Northern dry forest occurs on nutrient-poor sites with excessively drained sandy or rocky soils. The primary historic disturbance regime was catastrophic fire at intervals of 10-100 years. Dominant trees of mature stands include jack pine (Pinus banksiana), red pine (Pinus resinosa), and northern pin oak (Quercus ellipsoidalis). Large acreages of this forest type were cut and burned during the logging of the late-19th and early-20th century. Much of this land was then colonized by paper birch (Betula papyrifera) and/or trembling aspen (Populus tremuloides) or converted to pine plantations starting in the 1920s.
Today's forests have a greatly reduced component of pines, and a greater extent of aspen, red maple, and oaks compared to historic conditions. Common understory shrubs are hazelnuts (Corylus spp.), early blueberry (Vaccinium angustifolium), and brambles (Rubus spp.). Common herbs include bracken fern (Pteridium aquilinum), starflower (Trientalis borealis), barren-strawberry (Waldsteinia fragarioides), cow-wheat (Melampyrum lineare), trailing arbutus (Epigaea repens), and members of the shinleaf family (Chimaphila umbellata, Pyrola spp.). Vast acreages of cutover land were also planted to pine or naturally succeeded to densely stocked dry forests.
Factors affecting the current abundance and condition of northern dry forests include fire suppression and the spread of invasive species. On some sites (e.g., on richer sites where better growth is expected) silvicultural practices may maintain or even increase certain cover types such as red pine. For other cover types such as jack pine, the management efforts may be to eliminate interior gaps and edges and strive to make tree spacing and size as uniform as possible, in part to lessen the probability of severe budworm damage. Such practices reduce or remove habitat for native species dependent on aspects of the patchy nature of northern Wisconsin's dry ecosystems.
Northern dry forests most commonly occur on large, continuous glacial outwash or lake plain landforms. On these extensive dry plains, historic fires were large and intense, and were less likely to be halted by wetlands, hills, or mesic soils, creating ideal conditions for establishment this community type.
Three different seral stages are described for northern dry forest, based on the progressive stages of forest regeneration following harvesting or a major natural disturbance, from young forest to mid-seral to the attainment of reference conditions as seen in a mature stand. Stands with more than 50 percent aspen by basal area fall into the aspen-birch habitat type. For stands dominated by planted conifers, refer to the conifer plantation.
- Northern dry forest--late seral: Late-seral (may also be referred to as old or old-growth) northern dry forests are dominated by trees 40 feet tall (~5-10" dbh) or more and are dominated by jack pine (Pinus banksiana), red pine (Pinus resinosa), white and northern pin oak (Quercus alba and Q. ellipsoidalis), and pockets of trembling aspen (Populus tremuloides). Tall shrub (e.g. hazelnut (Corylus spp.) and serviceberry (Amelanchier spp.)) density is variable, ranging from sparse to dense thickets, but is typically greater in more mature stands, which provides important habitat for some Species of Greatest Conservation Need (SGCN). In addition, forest grasses, sedges, forbs, and mosses predominate in the groundlayer. Snag density is at its highest relative to other seral stages, providing habitat for woodpeckers and cavity-nesting birds. Stands may include those on the older end of those managed as part of a shifting barrens mosaic and those managed for old-growth characteristics.
- Northern dry forest--mid-seral: Mid-seral northern dry forests are dominated by trees 16 to 40 feet tall (~3-5" dbh). Like other seral stages of dry forests, species are primarily jack pine (Pinus banksiana), red pine (Pinus resinosa), and northern pin oak (Quercus ellipsoidalis), but can also include components of white pine (Pinus strobus), red maple (Acer rubrum), aspen (Populus spp.), and birch (Betula spp.). Depending on stand origin, scattered grassy or shrubby openings may be present, providing important habitat components for SGCN, such as Whip-poor-will and Common Nighthawk. However, in this seral stage, herbaceous vegetation shifts significantly away from barrens associates and toward forest grasses, sedges, and forbs. Structural complexity is slightly higher than in younger forests, with multiple size classes of trees developing, particularly where both oaks and pines are present, despite still being even-aged.
- Northern dry forest--young seral: Young-seral northern dry forests are dominated by trees 16 feet tall or less (~0-3" dbh). Species are primarily jack pine (Pinus banksiana), red pine (Pinus resinosa), and northern pin oak (Quercus ellipsoidalis), but can also include red maple (Acer rubrum), aspen (Populus spp.), and birch (Betula spp.). Stands typically originate from stand-replacing events such as clear-cutting or fire, but can also arise from mechanical soil scarification. They are mostly even-aged stands with few or no snags and little coarse woody debris. Structurally, young northern dry forests may have similarities with pine barrens--scattered openings with native grasses, scattered wildflowers, and patches of hazelnuts (Corylus spp.), dewberry (Rubus spp.), and blueberry (Vaccinium angustifolium) providing habitat for SGCN and other wildlife. However, tree density is higher and openings smaller than in true barrens, and ground flora is highly variable depending on how the forest was established.
In describing these stages, it is recognized that they exist and persist on the landscape due to a marked range of conditions depending on conservation or production goals and the nature and intensity of management. Even the most basic actions of promoting natural regeneration carry the weight of this reality---in an area where the purpose is strictly forest production, the decision to harvest and replant may be relatively straightforward, but on a site managed all or in part for ecological values the prescription and stand rotation is more complex.
Some of the important site characteristics that may determine how SGCN utilize this seral stage include pocket barrens, frost pockets, or other non-forested openings that provide important habitat, the landscape mosaic of barrens and forest across landscape, structural attributes, and diversity of other woody species and herbaceous plants.
