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  • Thinking Like a Forest: On Fire and Oak Woods
  • Thomas B. Simpson (bio)

I know of a white oak tree that is probably more than 500 years old. I have no way of proving this, but it is growing on a dry hilltop and thus growing very slowly owing to the lack of water, and dwarfs a 300-year-old white oak I cored for its age many years ago. Each time I go to these woods, I stop by and ponder this great mushroom-shaped tree, 130 cm in diameter, only 14 m tall, and with a 23 m crown spread. To the north, downslope on the Marengo Moraine, grew bur oak savanna a few centuries ago, and beyond that open prairie stretched for kilometers to the north and west across the glacial outwash plain. The woods are filled with young trees and shrubs of other species today, which cut off sightlines to the north, but it is easy to imagine standing on this knoll and looking out across the prairie—pretty heady stuff.

In its lifetime this oak has produced between one and two million acorns (Burns and Honkala 1990), yet within 40 m of the tree (squirrels rarely move acorns farther than this from the parent tree) grow only three mature white oaks. There are no younger white oaks nearby. Assuming these trees are the progeny of this one individual and not any other parent tree nearby, then about 0.0003% to 0.00015% of the offspring produced by this tree survive today as large adults. This means that 99.9997% to 99.99985% of all embryos died—eaten by predators, killed by disease, or consumed by fire or fungi.

For an oak woodland or savanna to have maintained a more or less constant density over the centuries, each old oak on average should have produced only one tree that lived to a similar age. To keep the math simple, if a tree lived to 100 years and during that time produced one offspring that lived to a similar age, and it had a heavy seed crop of 10,000 acorns every four years from age 50 to 100, then the survival rate was somewhat higher than that calculated above: about 0.001%. So between 0.0001% and 0.001% of acorns survived to produce large trees—all the rest died young.

Life for an oak tree was typically hazardous and short.

Mature oaks dominate the architecture of the brush-filled woods so common today, but they are functionally sterile artifacts of the past. One would expect a dozen large trees to be preceded by hundreds of saplings, and these by thousands of seedlings, yet hectares of woods have no white oak saplings today. Seedlings germinate and begin growth after a heavy seed crop, but die by midsummer of the next year owing to the lack of sunlight in densely shaded woods. So the dominance of white oak in these woods and in similar oak woods all over the Chicago Region, in fact all over the Midwest for thousands of years, was a result of the miniscule but significant survival rate of something like 0.0003%.

This low survival rate is not a bad thing; it is a good thing. That is hard to get your head around. It turns out that what is bad for oak trees, principally fire, can be good for the forest.

Allowing the free reproduction of elms, ashes, black cherries, maples, hackberries, and non-natives like shrub honeysuckles and common buckthorn doesn’t kill the old oaks. Their lower branches die from shading, but the trees command so much of the canopy that most get plenty of sunlight to survive. The old oaks will decline and die one by one regardless of what we do, succumbing to the stress induced by competition, insects, or disease. Setting fire to the woods, however, does tend to shorten their lives. Fire gets into the hollows of old trees or starts in dead branches and burns into the living part of the tree. I have seen fire climb up the bark of an old bur oak and kill half the tree. Not burning prolongs the life...

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