Physiognomy and Structure: These are mostly closed-canopy, uneven-aged forests, but may have patchy openings due to gap dynamics, primarily from windthrow (Tyrrell and Crow 1994a, 1994b). Forests on rocky substrates, more extreme sites, may have open or stunted canopies. Stands range from strong dominance by conifers to strong dominance by hardwoods. Some stands have a supercanopy of Pinus strobus. Shrub, sapling and ground layers vary from open to dense, often influenced by the pit-and-mound topography resulting from windthrows. Moss cover is patchy to continuous.

Floristics: Dominant tree species include the typical conifers and hardwoods of Acer saccharum, Betula alleghaniensis, Fagus grandifolia, Fraxinus americana, Picea glauca, Picea rubens, Pinus strobus, Tilia americana, and Tsuga canadensis; Abies balsamea is occasional. Acer rubrum, Betula papyrifera, Populus grandidentata, and Populus tremuloides may be common. Other prominent canopy associates may include Prunus serotina, Thuja occidentalis, and Quercus rubra. Southward, in the Appalachians part of the range, Liriodendron tulipifera may also occur (Whittaker 1956). Abies balsamea and Picea glauca are uncommon associates. Canopy composition is dependent on a variety of factors, including geographic location, past disturbance, soils, slope, and aspect. Smaller trees that may form an understory include Ostrya virginiana, Sorbus americana, Acer spicatum, and Acer pensylvanicum. Prunus pensylvanica may also be common, especially in openings. The density and composition of shrub, herb, and bryophyte strata vary with geographic location and degree of past disturbance (i.e., canopy openings). Common shrub species include Corylus cornuta, Diervilla lonicera, Dirca palustris, Hamamelis virginiana, Lonicera canadensis, Prunus virginiana, and Viburnum lantanoides. A diverse set of characteristic herbaceous species include Actaea pachypoda, Actaea rubra, Allium tricoccum, Aralia nudicaulis, Arisaema triphyllum, Athyrium filix-femina, Clintonia borealis, Coptis trifolia, Cornus canadensis, Dryopteris campyloptera, Dryopteris carthusiana (= Dryopteris spinulosa), Gaultheria procumbens, Huperzia lucidula (= Lycopodium lucidulum), Lycopodium obscurum, Osmunda claytoniana, Oxalis montana, Maianthemum canadense, Medeola virginiana, Mitchella repens, Polygonatum pubescens, Polystichum acrostichoides, Streptopus lanceolatus (= Streptopus roseus), Trillium undulatum, Trientalis borealis, Uvularia sessilifolia, and, in areas with a limestone or other more calcareous substrate, Adiantum pedatum, Caulophyllum thalictroides, and Deparia acrostichoides (= Athyrium thelypterioides). The bryophyte layer is generally very well-developed, characterized by Bazzania trilobata, Pleurozium schreberi, Dicranum spp., and many others. These forests may also be invaded by non-native shrubs, including Berberis spp., Lonicera morrowii, Lonicera tatarica, Frangula alnus, Rhamnus cathartica, and others (Curtis 1959, Cohen et al. 2014).

Dynamics:  The natural disturbance regime is characterized by frequent, small windthrow gaps, which allow for gap-phase regeneration of the shade-tolerant canopy dominants. Large-diameter coarse woody debris provides nurse logs for regenerating tree canopy species, and tip-up mounds for plant establishment and regeneration (Canham 1988, 1990). Catastrophic windthrow occurred infrequently (return intervals of >500 years) with several generations of trees passing between large-scale, severe disturbance events (Canham and Loucks 1984, Frelich and Lorimer 1991a, Frelich 2002). As a result, these mesic northern forests historically dominated vast areas of mesic uplands of the Great Lakes and were multi-generational, with old-growth conditions lasting many centuries (Cohen 2005).Fire is not an important factor, although it has been shown to occur at long return intervals (>500 years) in the presettlement landscape. Forests at higher elevations are affected by wind disturbance, debris avalanches, ice loading, and lightning fire, and can be susceptible to large blowdowns where the soils are shallow. Insect defoliation and selective herbivory by white-tailed deer can alter the composition and structure and favor browse-tolerant species. In the northern part of the range, the spruce budworm occurs in periodic outbreaks (~40-60 years) and causes significant defoliation and patchy blowdowns. Beech bark disease and emerald ash borer have locally killed canopy beech and American ash (Curtis 1959, Cohen et al. 2014).

Intensive and pervasive anthropogenic disturbance during the past 150 years has altered the extent, landscape pattern, natural processes, structure, and species composition of the mesic northern forest (Frelich 1995, Cohen 2005). In particular, old-growth and late-successional forests have been drastically reduced in acreage, or occur as remnant patches in a matrix of agricultural lands, early-successional forest, and young northern hardwoods (Leahy and Pregitzer 2003). Short-rotation timber management has replaced gap-phase dynamics as the dominant disturbance factor impacting structure and composition. Structural alterations include the reduction of large-diameter trees, snags, and coarse woody debris. Hemlock and white pine have declined in importance within these systems, and have been completely eliminated from many landscapes. Chronically high deer densities have limited tree recruitment and altered floral composition and structure (Frelich and Lorimer 1985, Alverson et al. 1988, Mladenoff and Stearns 1993, Alverson and Waller 1997, Rooney and Waller 2003, Cohen 2005).