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.

植被结构层之间的协变发生在某些森林中，但在森林湿地中很少发现。我们评估了39个森林湿地中6个植被层（树木，树苗，灌木和草药的3个大小等级）的共变，包括在甲虫翡翠灰bore（Agrilus planipennis）入侵之前和之后的5个中; EAB），位于美国西北俄亥俄州。在这39个湿地上，需要充分阳光照射的湿地草本植物覆盖物与树木密度成负相关，而耐荫性的草本植物与树木层的变化极小。几个湿地既包含大型树木，又包含相同树种的再生（小型树木和幼苗），但是大型树木和再生的分布常常不连贯。林下层的植物群落质量的变化（例如，非本地覆盖，植物区系）采用树的多变量组合进行了部分建模（方差的34％至75％）。低密度针叶栎（Quercus palustris）含有丰富物种，高品质林木的扁平木带动了植被层之间的大量协变，这表明层间的耦合可能仅发生在景观梯度的某些部分上。另一个因素是，几乎每个站点都有一个不同的优势草本物种，产生极端的组成异质性（地块之间有94％的相似性），限制了可能的共变，但产生了高β多样性。在五个长期的EAB站点上，灌木和草本植物成分在EAB入侵之前发生了变化，但在14年后灌木覆盖率却翻了一番。地点之间植被层的高度多样性表明，保护森林湿地（包括个别可能没有较高植物区系的地点）可能是景观多样性的主要贡献者。