Midwestern Wet Flatwoods

This small-patch system is found throughout the northern glaciated Midwest ranging east into Lower New England and the Champlain Valley. It usually occurs on somewhat poorly drained uplands or in depressions associated with glacial features such as tillplains, lakeplains or outwash plains. Soils often have an impermeable or nearly impermeable clay layer that can create a shallow, perched water table. Saturation can vary, with ponding common during wetter seasons, and drought possible during the summer and autumn months. Microtopography and fluctuating moisture levels can lead to complexes of forest upland and wetland species occurring within this system. Quercus palustris and/ or Quercus bicolor typically dominate the wetter portions and are often associated with Acer rubrum. Quercus alba, Quercus rubra, and Fagus grandifolia are common in the better-drained areas. Carya ovata is a characteristic tree in the Champlain Valley. Liquidambar styraciflua, Nyssa sylvatica, Acer saccharinum, Fraxinus americana, and Fraxinus pennsylvanica are also common associates, though their occurrence varies somewhat by region. Understory herbaceous and shrub species present in examples of this system can vary. Stands with more dense tree cover have less shrub and herbaceous cover, while those with moderate tree canopy cover tend to have a dense understory. Some common species in the wetter portions include Carex spp., Osmunda cinnamomea, Cephalanthus occidentalis, Alnus spp., and Ilex spp. Flooding, windthrow, drought, and fire can influence this system.

Quercus palustris and/or Quercus bicolor typically dominate the wetter portions and are often associated with Acer rubrum. Quercus alba, Quercus rubra, Fagus grandifolia, and Acer saccharum are common in the better-drained areas, seen in some examples around the southern Great Lakes and Lake Champlain. Carya ovata is a characteristic tree in the Champlain Valley. Liquidambar styraciflua, Nyssa sylvatica, Fraxinus americana, and Fraxinus pennsylvanica are also common associates, though their occurrence varies somewhat by region. Understory herbaceous and shrub species present in examples of this system can vary. Stands with more dense tree cover have less shrub and herbaceous cover, while those with moderate tree canopy cover tend to have a dense understory. Some common species include Carex spp., Osmunda cinnamomea, Cephalanthus occidentalis, Alnus spp., and Ilex spp. In the clayplain forests of Vermont, characteristic herbs include Waldsteinia fragarioides and Moehringia lateriflora (= Arenaria lateriflora).

This system usually occurs on poorly drained uplands or in depressions associated with glacial features such as tillplains, lakeplains, or outwash plains. Soils often have an impermeable or nearly impermeable clay layer that impedes waterflow. This favors flooding or ponding in the spring or after heavy rains. It also restricts subsurface water movement into the system and slows the growth of roots through it. Both of these factors lead to water deficits for the vegetation in the late summer and fall. These fluctuating moisture levels can lead to complexes of forest upland and wetland species occurring within this system. Overall topographic relief is very flat in this system though small tip-up mounds and depressions can occur from windthrow and often create small pockets with vegetation more typical of upland or swamp forest, respectively.

The large seasonal change in local available moisture is key to the development and maintenance of this system. Plants must be able to tolerate the excessive available moisture (surface flooding or saturation) and drought conditions that occur in most growing seasons. Fire can occur after the system dries, typically late in the growing season. Fires rarely start in this system but under favorable conditions can spread from nearby fire-prone systems (typically prairies, oak savannas, or oak woodlands). Under proper hydrologic conditions, this system can be self-maintaining (Tecic and McCain 2001). With the often shallowly-rooted trees, strong winds can create canopy openings. Small-scale windthrow is a characteristic disturbance in flatwoods that influences composition and structure by creating canopy gaps that are suitable for the colonization and growth of light-dependent tree seedlings and saplings, shrubs, and herbs. Windthrow also tips and uproots trees, creating pit-and-mound topography that provides suitable microhabitats for a diversity of plant species (Slaughter et al. 2010).

Changes to the hydrologic regime and conversion to agricultural or urban uses are the most common threats to this system. Road building and urban development can cut off or increase waterflow; drainage systems for nearby agriculture can remove water from the system. Fraxinus spp. and Ulmus spp. can invade and become common if the flooding/drying regime is not maintained and fires do not move through the ground layer (Bowles et. al 2003). Invasive shrubs are a problem in some areas. Very few examples remain as almost all have been converted to agriculture. Those sites that do remain typically occur as isolated woodlots in agricultural or urban landscapes, degraded by landscape-scale fragmentation and hydrologic alteration. Additional disturbances that have reduced viability of remnant flatwoods over the past century include the introduction of non-native pests and pathogens (e.g., elm blight and emerald ash borer), invasive plants, and excessive deer herbivory, which have significantly altered community structure, species composition, and successional trajectory (Slaughter et al. 2010). Invasive plants that threaten diversity and structure include Alliaria petiolata, Berberis thunbergii, Elaeagnus umbellata, Frangula alnus (= Rhamnus frangula), Ligustrum vulgare, Lonicera japonica, Lonicera maackii, Lonicera morrowii, Lonicera sempervirens, Lonicera tatarica, Lonicera x bella, Lonicera xylosteum, Rhamnus cathartica, and Rosa multiflora (Kost et al. 2007).

This system is found in the northern Midwest, southern Ontario, and portions of the northeastern U.S.