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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.

But it's been a cold spring so far, and when I went for a walk last week in the forest preserves at Devon and Caldwell little was in bloom, though the plants were ready to burst into flower as soon as we had a few warm days in a row. It looked and felt more like fall; I'd picked a 40-degree stone-gray day. Last year's dried savanna grasses and bare red dogwood stems still dominate the landscape. Some ruby crowned kinglets, one of the first birds to show up in spring migration, bounced between oak branches, though without binoculars they were hardly a heart-stirring sign of spring. They looked like just another bunch of what my birder mother-in-law calls "little brown jobs," or LBJs for short.

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.

The 1977 natural-area maps declare Bunker Hill Prairie to be on the west side of a small stream and Edgebrook Flatwoods to be on the east side. This looks clear enough on paper, but things are more complicated on land. If you walk south on the bike path from the Bunker Hill Forest Preserve parking lot, a mile north of Devon on Caldwell, Bunker Hill Prairie comes up quickly on the left. It's marked by a Forest Preserve District sign that explains what a prairie is and a small North Branch Prairie Project plaque that says the area is under the care of a volunteer group trying to restore natural areas to good health. On the right side of the trail the woods slope down about 30 feet to the edge of the Chicago River's north branch. The paved bike path feels like a sharp dividing line between two ecosystems.

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.

Farther still, almost to Devon, you walk down a slope to the stream that supposedly divides Bunker Hill Prairie from Edgebrook Flatwoods. But you really left the prairie a few minutes ago and have been walking under tree cover ever since.

In its pre-European-settler state the Chicago landscape probably ranged from wide-open wet prairies to savannas with a few big oaks to more densely wooded areas with lots of grasses and flowers growing under the trees. But the settlers began to simplify this spectrum of ecotypes into just two categories: forest and prairie. And not just rhetorically. They changed the landscape. When they arrived the bur oaks, hickories, and black maples were far enough apart that they could drive their covered wagons between them. Now the forests have filled in, becoming tangled jungles that are often impossible to walk in, much less drive a wagon through.

There are several reasons for this. For 10,000 years this region was swept by frequent fires set by lightning or by native people, who used fire as a hunting tool. And the plants and animals had thrived because of it. But the settlers, trying to keep their homes and barns from burning down, fought the fires.

They also brought buckthorn and other alien plants from Europe, Asia, and other regions of the United States. With no fires to thwart their growth, these intruders prospered in the rich soils, shading out the native species, including wild rye, culver's root, columbine, and fire pink.

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.

The plants that bloom from May through August have been most affected by the diminished light, but the spring flora have also been indirectly harmed. As the summer bloomers have slowly been killed by the increased shade, the amount of ground cover has decreased. That has caused slopes along streams and rivers to erode. And when the soil slides away, the little bulbs and corms of the spring flowers wash away too. Wilhelm knows of whole woodlands where the soil is almost gone. Huge tree roots stick out of the ground, with no soil around them. Rocks sit on dirt pedestals, all the surrounding dirt having washed away.

Knowing all of this can make a simple spring walk in a forest preserve depressing. I sometimes feel the same way about movies; understanding how they're made makes me see things that are wrong and keeps me from liking them the way I once did. Seeing green leaves coming out on low trees under oaks doesn't make me happy if I know they're all buckthorn leaves.

But going to Bunker Hill is like seeing a good movie. Things aren't perfect, yet the problems are being addressed. The North Branch Prairie Project is an amazing group of people who spend every Sunday morning correcting ecological wrongs. They've cleared brush out of the prairie and from under the trees in the savanna and woods. They've replanted the summer bloomers that have been missing for so many years. They've even crossed the asphalt dividing line and are working in the woods.

In doing their restoration work the volunteers have discovered that landscape distinctions are very fine. Grounded in reality rather than labels, they've been able to discern gradations among the natural communities that had eluded ecologists who were only studying them.

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.

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