Vegetation and invertebrate community response to Eastern hemlock decline in southern New England

Background/Question/Methods

The introduction of Adelges tsugae Annand (Hemlock woolly adelgid; ‘HWA’) to the eastern United States has had a devastating impact on Tsuga canadensis Carrière (Eastern hemlock). Although much research has been done to assess HWA impacts on ecosystem processes and vegetation structure, few researchers have examined community-level changes in highly infested forest stands. We assessed the impact of Eastern hemlock mortality on vegetation and terrestrial invertebrate diversity and community structure in heavily HWA-impacted stands in southern Connecticut with nearby (~50 km) northern Connecticut stands with little HWA-related damage. We sampled the vegetative and invertebrate diversity of eight sites (four low -impact and four high -impact) in the summer and fall of 2008. We used six invertebrate sampling methods and collected 8787 specimens of 623 morphospecies belonging to five different arthropod classes: Arachnida, Malacostraca, Diplopoda, Chilopoda, and Hexapoda.

Results/Conclusions

We found a shift in the understory plant community and the canopy and subcanopy arthropod communities. High-impact sites had more herbaceous and shrub species than low-impact sites (37 ± 3.7 and 27 ± 3.6 species respectively), and almost twice the species richness (14.25 ± 2.6 and 7.75 ± 0.8 species respectively). The most common species at high-impact sites in the herbaceous and shrub layers were Betula lenta L. (Black birch; 18% of individuals), Acer rubrum L. (Red maple; 17%), Maianthemum canadense Desf. (Canada mayflower; 8%), Hamamelis virginiana L. (Witch-hazel; 7%), Quercus rubra L. (Red oak; 7%) and Quercus prinus L. (Chestnut oak; 7%). A single hemlock seedling was found in the understory (<1m) at one high-impact site. Vegetation in the herbaceous and shrub layer at low-impact sites was dominated by T. canadensis (44% of individuals), A. rubrum (13%), and Quercus alba L. (White oak; 13%). Overall, forest invertebrate community diversity was greater in high- impact sites. Of the 21 indicator species significantly associated with a given forest type, 14 and seven species were associated with high- and low-impact forests, respectively. The most abundant indicator species was a Geophilomorpha sp. Centipede; all 223 individuals were collected in low-impact stands. The rarest indicator species were all found in high-impact sites and included species from the Leiobuninae, Nabidae, and Anthicidae families. Variation in arthropod community structure was driven by above-ground differences; ground-level arthropod community composition did not differ between high- and low-impact sites. These results demonstrate some of the biodiversity impacts that can result from the invasion of an exotic insect into forested systems.