In many ways the woods in the Preserve typify the broadleaf forest common to relatively moist, upland sites in southern Wisconsin. This accords with the soils in and around the Preserve, which consist of a silt loam cap overlying deep moraine laid down by the last glaciation.
You’ll find a common set of tree species occupying the overstory throughout the woods, but the relative abundance of each varies with slope and aspect (the direction the slope faces). The common species include Bur oak (Quercus macrocarpa), White oak (Quercus alba), Northern red oak (Quercus rubra), Shagbark hickory (Carya ovata), Slippery elm (Ulmus rubra), Hackberry (Celtis occidentalis), White ash (Fraxinus americana), Black cherry (Prunus serotina) and Basswood (Tilia americana).
These same species, all but the oaks, also inhabit the subcanopy, along with Green ash (Fraxinus pennsylvanica), Box elder (Acer negundo) and Sugar maple (Acer saccharum). Many of the overstory trees, especially the oaks, are impressive not only in terms of stature, but also longevity, as ages in some areas exceed 100 years. The ground cover of these woods is a mixture of hardy native species with some exotic invader species.
On north- and east-facing slopes, and at slope bases, Red oak, Basswood, Slippery elm and White ash are comparatively abundant in the overstory, and occasionally they’re joined by Sugar maple as you can see, for instance, in Muir Woods and Caretaker’s Woods.
On the somewhat drier south- and west-facing slopes, you’ll encounter proportionally more White oak, Black cherry, and Bur oak, as well as a contingent of Black oak (Quercus velutina), for instance in Bill’s Woods and the south side of Eagle Heights Woods.
Yet another set of species can be seen in the lowlands and shoreline areas, including Red maple (Acer rubrum), Silver maple (Acer saccharinum), Cottonwood (Populus deltoides), American elm (Ulmus americana) and Black willow (Salix nigra), easily observed along the Temin Lakeshore Path. Sugar maple, Hackberry and Box elder are also common in these areas.
Particularly along trails and edges of openings, an assortment of other tree species has been planted at various times during the past century. These plantings are composed of native and non-native conifer and deciduous broadleaf species, and many, such as those of Black locust (Robinia pseudoacacia), have been quite successful.
Spring and summer in the woods
The initial sign of life in the overstory each spring is the opening of flower buds. Elms, Hackberry, maples, aspens, Cottonwood and willow are the first to produce flowers. Whereas those of elm and Hackberry are fairly inconspicuous, flowers of maples are easy to see, especially since they precede the onset of leaf development. Flowers of Red and Sugar maple, for example, imbue the forest overstory with crimson and pale yellow hues, respectively.
These early birds are followed by a procession of other species, such as Red and White oak, which produce orange and light-green male catkins, respectively. Later in the summer, after leaf flush, spikes of little white flowers will appear on Black cherry, yellow clusters will emerge on Basswood, and larger, fragrant white groups of flowers will adorn Black locust. These summer flowers depend on insects for pollination, while most of the spring flowers rely on wind.
If successfully pollinated, these flowers will give way to fruit development and ripening, which for certain species is a hasty affair. For example, the fruit of elm, as well as Red and Silver maple, will ripen and disperse in early summer. Cottonwoods, aspens and willows are also early summer seeders. The seeds in these fruit, in turn, will germinate immediately if they land in a sufficiently moist microenvironment.
Fruit of other species, such as Bur and White oak, Sugar maple, ashes, hickories, Hackberry, Basswood and walnuts, require a full summer to develop, and are dispersed in the fall. Here, Red oak is a standout, as its acorns require a second full growing season to mature before dispersal. Offspring of these autumnal seeders will not germinate until spring arrives.
In the spring, large clumps of Trout-lily (Erythronium albidum), Toothwort (Dentaria laciniata), and Virginia waterleaf (Hydrophyllum virginianum) interspersed with Jack-in-the-pulpit (Arisaema triphyllum) adorn the woodland understory. In the summer, plants with inconspicuous flowers, such as White snakeroot (Eupatorium rugosum), compete with the woody vine Virginia creeper (Parthenocissus quinquefolia) and with tree seedlings for growing space on the woodland floor. The heavy leaf litter of early spring, host to a community of tiny invertebrate animals, decays quickly with the aid of exotic earthworms, so that by middle to late summer much bare ground is visible.
