Co-Variation among Vegetation Structural Layers in Forested Wetlands

Abstract

Co-variation among vegetation structural layers occurs in some forests but has been minimally found in forested wetlands. We assessed co-variation in six vegetation layers (three size classes of trees, tree seedlings, shrubs, and herbs) in 39 forested wetlands including in five before and after invasion by the beetle emerald ash borer (Agrilus planipennis; EAB) in northwestern Ohio, USA. Across the 39 wetlands, cover of wetland herbs requiring full sunlight was negatively related to tree density, while herbs tolerant of shade minimally co-varied with tree layers. Several wetlands contained both large trees and regeneration (small trees and seedlings) of the same tree species, but often distributions of large trees and regeneration were disjointed. Variation in plant community quality (e.g., non-native cover, floristic quality) for understory layers was partly modeled (34–75% of variance) using multivariate combinations of tree layers. Low-density pin oak (Quercus palustris) flatwoods, which contained species-rich, high-quality understories, drove much co-variation in vegetation layers, suggesting that coupling of layers might occur on only segments of landscape gradients. Another factor was that nearly every site had a different dominant herbaceous species, producing extreme compositional heterogeneity (94% dissimilarity among plots), limiting possible co-variation, but creating high beta diversity. On the five long-term EAB sites, shrub and herb composition co-varied before EAB invasion, but not 14 years later after shrub cover doubled. High diversity in vegetation layers among sites suggests that conserving forested wetlands, including sites that individually might not have high floristic quality, can be a major contributor to landscape diversity.