Black ash forests are found in wet areas, occurring in narrow bands at wetland margins and along streams or as larger tracts in depressions or where groundwater seeps to the surface. The most abundant trees in black ash forests are, not surprisingly, black ash. These grow in almost pure stands or are mixed with green ash, red maple, and sometimes bur oak. In northern Minnesota black ash forest can include white cedar and balsam fir.
Old-growth forests consist of a mixture of tree sizes and ages, and young black ash can become established and grow beneath their parent trees. Black ash trees in old growth may be 200 years old or more. The size of a tree varies depending on the soil moisture and latitude of a site. For example, in continuously moist soils in central and northern Minnesota, black ash easily reaches 15 inches in diameter, but on similar sites in far southern Minnesota it is common to encounter 20–inch–diameter trees.
Due to wet soils, the shallow-rooted black ash trees are easily blown down in storms when heavy cutting has taken place around the old-growth forest perimeter. White cedar often has been removed from black ash forests, and restoration of this species can be difficult where deer numbers are high and only small amounts of fallen large wood exist to serve as nurse trees.
Lowland hardwood forests are found on stream and river floodplains and in swamp settings away from flowing water. The make-up of the trees that grow in this forest type varies moving from south to north. In the south, lowland hardwood forests consist of silver maple, American or red (slippery) elm, and green ash, together with a large variety of other tree species. In the north, the elms are being attacked by Dutch elm disease and many have recently died. Ash is the most common species there, often growing with basswood and oak.
In addition to old and medium-aged trees, old-growth lowland hardwoods will have some seedlings and young trees. The ages and sizes of the oldest trees are similar to those found in black ash old-growth forests. It is rare to find species that require large amounts of light, such as willow, cottonwood, aspen, birch, and balsam poplar. At most these species grow in small patches within the old-growth forests. In some old-growth lowland hardwoods, the elm trees were cut as they were dying.
When old-growth lowland hardwood forest occurs in small patches and when the soil is disturbed, nonnative plants (e.g., reed canary grass, garlic mustard) can invade and disrupt the ground vegetation. Floodplain forests receive water from upstream. The amount of water, the length of flooding, and the frequency of flooding are increased when the upstream watershed is developed into residences and commercial land or excessively drained for agriculture. Due to past cutting, some old-growth stands of this type have fewer snags and less fallen large wood than historically typical of this type. Dealing with Dutch elm disease is problematic because removal of diseased trees may prevent other trees from being affected, but can change the character of the old-growth stand.
This forest type can be subdivided into maple-basswood forest and northern hardwoods, the latter being the most common in northern Minnesota. Sugar maple and basswood are most abundant, but yellow birch is common in the north, and red oak is common on the driest sites. American elm and black ash are found on the wettest sites. Northern hardwoods also support conifers—white pine, balsam fir, white spruce, and white cedar. Because it was so desirable, white pine has been largely removed from northern hardwood forests. Maple-basswood forests have a larger variety of tree species than northern hardwoods, including red (or slippery) elm, bur oak, white oak, white ash, green ash, bitternut hickory, and walnut. In forests that have not been grazed or suffered soil disturbance, a beautiful and diverse collection of wildflowers appears each spring. Because they bloom only briefly and many wither by midsummer, these wildflowers are called "spring ephemerals."
Northern hardwood old-growth has the classic look of old growth. Forests appear dense with layers of vegetation, and seedlings and saplings grow vigorously, especially those of sugar maple. These forests perpetuate themselves over many centuries if they are not disturbed by severe storms or cutting. The young trees tolerate living for years in deep shade until a gap opens in the canopy. Then the young trees under that canopy gap race upward to take the place of the fallen giant that previously occupied the space. The oldest trees can live for 300–400 years and grow to diameters of at least 20 inches in the north and 26 inches in the south. Sun-requiring species, such as aspen, are rare or occur in scattered small clumps of a few trees. These trees are often very old. White pine and yellow birch were favorite trees for building materials, and therefore they are much rarer than they were historically.
Small patches are vulnerable to invasion by nonnative species, especially if the canopy has been opened or the soil disturbed (usually by grazing). Common buckthorn and honeysuckle are particularly good at invading and changing the understory of northern hardwoods. Other species, such as exotic earthworms, are a threat to the understory herb layer of old-growth forests. Regeneration of white pine and yellow birch presents a challenge if deer numbers are high, mature trees of these species are rare, and large fallen wood (serving as nurse sites for germination of yellow birch) is uncommon.
Oak forests grow on the driest places of a landscape. They are dominated by red oak and white oak individually or in combination. Aspen, paper birch, and black cherry often grow with these oaks, and in southern Minnesota black oak, bur oak, bitternut hickory, and pignut hickory increase in importance. Without some kind of disturbance (e.g., fire) that clears away old leaves, shrubs, and saplings, oak forests will be invaded by northern hardwoods. Oak forest is made up of species that do not tolerate deep shade, and consequently oak forest is called a "seral" forest community—one that gives way to more shade-tolerant species over time. For over 100 years, fires have been suppressed in the oak regions of Minnesota. This has made oak forest an increasingly rare type, especially on moister soils where maples and other shade-tolerant species easily invade. Complicating matters, before 1850 most of the oak trees in Minnesota grew in conditions that did not resemble forests. In the brush prairie regions of the state, oaks grew as short trees having many stems which, after fire, sprouted out of a large root mass called a grub. In the savanna regions of the state, oaks grew scattered or in small clumps with light-loving wildflowers, grasses, and shrubs interspersed among them. In both settings fires swept over the ground at least once every two years, less often in more rugged terrain and locations with many wetlands.
