Emerald ash borer (EAB) (Agrilus planipennis), non-native shrubs, and white-tailed deer (Odocoileus virginianus) may profoundly affect forest seedling composition. Each stressor has independently been shown to affect forest seedling layers. Our objective is to determine their relative importance and additive effects.
We sampled 24 sites (three plots each), representing a natural gradient in ash mortality, variation in deer browse, and extent of introduced shrub invasion. Nested within each plot (400m2) was a sub-plot (200m2) and four micro-plots (4m2). Within plots we assessed ash condition and measured all trees ≥10 cm diameter at breast height, and Sugar Maple Browse Index was used to assess deer density. In sub-plots we measured introduced shrub cover and basal area (BA) of the eight largest shrubs of each species. Within micro-plots, all seedlings 20-100 cm were identified. Our analyses were conducted using model averaging on our full set of models using Akaike’s Information Criterion framework. Models were composed of the effects of ash death, deer, and introduced shrubs on native and introduced seedling abundance, species richness, and Shannon diversity. Models also included canopy tree BA and weighted mean waterlogging tolerance of canopy species. Ordination using distance-based redundancy analysis w/ Bray-Curtis dissimilarity was employed to test roles that these same predictors had on seedling community composition.
Results/Conclusions
Seedling community composition was shaped primarily by stand BA, waterlogging index, and non-native shrub BA, based on ordination. Seedling communities with more stand BA were characterized by spicebush (Lindera benzoin), whereas sites with less were characterized by ash (Fraxinus spp.) and hawthorn (Crataegus spp.). Sugar maple (Acer saccharum), hackberry (Celtis occidentalis), and pawpaw (Asimina triloba) were associated with drier sites. Seedling communities in sites with more non-native shrub BA were characterized by Amur honeysuckle (Lonicera maackii).
Our findings indicate that site characteristics such as soil wetness and non-native shrub invasion are more important for shaping woody seedling communities than EAB-caused ash mortality. Furthermore, suggesting that underlying site specific factors may play a larger role in shaping future forest composition than acute stressors like EAB.