The Somme preserves contain fine examples of some of the rarest kinds of forest on Earth. They are a rich mosaic of savanna and open oak woodland—ancient natural communities that are gradually being lost throughout their range. The natural ecosystem that survives here once covered large parts of central North America. Between the plains and prairies of the west and the dense forests of the east were millions of acres of oak forest and oak savanna, some of the most beautiful and most productive parts of the ancient landscape that had been in this region for thousands of years and had evolved in North America for about five million years. The keystone species of trees in most of this region’s forests were the oaks. Rich in wildlife and studded with bright flowers and birds, this ecosystem was dependent on occasional fire for good health. Today, with the suppression of natural fires and the invasion of weedy shrubs and trees, the species of this ancient forest system need help to recover.
Three Oaks and a Hickory
The Public Land Survey of 1838 and a census of the oldest trees that survive here today tell the same story. Most of the large trees here naturally are of four species—bur, scarlet, and white oak and shagbark hickory. Because they are the most adapted to fire, bur and scarlet oak are the major trees in the most open savanna areas. White oak is found where, for a variety of reasons, the fires are less intense.
Bur Oak — thick, deeply furrowed bark, thick and corky twigs, leaves that are thickest near the end away from the stem and have a deep lobe below the middle, lobes rounded at the tip, acorn caps are covered with a “bur” of rough filaments.
Scarlet Oak — smooth, thin bark, deeply cut leaves, each lobe ending with a sharp point. Acorns small. Most older trees have a scraggle of dead, often downward-pointing branches below the crown.
White Oak — smooth flaky white bark, deeply lobed leaves with rounded tips. Acorns long and thin.
Shagbark Hickory — easily recognized by the large curved plates of bark that curve out from the trunks of mature trees.
More Trees of the Ancient Woodlands
A student of trees will find a great many additional species at Somme Prairie Grove. Among the oaks are black, chinquapin, red, pin, and swamp white oaks. There are also hybrid oaks including the Bebb oak (a hybrid of white and bur oak). Two other nut trees include black walnut and bitternut hickory. Other species that may have been present in open prairie groves (but which may be more common today) include choke cherry, wild black cherry, American elm, and basswood. In the ponds and wetlands large cottonwood and various willows grow.
Small Trees and Shrubs
American hazelnut—once the commonest shrub in the region—has declined in the absence of fire even more than the oaks. Both the savanna and woodland naturally had shrubby components or phases—perhaps changing dramatically over time in height and density as a response to changing fire frequency and intensity.
In addition to hazelnut, shrub species present at Somme include gray dogwood, silky dogwood, nannyberry, smooth sumac, wild plum, Iowa crab apple, wafer ash, prickly ash, and many species of hawthorn.
Many species of herbaceous plants and animals (especially birds) appear to be associated with shrubby areas. Birds found in the shrubby areas at Somme include the indigo bunting, brown thrasher, catbird, yellow warbler, common yellowthroat, black-billed cuckoo, kingbird, cedar waxwing, song sparrow, field sparrow, and the ruby-throated hummingbird.
Exotic and Invasive Trees and Shrubs
Some trees from outside the region were planted at Somme by the Forest Preserve District many decades ago, in the days before the District (or any other agency) had developed detailed principles for the restoration of the county’s natural forests. Some of these include black locust, silver poplar, honey locust, pine, birch, and post oak. Most of these have dropped out over the years, but black locust and silver poplar have become serious problem species.
Trees natural in this region, but not to the oak woods can also be problem invasives. Box elder and quaking aspen are examples. These are species of disturbance that may increase dramatically in the absence of fire. Other species of more stable, closed forest communities may also be invasive in the oak woodlands. These include silver maple and green ash of the floodplains, along with sugar maple and white ash of the upland closed forest. Under modern conditions, these species may entirely crowd out the natural species of the Somme preserves, eliminating oak reproduction and killing off the grasses, wildflowers, butterflies, birds, salamanders, and other species dependent on the sunny open oak woods. The presence of some of these species is natural, but their over-abundance is the problem. Good stewardship of oak woodlands thins selected individuals of these species to reduce their abundance sufficiently that the oaks and their companion species can reproduce and thrive.
The single most damaging invasive in the Somme preserves is European buckthorn. It successfully invades prairie, savanna, and woodland—in some areas so densely that essentially no other plant species (and few species of animals) survive. It is the goal of the Forest Preserve District’s management plan to eliminate such invasive exotics from the site.
- Protecting young oaks from deer browse by exclusion cages (only necessary until the oaks are six feet tall)
- Protecting oak saplings from deer antlers by placing brush barriers until the oak are large enough (3″ or 4″ in diameter).
- Oak rescue—cutting away invasive species that are shading out the young oaks.
- Harvesting, planting and protecting hazelnuts. This species is not naturally reproducing on the site. With a few years of help, it will likely be able to reproduce naturally. Needed work includes protecting hazel from deer browsing and beating the squirrels to the nuts.
- Propagating wild plum. This species is now common enough at Somme that in probably needs little help except occasional rescue from overgrowth by buckthorn.
- Monitoring. Ongoing studies are documenting changes in the populations of trees (and shrubs, grasses, wildflowers, birds, butterflies, frogs, etc. etc) in response to management (or the lack thereof). Help is needed.
- Work party volunteers. On most weekends throughout the year there are Saturday or Sunday morning volunteer opportunities at Somme Prairie, Prairie Grove, or Woods. Check out the calendar. Everyone is invited to help out.
Acknowledgements: The following people have contributed to the development of the ideas presented in this summary. George Ware and Marlin Bowles (Morton Arboretum), Gerould Wilhelm (Conservation Design Forum), Wayne Lampa, (Forest Preserve District of Du Page County), Roger Anderson (Illinois State University), Tom Vanderpoel (Citizens for Conservation), Jim Steffen (Chicago Botanic Garden).