Abstract
Adelges tsugae Annand (hemlock woolly adelgid, HWA, an invasive insect native to Japan), which causes defoliation and death of Tsuga canadensis (L.) Carrière (eastern hemlock), was introduced to the United States in the early 1950s and has spread throughout much of the range of T. canadensis causing widespread mortality. In 2016 and 2017, we resampled long-term vegetation monitoring plots across five forest types (ecogroups) within Great Smoky Mountains National Park that contained T. canadensis in 2003 (prior to the spread of HWA within the park) to examine changes in the species composition and diversity of the regeneration layer. We hypothesized that compositional changes in the seedling and sapling strata would be driven primarily by the pre-HWA importance value of T. canadensis and relative dominance of Rhododendron maximum L. (rosebay rhododendron), and that species diversity metrics would differ across year depending on ecogroup and the relative dominance of R. maximum. Non-metric multi-dimensional scaling (NMDS) revealed that the seedling and sapling strata of plots with greater pre-HWA importance of T. canadensis and lower basal area of R. maximum generally exhibited greater compositional change between 2003 and 2017. Topo-edaphic variables were also significant in both NMDS ordinations and were associated with the distribution of multiple hardwood species. Species richness, evenness, and Shannon-Wiener diversity varied with strata following the loss of T. canadensis, with the degree and direction of change varying with the dominance of R. maximum.
Highlights
With increased globalization and movement of material, humans have aided the invasion of forests by non-native pests and pathogens [1,2,3] that contribute to biodiversity loss [4] by altering ecosystem structure and function [5,6,7]
Overstory basal area declined between years across ecogroups (F = 6.87, p = 0.014; Table 1), ranging from no change in acid cove forests to a 24% decline in montane oak-hickory forests (35.4 ± 6.0 m2 ha−1 in 2003 to 27.0 ± 4.8 m2 ha−1 in 2017)
While treated T. canadensis trees survive within conservation areas, we observed few surviving trees outside of these areas, and those that were alive showed obvious signs of decline
Summary
With increased globalization and movement of material, humans have aided the invasion of forests by non-native pests and pathogens [1,2,3] that contribute to biodiversity loss [4] by altering ecosystem structure and function [5,6,7]. Over 450 non-native forest insects have been detected in the United States since 1635, sixteen of which are considered high-impact due to regulatory significance or documented negative effects on forest trees [6]. Invasive insects have both direct and indirect effects [7,8,9] that range in scale from the defoliation and death of individual trees, to long–term changes in forest composition, structure, productivity, and nutrient cycling that may cascade throughout the ecosystem [8,9]. Soon after the loss of C. dentata in the southern Appalachians, canopy gaps were filled by co-dominant species such as Quercus prinus L. (chestnut oak), Quercus rubra L
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