Modern oak forests that began as savannas and open woodlands have oaks with wide-spreading branches, even on their lower trunks. This indicates that they matured without other trees shading them. These are not considered old-growth forests, even if they possess ancient trees. The oldest trees in an actual old-growth oak forest are often 150–200 years old or older with diameters of 20 inches in the north and 30 inches in central and southern Minnesota. Seedlings and saplings of northern hardwood species can be found in moist oak forests. Without fire these locations will eventually become maple-basswood forest. Because they are so rare, these forests often are considered old growth if they have not been disturbed by people for 20 years. However, nearly all oak forests in the state were pastures at some point. If the number of cattle in a woods was not excessive, an oak forest will have few nonnative species and only small amounts of shrubs bearing prickles and thorns. It will also support a diverse group of wildflowers, grasses, sedges, and shrubs.
Oak forest that grows in small patches or has disturbed soils will be invaded by nonnative species and native weedy plants. To perpetuate old-growth oak forest, some disturbance is needed. Fire was the natural disturbance, but grazing has kept many oak forests from succeeding to northern hardwoods. Grazing, however, creates opportunities for nonnative species to invade. For example, due to past grazing in combination with timber harvesting, common buckthorn is now one of the only plants growing beneath the canopy of many oak forests. Achieving a balance between disturbance and maintenance of old-growth oak forest conditions is an immense challenge.
This forest type is largely restricted to northeastern and north-central Minnesota, with the white pine subtype also found along steep slopes and knobs of creek and river valleys in the southeast. Red and white pine form a canopy either together or separately. White pine tends to grow on moister sites than red pine. Paper birch, red maple, and other deciduous trees occur with white pine on the moistest soils. On the driest sites red pine is joined by jack pine, and where red pine grows on moister sites it is accompanied by white spruce or balsam fir. Historically frequent surface fires and rare canopy fires perpetuated this type for many centuries, but in the absence of fire, maple, balsam fir, or white cedar will eventually take over on many sites. Being somewhat tolerant of shade, white pine can grow beneath a pine canopy and its saplings will grow up to fill canopy gaps. Red pine, however, requires nearly direct sunlight in all its life stages. Formerly common, old-growth red and white pine forests are now rare due to their high demand for building material from 1850 to 1920. The abundance of white pine has declined further due to the belief that other trees are easier to grow.
Old-growth pine forests usually consist of distinct age groups that are established after a disturbance that clears the ground of old needles, leaves, and brush, or opens the canopy. Trees can be as old as 400 years with diameters of 3 feet or more. Most remaining pine stands have experienced some recent cutting. In some cases, this occurred relatively recently, and such forests are not as high in quality as those disturbed long ago or never harvested. Future old-growth pine forests have also been identified. Here the pines are vigorously growing following harvesting and fire early in the last century, in some cases, beneath a different type of tree canopy, such as aspen, and will eventually replace it as the most dominant species in the stand.
Long-term management of old-growth pine forests is difficult. Several factors must be balanced. Fires naturally created ideal conditions for the pines to germinate and grow. However, fire exclusion in the last century has led to an excessive buildup of needles, fallen branches, and conifer saplings, making these forests prone to catastrophic fires that destroy the forest canopy, especially on dry, sandy soils. To perpetuate old-growth pine forests may require an initial mechanical removal of these fuels before quick-running surface fires can be lit to regenerate the trees. White pine is favored by deer as food in winter and early spring. Where deer are abundant, they will hamper efforts to establish young white pine trees.
Along with white spruce, a varying combination of species is commonly found in white spruce forests, including balsam fir and paper birch. This type is restricted to north-central and northeastern Minnesota.
Because white spruce does not live as long as the trees of other forest types, old growth is considered to begin when a forest reaches 90 years of age. The oldest trees reach 150 years of age. During the lifetime of the oldest spruce trees, several generations of balsam fir will have come and gone. As a result these types of forests are filled with medium-size and small pieces of fallen wood.
White cedar often grows in pure stands over large areas of moist soils. Cedar trees growing in swamp settings are not considered part of the upland white cedar type. It also is found mixed in extensive tracts with balsam fir, yellow birch, paper birch, white spruce, and black spruce. Upland white cedar forest is most common on Lake Superior's north shore, but occurs throughout northeastern and north-central Minnesota. White cedar has been in high demand for shingles and other specialty wood products, and as a result the largest trees in most cedar forests are continuously removed.
Although it is a long-lived and shade-tolerant tree, white cedar has difficulty reproducing itself. Seedlings and young saplings are rarely found even in old-growth upland white cedar forests. One reason is that deer shelter in white cedar forest in winter and early spring eat young white cedar. Many trees are at least 200 years in age, and commonly over 400 years old. In one old-growth forest, researchers found a cedar tree nearly 600 years old. Because their wood is dense, even old white cedar trees may not be large. The oldest trees can be 3 feet across. The cedars often grow in clusters with spaces between them, giving the impression of an open forest.
The greatest challenge in managing old-growth upland white cedar is to establish the next generation of canopy trees. Preventing deer from eating seedlings and young saplings would increase the numbers of young trees. In addition, white cedar germinate on fallen large wood, which meters out water to the delicate roots of the seedlings at a constant rate, even during periods of low rainfall. The common practice of removing the oldest trees has eliminated much of the fallen large wood in white cedar forests. Consequently there are few sites for white cedar to get started on.