engelm.) is an eastern North American endemic hemlock species found in the Ridge and Valley, Blue Ridge, and Piedmont of the southeastern United States.  While more closely related genetically to the Asian hemlocks, than to  North American western and eastern hemlocks, the Carolina hemlock  has been severely impacted by the hemlock woolly adelgid (Adelges tsugae Annand; HWA).  We investigated three Carolina hemlock communities near the species’ northern range in the Virginian Ridge and Valley (Sinking Creek Mountain) and Blue Ridge (Camp Powhatan and Bottom Creek Gorge) to better understand the structure and composition of this species and the impact of the HWA.  We identified, inventoried, and measured the diameter at breast height, total height, and crown canopy diameter for all woody stems within randomly selected 20 m by 50 m plots.  If we encountered live Carolina hemlocks with the HWA, we measured the ratio of live to dead crown.  Tree seedlings and saplings were also inventoried in 20 m by 20 m subplots and canopy cover was also documented for all plots.  Bottom Creek Gorge was sampled in 1997 and was not infested with the HWA; we resampled this site to determine the status of the HWA and its current structure and composition.

Results/Conclusions

While all three communities differed in topography, occurring along a ridgeline, a creek, and on a steep slope in a gorge, all sites were constricted to northwest or northern facing slopes with shallow soils.  All the Carolina hemlocks within our plots at Bottom Creek Gorge were either snags or had fallen, with hardwoods recruiting into their gaps.  At Sinking Creek Mountain and Camp Powhatan, Carolina hemlock was most prevalent in cover and dominance with chestnut oaks or northern red oaks sporadically piercing through the Carolina hemlock canopy.  Carolina hemlock was not recruiting at Sinking Creek Mountain as the HWA had killed all seedlings, saplings, and subcanopy Carolina hemlocks with hardwoods recruiting in.  However, recruitment of Carolina hemlock was occurring at Camp Powhatan due to HWA eradication.  While the HWA has affected all three communities, recruitment patterns suggest that if the HWA continues to kill off Carolina hemlocks, these communities will be replaced with oaks, maples, and other hardwood species.