Great Lakes Alkaline Conifer-Hardwood Swamp

These forested wetlands are found across northern New England and the upper Midwest and eastern to south-central Canada in basins or floodplains where higher pH and/or nutrient levels are associated with a rich flora. The substrate is typically mineral soil, but there may be some peat; often, there is an organic epipedon over mineral soil. Thuja occidentalis is a diagnostic canopy species and may dominate the canopy or be mixed with other conifers or with deciduous trees, most commonly Acer rubrum or Fraxinus nigra but also Tsuga canadensis, Larix laricina, and Betula alleghaniensis. Some examples can be almost entirely deciduous and dominated by Fraxinus nigra. Cornus sericea is a common shrub. The herb layer tends to be more diverse than in acidic swamps. Small open fenny areas may occur within the wetland. Seepage may influence parts of the wetland, but the hydrology is dominated by the basin setting.

This system is typically found in basins or in floodplains with higher pH and/or nutrient levels. Groundwater typically keeps these sites saturated or nearly so through most of the growing season. Surface water, either overland flow or from nearby lakes and streams, often contributes to the hydrologic regime, especially through flooding in the spring or after heavy rains. Some movement of groundwater is important in maintaining the dominant trees in this system (Schwintzer 1981, Johnson and Booker 1983). Soils are mineral or muck (well-decomposed peat) with sometimes a thin layer of peat over mineral soil. There is often pronounced microtopographic relief between hummock/mounds created by tree boles and roots and rotting fallen logs and small depressions. These provide different microhabitats and contribute to the diversity of the system.

Cold, nutrient-rich and alkaline groundwater is important in maintaining this system. While water chemistry is similar to alkaline fens (Laurentian-Acadian Alkaline Fen (CES201.585)), this is a treed conifer, conifer-hardwood, or hardwood swamp versus a shrub- or graminoid-dominated fen, implying other factors beyond just water chemistry are important in creating differing vegetation (Schwintzer and Tomberlin 1982). Other factors are likely hydrologic regime (length and degree of soil saturation), site history, and degree of water movement. Patchy windthrow creates small-scale canopy gaps. These swamps often occur on structurally weak organic soils where trees root shallowly due to anaerobic conditions and are thus particularly susceptible to windthrow (Slaughter et al. 2007). Fire was very infrequent in this system but could occur in very dry periods. If other factors remain the same, this system could regenerate after fire since Thuja occidentalis and many other dominants grow well on exposed mineral soil (Johnson and Booker 1983). Beaver (Castor canadensis) flooding can also shape conifer-hardwood swamp structure, species composition, and direct successional pathways.

Alterations in wetland hydrology, logging, excessive deer browse, and physical destruction of sites 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 an 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. Deer prefer Thuja occidentalis stands as wintering yards and can have significant impacts on Thuja occidentalis and other species through over-browsing (Rooney 2001, Rooney et al. 2002). Logging can negatively impact this system through removal of trees, compaction of the soil, and creation of ruts. This system is slow to recover from perturbation so disturbance can accumulate over time. 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 structure of conifer- hardwood swamps include Elaeagnus umbellata, Frangula alnus (= Rhamnus frangula), Lythrum salicaria, Phalaris arundinacea, and Phragmites australis (Kost et al. 2007).

Scattered locations from New England and adjacent Canada west to the Great Lakes and northern Minnesota.