Typically by mid-April a lot of native woodland flowers are in bloom in the Cook County forest preserves. The reproductive strategy of these plants, called “spring ephemerals,” consists of coming into blossom, getting laid, and setting seed while the sun still shines. The bare tree branches let a lot of light through to the forest floor where these plants live; if they waited until May to bloom, the tree leaves would make it too dark. By June these ephemerals–trillium, bloodroot, mayapples–will turn yellow and rot back into the earth, disappearing from view entirely, having already stored enough energy for next year in underground bulbs and corms.
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There aren’t really dividing lines in nature. Just as spring can’t be extricated neatly from winter, forests intermingle shamelessly with prairies. This is particularly true at Bunker Hill Prairie and Edgebrook Flatwoods, the names ecologists have given to portions of this forest preserve. They were declared two separate sites in the 1977 inventory of Illinois “natural areas,” generic-sounding words that mean something very specific to ecologists. To call an ecosystem a “natural area” is to pay it the highest compliment, marking it as a place of ecological integrity, with a large percentage of its native species of plants and animals intact.
A few hundred feet farther down the trail the prairie grades into an open oak savanna dominated by white oaks. You can see the white rumps of flickers as they swoop among the trees, last year’s grasses rippling beneath them. Though it probably looked much like this a thousand years ago, this area is the product of recent change: the North Branch Prairie Project volunteers expanded their restoration work into the trees about five years ago. For 40 years this section had been gradually filling in with brush and European weeds. Its savanna origins were difficult to detect; it looked a lot more like the dense woods on the right side of the trail.
Gerould Wilhelm, a field taxonomist from the Morton Arboretum and one of the people most knowledgeable about the decline of quality in Illinois woodlands, estimates that the light levels in a typical Cook County forest preserve have decreased by ten times since the European settlers came. Plants used to bloom all during the growing season in the sunny, open woods. But in most Cook County woods only 1 percent of available light reaches the ground after the leaves come out. The only thing under the thick stands of European buckthorn is bare dirt.
I could see from the absence of leaf litter that the Edgebrook Flatwoods side of the stream had been burned recently, either last fall or this spring, something the volunteers wouldn’t have attempted when they started managing the preserves 16 years ago. The walls dividing the natural communities are falling, on the land and in the minds of the people taking care of it.
