PROTECTING WISCONSIN'S BIODIVERSITY

Prior to European settlement, northern mesic forests covered the largest acreage of any Wisconsin vegetation type. While still extensive today, their character is very different from that seen by early settlers in the late-19th and early-20th centuries, primarily as a result of past and current management (see Seral Stages). Scattered small pockets of older northern mesic forest persist today and some second-growth examples are beginning to develop old forest attributes. Large acreages are managed for pulp and sawtimber. Collectively, northern mesic forests provide important habitat for wildlife and plants across large portions of Wisconsin.

Northern mesic forests form the matrix for most of the other community types found in northern Wisconsin. They are found primarily north of the climatic Tension Zone on loamy soils of glacial till plains and moraines deposited by the Wisconsin glaciation. Sugar maple (Acer saccharum) is dominant or co-dominant in most stands, regardless of their age or origin. Historically, eastern hemlock (Tsuga canadensis) was the second most important species, sometimes occurring in nearly pure stands with eastern white pine (Pinus strobus). Both conifer species are greatly reduced in today's forests. American beech (Fagus grandifolia) can be a co-dominant with sugar maple (Acer saccharum) in the counties near Lake Michigan. Other important tree species are yellow birch (Betula alleghaniensis), basswood (Tilia americana), and white ash (Fraxinus americana), although yellow birch reproduction has become scarce in most stands.

Characteristic subcanopy trees include balsam fir (Abies balsamea), ironwood (Carpinus caroliniana), and American elm (Ulmus americana). The shrub layer includes species such as alternate-leaved dogwood (Cornus alternifolia), beaked hazelnut (Corylus cornuta), leatherwood (Dirca palustris), American fly honeysuckle (Lonicera canadensis), prickly gooseberry (Ribes cynosbati), red elderberry (Sambucus racemosa), and maple-leaved arrow-wood (Viburnum acerifolium). Historically, Canada yew (Taxus canadensis) was an important shrub, but it is now absent from nearly all of its previous range, mostly due to deer browse. The groundlayer varies from sparse and species poor in hemlock stands with wood ferns (Dryopteris intermedia), blue-bead lily (Clintonia borealis), club-mosses (Lycopodium spp., Dendrolycopodium spp., etc.), and Canada mayflower (Maianthemum canadense), to lush and species-rich with fine spring ephemeral displays of species like large-flowered trillium (Trillium grandiflorum), Dutchman's-breeches (Dicentra cucullaria), spring beauty (Claytonia virginica), and trout lilies (Erythronium spp.). Other characteristic species include white baneberry (Actaea pachypoda), downy Solomon's-seal (Polygonatum pubescens), wild sarsaparilla (Aralia nudicaulis), rose twisted stalk (Streptopus roseus), starflower (Trientalis borealis), maidenhair fern (Adiantum pedatum), and lady fern (Athyrium filix-femina).

The predominant historic disturbance regimes consisted of windthrow that semi-regularly created small forest gaps and, less frequently, large windstorms that downed large acreages of trees. Windthrow still occurs today and is an important source of coarse woody debris, which is crucial as a seed bed or nurse log for species like hemlock and yellow birch, and important in nutrient cycling, and for wildlife habitat. After old-growth stands were harvested during the cutover (late 1800s to 1932), slash fires affected many areas, resulting in a shift towards species such as aspen, paper birch, and red maple. These tree species are still commonly found in many second-growth northern mesic forests today. In general though, many stands currently lack tree species diversity after many decades of traditional hardwood management that tends to favor the extremely shade-tolerant sugar maple.

Four different seral stages are described for northern mesic forest, based on the progressive stages of forest regeneration following harvesting or a major natural disturbance from young forest to the attainment of reference conditions as seen in a mature stand. Stands with more than 50% aspen by basal area fall into the aspen-birch habitat type. For stands dominated by planted conifers, refer to the conifer plantation habitat type.

