Deciduous woods description
Description | Landscape areas
The deciduous woods is a species-rich extension of the eastern deciduous forest, with numerous plant species occurring here at the very western edge of their range. The deciduous woods landscape includes the forests of southeastern Minnesota and extends through the prairie-coniferous transitional zone, up to the Aspen Parkland in northwestern Minnesota.
The Big Woods--an area of dense forest characterized by maple-basswood forests--represent the peak of deciduous forest development. Minnesota had extensive stands of this woodland community at the time of European settlement. Today only a tiny fraction remains. Common tree species in Deciduous Woods include sugar maple, basswood, various oak types, ironwood, elm, hickory, butternut, birch, and aspen.
During the last period of glaciation, the ice sheet sculpted portions of this geologically unique landscape, but missed the southeastern "driftless" portion. Most of the region's geological character is glacial, including glacial moraines, the Mississippi River Valley and its sand plain outwash, and the St. Croix River with its valley, kames, and kettle lakes. Also included is the Twin Cities metro area, cupped in a gently sloped basin formed of Paleozoic sedimentary rocks. Channels of pre-glacial rivers cut through these formations, then were filled by glacial till that later settled, forming the chains of lakes that meander through the cities.
The driftless area in southeastern Minnesota features caves, ravines, and sinkholes, with clear, spring-fed trout streams coursing through the steep and hilly countryside.
Natural plant communities in this area are mostly influenced by climate, topography, soils, and fire. Gulf air masses bring warm summer temperatures and humid, sunny days that provide an ample growing season. This rich energy budget allows deciduous trees to drop their leaves each fall, then grow an entirely new crop each spring and still produce luxuriant growth each year.
The landscape includes a mosaic of prairie, forest, and wetland communities. Prairie grasslands have historically occupied the flat lands that today are agricultural fields. Today most prairies are found on steep slopes with thin soils, or on sandy or wet areas unsuitable for agricultural production. Forests developed around lakes and wetlands along winding rivers, where the effects of fire were limited. Forests also developed on the north sides of hills, ravines, and other areas where temperatures were cooler and moisture more available. Wetlands allowed wet prairie and specialized forest communities to develop.
Deciduous forest communities themselves are distinguished by degree of soil moisture:
- Xeric forests, found on dry sites, feature drought-tolerant species, typically dominated by white, red, and black oak canopies.
- Mesic forests, found on sites with moderate soil moisture, are the stable maple-basswood forests we know as Big Woods.
- Lowland forests, found along floodplains and swamps, are adapted to the greatest extremes in moisture, ranging from spring flooding to summer drought. Cooler air settles or drains through these areas. Canopy species vary widely. Floodplain forests include silver maple, cottonwoods, black willow, American elm, green ash, and bur oak. Hardwood swamp forests include black ash, paper birch, yellow birch, red maple, American elm, slippery elm, and green ash.
Plant species associated with these communities are adapted to the stress of drought, excessive moisture, and shade in a variety of ways. Consider these examples:
- Rich, mesic forests support a variety of spring ephemerals that avoid the stress of deep shade by emerging, flowering, and fruiting before the forest canopy leafs out. Other herbaceous plants tolerate the shady environment, beginning their growth flush upon closure of the leaf canopy, and fruiting during the summer months.
- In the extremes of the floodplain forest, tree species tolerate inundation as well as abrasion by debris--ice, fallen trees, or manmade objects--in floodwater currents. Frequent treefall in these conditions create canopy openings that optimize individual tree growth, resulting in individual trees that are large in diameter and height. Trees often develop multiple stems. Floodplain understory is highly variable from year to year, depending on the duration and depth of flooding: vines and short-lived opportunists are most successful in these communities.
