Midwestern and Appalachian Rich Swamp

These forested wetlands are scattered throughout the north-central Midwest (south of the Laurentian region), the north-central Appalachians and southern New England at low to mid elevations. They are found in basins where higher pH and/or nutrient levels are associated with a rich flora. Species include Acer rubrum, Fraxinus nigra, as well as calciphilic herbs. Conifers include Larix laricina, but typically not Thuja occidentalis, which is characteristic of more northern wetland systems. There may be shrubby or herbaceous openings within the primarily wooded cover. The substrate is primarily mineral soil, but there may be some peat development.

Water can come from nutrient-rich groundwater or surface runoff. Sites are basins or low areas in floodplains, usually near the edge of the floodplain in a localized basin or at the base of a bluff where groundwater emerges. Soils are muck or fine-textured mineral. Small hummocks and depressions, created from tree tip-ups, sluggish streams, or tree root build up, create drier and wetter microsites within the system. Sites are usually flooded in the spring, and low areas may remain wet for all or most of the growing season, but if stands remain under water for multiple years, the trees die (Kost et al. 2007). The microsite differences allow a mixture of wet-mesic upland species and wetland species to exist in the herbaceous layer of this system (WDNR 2015).

The hydrologic regime is critical to maintenance of this system. Sites must be wet or flooded for part of the growing season but not completely saturated or under water for too long over a large portion of the site. Periodic sustained floods or droughts can kill canopy trees and allow the mostly shade-intolerant canopy trees (Fraxinus nigra, Fraxinus pennsylvanica, Larix laricina) to regenerate. Trees are shallowly rooted in this system so wind can blow canopy trees over relatively easily. This creates gaps in the canopy and allows smaller trees enough light to reach the canopy. Windthrow contributes to hummock-and-hollow microtopography, which generates small-scale gradients in soil moisture and chemistry, contributing to floristic diversity.

Alterations in wetland hydrology, logging, invasive plants, and emerald ash borer (Agrilus planipennis) are the prime threats to this system. Hydrologic alterations can occur due to ditching, road construction, or quarrying/mining that affect groundwater or surface waterflows into sites. Both reductions and increases in groundwater or surface water input can negatively affect this system. Partial drainage of a site can allow upland species to colonize. Increased surface waterflow can flood these swamps, changing both the hydrologic regime and water chemistry. This would likely lead to tree death and the development of a herbaceous marsh or shrub swamp. The proximity of roads has been shown to be negatively correlated with black ash health in Minnesota (Ward et al. 2006). Increased flooding can also transport sediment and higher nutrient loads. Logging can negatively impact this system through removal of trees, compaction of the soil, and creation of ruts. A serious threat to stands of this system that contain Fraxinus spp. is emerald ash borer (Agrilus planipennis). This exotic beetle has seriously affected Fraxinus spp. trees in southern Michigan and is projected to continue to spread throughout the range of Fraxinus spp. in the Midwest and Northeast by 2045 (DeSantis et al. 2012). After prolonged infestation, mortality of Fraxinus spp. is nearly 100% (Herms et al. 2010). Invasive plant species that can reduce diversity and alter community structure of this system include Elaeagnus umbellata, Frangula alnus (= Rhamnus frangula), Lythrum salicaria, Phalaris arundinacea, Phragmites australis, Rosa multiflora, Typha angustifolia, and Typha x glauca. Frangula alnus is especially problematic because it is capable of completely dominating the shrub and ground layers and altering a sites hydrology and soil nutrient characteristics (Kost et al. 2007).

This system is found from central New England to the southern Great Lakes and south-central Minnesota south to northern Illinois, Indiana, Ohio, and Pennsylvania. It is not known to extend south into the Southern Blue Ridge.