This nutrient-rich northern hardwood forest is dominated by sugar maple with basswood and occasional yellow birch, which form a closed, continuous canopy. White ash is also often present in the canopy of sites not yet impacted by emerald ash borer. Red oak and hemlock may be local but are not dominant, and white pine can be present as a supercanopy tree. Ironwood and occasionally musclewood are present in the understory, along with species such as sugar maple, other gap-loving saplings such as white ash, or moderately shade-tolerant trees like bitternut hickory.
Rich northern mesic forests occur in northern Wisconsin primarily on moraines and ice-walled lake plains but have also been documented on silty alluvial deposits just above river floodplains. Soils are predominantly silt loam but can also include loam and sandy loam. Nutrient levels are high, especially cations such as calcium and magnesium. High-quality sites also have a thick duff layer of partially decomposed leaves, though duff may be reduced or lacking in sites with non-native invasive earthworms.
Shrubs are typically sparse due to dense shade but can include prickly gooseberry (Ribes cynosbati), American fly honeysuckle (Lonicera canadensis), choke cherry (Prunus virginiana), beaked hazelnut (Corylus cornuta) and leatherwood (Dirca palustris).
The groundlayer is the most notable feature and is lush with abundant spring ephemerals, including spring-beauty (Claytonia virginica and C. caroliniana), false rue anemone (Enemion biternatum), trout-lily (Erythronium americanum and E. albidum), Dutchman’s breeches (Dicentra cucullaria), squirrel corn (Dicentra canadensis), toothwort (Cardamine concatenata and C. diphylla), and occasionally false mermaid-weed (Floerkea proserpinacoides). Other rich site indicators include blue cohosh (Caulophyllum thalictroides), trillium (Trillium grandiflorum and T. cernuum), bellwort (Uvularia grandiflora), miterwort (Mitella diphylla), dwarf ginseng (Panax trifolius), wood nettle (Laportea canadensis), bloodroot (Sanguinaria canadensis), wild leek (Allium tricoccum and A. burdickii), and yellow violet (Viola pubescens). Sedges such as hairy wood sedge (Carex hirtifolia), white bear sedge (Carex albursina), and plantain-leaved sedge (Carex plantaginea) are also good indicators along with ferns such as maidenhair fern (Adiantum pedatum), silvery glade fern (Deparia acrostichoides), and rarely, Goldie’s fern (Dryopteris goldiana). Horsetails and scouring rushes (Equisetum spp.) can also be locally common. In spring, ephemerals and associated plants sometimes form a continuous carpet in the groundlayer.
More common northern mesic forbs and ferns such as wild sarsaparilla (Aralia nudicaulis), blue-bead lily (Clintonia borealis), downy Solomon’s-seal (Polygonatum pubescens), hairy sweet cicely (Osmorhiza claytonii), and lady fern (Athyrium filix-femina) can also be abundant.
Rich northern mesic forests have long been recognized as a unique feature on the landscape and were recently split from the more common northern mesic forest due to their conservation significance. Several rare plants and invertebrates are strongly associated with this community and have significantly more populations on rich sites compared to more typical northern mesic forests. Examples include foamflower (Tiarella cordifolia) and the West Virginia white butterfly (Pieris virginiensis), which relies on toothworts as a larval host plant. Adults also utilize nectar from other spring ephemerals such as spring beauty.
Rich northern mesic forests are globally vulnerable (G3), being uncommon on the landscape and threatened by high deer populations and earthworms, all of which are non-native and invasive in Wisconsin. Earthworms consume the protective duff layer, which causes the decline of many herbaceous plants and increase soil drying. Invasive plant species are also a major threat, especially garlic mustard (Alliaria petiolata), hemp-nettle (Galeopsis tetrahit) and European marsh thistle (Cirsium palustre). High deer populations, earthworms, and invasive plants often interact in a negative feedback loop that causes steep declines in native plant abundance as well as the local extirpation of species sensitive to browsing such as plants in the lily family (e.g., Trillium spp., Polygonatum spp., etc.) or those dependent on a thick duff layer such as moonworts and grape-ferns (Botrychium spp., Sceptridium spp., etc.)
Certain types of timber harvests can also be a threat, particularly high grading and even-aged management. Two-tracks and skidder trails can also cause rutting, soil compaction, and spread invasive plants and earthworms. Due to their nutrient-rich soils and productive timberland, there have historically been more resource conflicts compared with other subtypes of northern mesic forests. As a result, rich mesic forests are under-represented in protected areas relative to subtypes such as hemlock stands, which are typically less economically desirable and therefore easier to designate for conservation. However, ecological silviculture designed to meet multiple co-benefits, ideally with harvests conducted on frozen ground, can be compatible with site conservation.
