Physiognomy and Structure: Deciduous, mixed, or coniferous canopy ranging from dense to partially open; shrub layer generally well-developed but can be sparse under dense canopy and where deer browse pressure is high. Hummock-and-hollow microtopography is characteristic, and mosses, especially species of Sphagnum, are common and usually abundant. Hummocks and decaying nurse logs provide critical substrate for plant establishment and growth.

Floristics: Acer rubrum is a relatively constant species throughout the range of these swamps. At the northern end of the range, canopy trees Betula alleghaniensis, Larix laricina, Picea rubens, Pinus strobus, and Tsuga canadensis are characteristic. Nyssa sylvatica can be more important toward the southern end of the range. Occasionally, colder conditions favor Abies balsamea, Picea glauca, or Picea mariana, mixed with temperate trees, shrubs, and herbs. Typical shrubs include Alnus incana, Carpinus caroliniana, Cornus spp., Ilex verticillata, Lindera benzoin, Ilex mucronata (= Nemopanthus mucronatus), Ribes spp., Rubus pubescens, Salix spp., Vaccinium corymbosum, and Viburnum nudum var. cassinoides. On the coastal plain, Picea rubens and Larix laricina are replaced by Chamaecyparis thyoides or Pinus rigida, with typical shrubs including Clethra alnifolia, Gaylussacia dumosa, Ilex glabra, Eubotrys racemosa (= Leucothoe racemosa), and Rhododendron viscosum. Typical herbs across the range of acidic swamps include Carex folliculata, Carex intumescens, Carex scabrata, Carex stricta, Chelone glabra, Dryopteris cristata, Onoclea sensibilis, Osmunda spp., Saxifraga pensylvanica, and Symplocarpus foetidus, among others. Alkaline swamps are limited to the northern portion of the range of this macrogroup and are characterized by Acer rubrum, Fraxinus nigra, Larix laricina, Thuja occidentalis, and Ulmus americana, with shrubs including Rhamnus alnifolia and Cornus sericea in addition to the shrubs of northern acidic swamps. Herbaceous species of alkaline swamps include Caltha palustris, Carex bromoides, Carex leptalea, Geum rivale, Impatiens capensis, and Packera aurea, in addition to many of the acidic swamp species listed above.

Dynamics:  From Slaughter et al. (2007): "The primary natural processes structuring hardwood-conifer swamp are small-scale windthrow and dynamics of surface water and groundwater. Patchy windthrow is the dominant natural disturbance, creating small-scale canopy gaps (Forrester et al. 2005).

"Seedlings of several characteristic hardwood-conifer swamp canopy species (e.g., yellow birch, white pine, northern white-cedar, and hemlock) preferentially germinate and establish on root hummocks and/or decaying logs versus muck or litter-covered depressions (i.e., hollows) (Holcombe 1976, St. Hilaire and Leopold 1995, Rooney and Waller 1998, Allison and Ehrenfeld 1999, McGee 2001, Rooney et al. 2002). In comparison to hollows, hummocks and decaying logs have high moss cover, high moisture content, coarse substrate texture, and stable hydrology, characteristics which may favor the germination and establishment of small seeds with low nutrient reserves (Coffman 1978, St. Hilaire and Leopold 1995, McGee 2001).

"Groundwater and surface water dynamics also shape hardwood-conifer swamp structure and impact succession. Significant hydrological processes impacting hardwood-conifer swamp include groundwater seepage, water table fluctuation, seasonal inundation, and flooding events. "

Logging, especially of Thuja occidentalis and Chamaecyparis thyoides, has influenced the structure and dominance of this macrogroup. Regeneration of Chamaecyparis thyoides is stimulated by fire.

At the present time, excessive deer herbivory threatens the viability of hardwood-conifer swamp throughout its range. High white-tailed deer (Odocoileus virginianus) density is leading to considerable browse pressure on conifer seedlings and saplings throughout Michigan and the Great Lakes region (Frelich and Lorimer 1985, Mladenoff and Stearns 1993, Alverson and Waller 1997, Long et al. 1998, Rooney and Waller 1998, Rooney et al. 2002, Krueger and Peterson 2006). Deer browse also reduces frequency and cover of understory shrubs and herbs (Balgooyen and Waller 1995, Augustine and Frelich 1998, Rooney and Waller 2003, Kraft et al. 2004). The result of heavy deer browse is significant alteration of community structure consisting of impacts to all vegetative strata.