• Northern mesic forest--late seral: Late-seral (may also be referred to as old or old growth) northern mesic forests have older trees, high structural diversity, higher species diversity, and may have scattered, long-lived conifers. Trees of all sizes and age classes are present, including scattered individuals 18-24" or more dbh. Old-growth canopy trees can range in age from 75 to 300 years, the average 115-175 years. Late seral mesic forests often have a complex, multi-layered canopy with natural gaps present. Other important structural attributes include abundant snags and cavity trees and significant coarse woody debris in various stages of decomposition, which contribute significant habitat for SGCN and sites for seedling establishment of hemlock (Tsuga canadensis) and yellow birch (Betula alleghaniensis). Sugar maple (Acer saccharum) dominates most sites, but large basswood (Tilia americana) and red oak (Quercus rubra) may also be present, along with scattered yellow birch and white ash (Fraxinus americana). Hemlock and white pine (Pinus strobus) may occur as scattered individuals. Other stands may be dominated by hemlock, and small groves of older hemlock can be dotted throughout older hardwood stands. Conifers are an important component for many SGCN, providing thermal cover, nest and den sites, nesting material, as well as decay-resistant snags and coarse woody debris. Late-seral mesic forests includes older passively managed stands, stands actively managed for old-growth conditions and virgin "reference condition" forests. Although the latter is exceptionally rare on the Wisconsin landscape, it provides a glimpse of the range of structural diversity possible in this forest type, particularly the size and density of cavity trees, snags, and coarse woody debris. Important site-level characteristics that benefit SGCN at this seral stage include large trees that serve as nest sites for forest raptors, standing live and dead trees, an abundance of decaying coarse woody debris, and a diverse understory (e.g., for northern flying squirrel), coniferous trees in the understory and overstory (e.g., for Swainson's Thrush and Evening Grosbeak), and soils with thick duff layer and minimal damage from non-native earthworms that support a wide variety of snails as well as rare plants.
• Northern mesic forest--mid seral: Mid-seral northern mesic forests are dominated by trees 11-15"+ dbh, though occasional older, larger trees may also be present. Young saplings may be present as stands transition into what foresters term the understory re-initiation phase, and the forest takes on uneven-aged characteristics, though these forests typically still lack the complex structural and species diversity found in older stands. While most sites are dominated by sugar maple (Acer saccharum), other species such as basswood (Tilia americana), red oak (Quercus rubra), American elm (Ulmus americana), white ash (Fraxinus americana), and yellow birch (Betula alleghaniensis) may also be present. Groves of old hemlocks (Tsuga canadensis) may be embedded within mid-seral forests as well. If trees are allowed to age beyond typical rotation age, stands will mature and may "break apart," creating snags, coarse woody debris and multi-aged structure that benefit SGCN that prefer mature forests. Techniques can be applied to managed stands to try to achieve these results, as well. Important site-level characteristics that benefit SGCN at this seral stage include large trees that serve as nest sites (e.g., for forest raptors), trees with cavities or cracks that serve as roost sites (e.g., for several species of bats), and rich soils with thick duff layer that support host plants such as the two-leaved toothwort (Cardamine diphylla), the host plant for the West Virginia white butterfly.
• Northern mesic forest--early seral: Early-seral northern mesic forests are dominated by trees 5-11" dbh and may be even aged or two-aged, fitting into what foresters term the stem exclusion phase as competition inhibits new saplings and shrubs. Stands may provide relatively high, consistent canopy cover, but lack the larger trees as well as the species and structural complexity of older forests. Snags and coarse woody debris are typically sparse except for legacy trees. Sugar maple (Acer saccharum) is often dominant, while red oak (Quercus rubra), red maple (Acer rubrum), basswood (Tilia americana), and white ash (Fraxinus americana) may also be present. Aspen (Populus spp.) and birch (Betula spp.) may be present in small patches as well, especially in forests specifically managed to promote them. Important site-level characteristics that benefit SGCN and rare plants at this seral stage include coarse woody debris and mossy logs around ephemeral ponds and seeps (e.g., for four-toed salamander) closed canopy forest (e.g., for Least Flycatcher), and a thick duff layer with minimal damage from non-native earthworms (e.g., for snails and rare ferns).
• Northern mesic forest--young seral: Young northern mesic forests are dominated by trees ranging from 0-5" dbh and typically originate from stand-replacing events such as clear-cutting, coppicing, or a catastrophic blow-down, creating an even-aged stand through what foresters term the stand initiation phase of forest development. Typically, tree species diversity is low and dominated by sugar maple (Acer saccharum), sometimes with an aspen (Populus spp.) or birch (Betula spp.) component. Other northern hardwoods tree species may be present as well, including red oak (Quercus rubra), red maple (Acer rubrum), basswood (Tilia americana), and white ash (Fraxinus americana), depending on the site. Coarse woody debris is typically sparse except for old, highly decayed legacy logs on the forest floor. However, fresh coarse wood may be abundant in stands originating from blow-down, provided the stands have not been salvage logged. Although unusual, such unsalvaged blow-down stands may have hemlock (Tsuga canadensis)and yellow birch (Betula alleghaniensis) reproduction where seed source is abundant (e.g., Kemp Natural Resources Station in Oneida County). However, factors such as local deer abundance and weather conditions may limit natural regeneration of these species. Important site-level characteristics that benefit the most SGCN at this seral stage include proximity to more mature forest for foraging, dense groundcover and abundance decaying coarse wood (e.g., for woodland jumping mouse) and a thick duff layer with minimal damage from non-native earthworms (e.g., for snails as well as rare ferns).