Xeric Forest Communities
Sandy, porous, nutrient-poor soils on southern and western slopes or on hilltops and ridges support xeric forest communities typically of oak and aspen. Oak leaves are low in nutrients, high in acid, and can take from three to five years to decay, further affecting soil development. After a fire, oaks "stump-sprout," while aspen produce profuse suckers, both rapidly perpetuating their species as an adaptation to fire. The irregular oak-aspen canopy of southern forests creates openings for sunlight to penetrate to the forest floor, where a variety of saplings develop, opening the way for succession.
Shrub diversity is highly variable throughout this extensive landscape area, providing varied food and habitat for animal species. The ground layer under this relatively sunny canopy tends to bloom in mid-summer: orchids, rattlesnake plantain, pink shinleaf, smooth bedstraw, wild geranium, and false Solomon's seal are just some of the native flowers in this community.
Mesic Forest Communities
Mesic forest communities have developed on cool north slopes whose rich soils formed from glacial till and loess, with the fertile leaf litter of the maple-basswood forest. Adequate soil moisture and protection from fire by wetlands, rivers, and topography have promoted development of this fire-sensitive community.
The closed maple-basswood canopy intercepts most of the sunlight, impoverishing the understory's supply of light. Spring ephemerals have evolved to capitalize on the early spring sun before the tree canopy emerges. It is within the "Big Woods" mesic forest remnants that spring ephemerals are at their best. Trout lilies, Dutchman's breaches, spring beauty, toothwort, and false rue anemone are among this group of forest wildflowers. Shade tolerant wildflowers, however, retain their leaves after the canopy emergence, and ripen their fruit in mid-summer: mayapples, bloodroot, Jack in the pulpit, wild ginger, hepatica, and trilliums constitute this group. Groundcover in the mesic forest community is typically patchy in distribution.
The sparse shrub layer in the mesic forest is dominated by shade tolerant specialists, particularly saplings of the canopy species, awaiting their day in the sun when a mature tree falls, leaving its legacy of sunlight to the younger sibling. This homogeneity accounts for the long-term stability and continuity of an established maple-basswood forest. Shade-tolerant shrubs such as leatherwood, American hornbeam, ironwood, bitternut hickory, and pagoda dogwood complete the understory.
Lowland Forest Communities
Lowland forest communities occur throughout the length of this landscape wherever conditions of abundant soil moisture are found. In the southern portion, high summer temperatures, long frost-free periods, and high humidity create optimum growing conditions for lowland forests. Most extensive examples occur along floodplains where the extremes of flooding and drawdown, along with abrasion by floodwater debris, are annual events. Ice scars on trees, along with windrows of debris on the forest floor and abandoned channels of stagnant water, provide evidence of the floodplain dynamics. Hardwood swamps, though not subject to these extremes, occupy areas of poor drainage on peaty soils. In both cases, soil improvement is limited, and the communities are stable.
Floodplain communities of northwestern Minnesota include American elm, slippery elm, green ash, cottonwood, and bur oak. In southern Minnesota, silver maple, black willow, and cottonwood are more common, with scattered patches of river birch, American elm, slippery elm, green ash, and swamp white oak. Tree and shrub seedlings are limited by flooding, resulting in an open understory. Vines such as wild grape and Virginia creeper seek out light gaps and open areas, where they escape the ground layer and join the canopy. The ground layer is low in diversity, and comprised of short-lived opportunists such as cleavers, sedges, and wood nettle.
Wet sites with muck and shallow peat substrates support hardwood swamp forests dominated by mixed hardwoods, black ash, American elm, slippery elm, green ash, quaking aspen, or balsam poplar. In the north, tamarack is common, with occasional white pine or white cedar. Canopy density varies considerably, and communities occur in isolated pockets, reflecting highly localized variations.
The Deciduous Woods biome is made up of lake and outwash plains, moraines, and drumlin fields. Topography ranges from relatively level plains, to very steep gradients in southeastern Minnesota along the edge of the Paleozoic Plateau. Containing a mixture of grassland and deciduous woodlands, it forms a transition between the Prairie Grasslands and Coniferous Forest.