Fall and winter in the woods
In a forest full of mast-fruiting species, such as the oaks, fruit dispersal can be a raucous event in the fall. Especially on windy days, hundreds of acorns will rain down from the crown of every overstory oak, causing a cacophony of taps, clatters and thumps. In southern Wisconsin, this seed rain typically occurs in September or early October. By comparison, the fruits of Sugar maple, helicopter-like samaras, quietly spin to earth earlier in September. Earlier still, you’ll see, hear or feel the impact when walnuts drop their big greenish-yellow fruits.
Usually, but not always, leaf color change and abscission (leaf drop) will follow fruit dispersal in the fall. Such is the case with the oaks, which are among the last to undergo leaf senescence. As far as showy displays are concerned, Sugar and Red maple are typically the first to set the forest canopy ablaze in oranges and reds. The maples are followed by golden displays of Basswood, elms, ashes and aspens. During this time, walnuts and hickories sneak into the lineup with dull rusty-yellow (and sometimes brown) foliage. Last but not least are the crowns of Red and White oak, which, when conditions are right, will turn deep red to crimson. For all species, leaf color change and abscission are but two steps in a complex process of cold-acclimation that ensures the survival of all tree tissues during the harsh Wisconsin winter.
By fall, the woodland understory is characterized by foot-high clumps of White snakeroot with their small pale tufts of wind-dispersed seeds, the occasional Jack-in-the-pulpit stalk topped by a short cone of bright red berries, and the Virginia creeper leaves turning pink to bright or dark red. Then all is covered by a thick layer of fallen leaves that insulates the ground during the winter.
Origins of the woods
Most of the overstory trees in the Preserve are straight and tall with umbrella-like crowns, but a few still have large, low-hanging branches spreading in all directions. These “wolf” trees serve as testament to the origins of the woods, which were once embedded in a vast complex of prairies, savannas and open oak woodlands. At the time of European settlement in the early 19th century, crowns of these wolf trees grew in all directions, as they enjoyed an existence free of arboreal neighbors. Actually, neighboring trees were abundant, but the vast majority of these were “grubs,” low-stature sprout clumps of oaks and, less commonly, other species such as Shagbark hickory and Black cherry. These grubs routinely re-sprouted after top-kill by fires that swept through savannas and prairies of southern Wisconsin.
Many oaks in the Preserve have multiple stems, which is a manifestation of their sprout clump origin. So, the beginnings of a forest were most likely in place when burning ceased on the landscape. One feature of the “grubs” is that they grew with amazing speed after the fires were stopped because they had developed extensive root systems over the years that their tops were continually set back by fire. Thus, as occurred throughout southern Wisconsin, oak-dominated forests on the Preserve developed quickly in the 1800s on lands that weren’t pastured or plowed.
Future of the woods
Through the years an increasing number of Sugar maples have recruited into the overstory and subcanopy in the Preserve woods. These trees provide a glimpse of the woods’ future. Sugar maple is one of the most shade-tolerant of the hardwoods in eastern North America. Accordingly, it has a firm grip on the tail end of the successional sequence common in forests on relatively moist, fertile soils throughout southern Wisconsin. In all likelihood, Sugar maple and other shade-tolerant species such as Basswood will eventually dominate the overstory in most of these woods.
This trajectory may be preempted, however, by exotic species invasions, which are common concerns for forests located in or near urban areas. In the case of our Preserve, several exotic woody species pose potential problems for the future of the woods. The greatest challenge probably stems from the invasion by Norway maple, which, like Sugar maple, is shade tolerant. It is also a prolific seeder and fierce competitor, and hence you will notice in certain woods a carpet of fast-growing Norway maple seedlings and saplings.
Invasive shrubs, primarily buckthorn and honeysuckle, have also captured significant portions of the forest understory and mid-story layers. Controlling them and the Norway maples will be important for the future of the woods. Since oaks cannot regenerate in shade, maintaining oaks in the Preserve over the long term will require the use of prescribed fires or creating open areas in other ways.