Winter 2005

Ice and Fire

In this photo, notice six sinuous layers. Ice, open water, a snow-covered band of wetland grasses and wildflowers, little trees holding dried leaves, bigger trees that are pretty much of equal size, and a sky of oxygen-rich air.

Water freezes slowest and thaws fastest near the shore. It's a reminder that what appears as an isolated pond is merely the part of the water table that we can see. Under the soil and the tree roots, the underground reservoir of warmer water continues to flow — and to melt the ice along the pond's edge.

Here in Spears Woods, many of the old trees are bur oaks. Bur oaks indicate a history of frequent fire; thanks to their thick, corky bark they have better protection from flames than any other tree. Often bur oaks have horizontal limbs well down the trunk, telling us that they spent their youths in lots of sun, in a grassland, with ample treeless space around them. Yet the tallest trees in this photo are mid-sized, close together, and clearly, when young, raced each other for the light. They have few branches except near their tops. These trees began all at once — after being cut over or after a catastrophic fire.

Note how the little trees here hold their dead leaves in winter. Most of these are scarlet oak and some white oak. The bur oaks are bare, since these are the first to drop their leaves each fall. Is this an adaptation to promote as much fire as possible, since the bur is so dependent on it? Scarlet oak, with thin fire-sensitive bark, holds its leaves. "Cool fires" — those on damp days with low wind — often skip by patches of scarlet oak, leaving them unburned and the young oaks intact. The hottest fires — those on the driest, hottest, windiest days — consume whole scarlet oaks and everything above them. Is this, too, an adaptation? Scarlet oaks have an interesting ecology; they thrive best in the fieriest landscapes despite their thin bark. They're champion re-sprouters. Might they "torch" their tops as a weapon against getting overgrown?

White and red oaks are variable about dropping their leaves. Many young white oaks hold their leaves in winter while nearby older whites drop theirs. Adaptation for something? No one seems to know.

One thing we do know: to conserve the teeming diversity of animal and plant species of our oak woods, land managers need more resources and more support for controlled burns. Under a diminished fire regime, the bur oaks stop reproducing first, then go the scarlet, then the white. In the short run, the red oaks invade to take their places, but soon all oaks are replaced by cherry, or basswood, or ash, or maple, or just buckthorn. None of the birds, butterflies, native bees, grasses, orchids or asters of the oak woods survive in the gloom of an invader "forest." The poet Robert Frost worried about the world ending in fire and ice. I'm more concerned about the cold, dark, silent shade of buckthorn.

But forests like Spears Woods are getting controlled burns once again. Many plants and animals (including Homo sapiens) rejoice. Joy to the natural world!

Photo by Mike MacDonald. Words by Stephen Packard. Spears Woods (south of Willow Springs) thrives today thanks to the Forest Preserve District of Cook County.

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