Long term monitoring of vegetation and avian communities in hemlock stands

Background/Question/Methods

Introduced forest pests such as the hemlock woolly adelgid (Adelges tsugae; HWA) can substantially change native forest ecosystems and the wildlife communities they support.  In 1998, prior to HWA infestation, we initiated a long-term hemlock ecosystem monitoring study on the New River Gorge National River (NERI) and Gauley River National Recreation Area (GARI), in Nicholas, Fayette, and Raleigh counties, West Virginia, to quantify effects of HWA on forest ecosystem dynamics.  The study was designed to measure change across the broad landscape represented by NERI and GARI, covering an extensive area spanning ~97 km north to south and elevations ranging 427–853 m.  Hemlock trees occur throughout in moist coves, along stream and river corridors, and in mixed conifer/deciduous forests on rock outcrops at the rim of gorges.  Hemlock tree vigor and degree of adelgid infestation were sampled in each fall 1998-2009 except 1999 and 2005; vegetation structure and composition were sampled in summer 1999 and 2007; and avian populations were sampled each summer 1999-2009. 

Results/Conclusions

HWA was first detected on sampling plots in 2004 on 8 of 36 plots.  As of 2009, all plots have had trees with signs of HWA infestation.  Xeric and mesic plots were infested more heavily than hydric plots.  Although hemlock crown vigor has declined over time, little hemlock tree mortality has occurred and consequently, vegetation structure had changed little in 2007.  Since 1999, 76 different avian species have been detected on and adjacent to the sampling plots; the majority (65) were songbird species.  Overall species richness of songbirds declined over time, as did richness and abundance of the forest interior and interior-edge habitat guilds and the majority of nesting guilds.  In contrast, species richness and abundance of the early successional habitat guild had increasing trends as did species richness of the shrub nesting guild.  The limited change detected in vegetation structure on the plots suggests that changes in the bird community likely were not related directly to changes in vegetation structure but were due to other factors.  The literature suggests that hemlock tree mortality can occur 4-6 years after infestation; thus, we expect hemlock mortality to begin occurring within the plots in the near future.  Continued monitoring is needed to document response of the hemlock ecosystem to HWA and to improve our understanding of the impacts of the decline or elimination of this habitat type.