Clearcutting, Nature's Way
If tornadoes are death to buildings, they can be equally cataclysmic to the natural world, and to standing trees in a forest. That kind of "disturbance" interests ecologists who have come to realize that nature is not some form of "steady-state" system, but a fluid, ongoing "discussion" between many factors, including climate, nutrient cycles and the influx and departure of species.

This catastrophic blowdown occurred in the Flambeau River State Forest, in northern Wisconsin. 1977 photo courtesy Craig Lorimer.

Large disturbances have posed a challenge to ecologists and their understanding of how forests change over time. Earlier in this century, ecologist Frederick Clements promulgated the idea that forests would reach a "climax" condition. In other words, their species would perpetuate themselves in similar proportions, over the years.

While Clements was referring to a stable mix of species, Craig Lorimer, a professor of forestry at the University of Wisconsin-Madison, says "climax forest" was gradually redefined over the next few decades to mean a steady state of development.

By the 1960s, Lorimer says that concept "really went awry. People began to define climax forest as a steady state, with births balancing out deaths, and nutrient cycling quite stable. That was the death knell for the whole climax forest concept. It was so unrealistic, so perfect that no forest in the world could qualify."

A primary reason why natural forests did not resemble this steady-state forest was disturbances like tornadoes and other windstorms. (Other disturbances include fire, flooding, insects, disease, drought, ice storms, lava flows and glaciers. Even a simple tree-fall may create a "gap" with enough light for the regeneration of "shade-intolerant" tree species.)

In forests in the Upper Peninsula of Michigan, Lorimer says, winds will cause any given acre to suffer 60 percent loss of the forest canopy once in 1,200 to 1,500 years.

Using a lower standard of destruction -- 30 to 50 percent blow-down -- 20 to 40 percent of all tracts would be affected during the 300-year life span of many of the trees in these forests, he adds.

Blowdowns and other large disturbances are an important source of information on ecological succession. A recent study of a tornado's aftermath (see Forest Reorganization:...) showed a "dramatic" response by the plant community to the blowdown, with increases recorded in "species richness, tree seedling density and total percent cover." Six years after the windthrow, the new forest was dominated by seedlings and sprouts that started growing within a year of the tornado.

But these disturbances also pose a challenge for preservation. Even relatively large preserves are likely to suffer disturbances large enough to lead to drastic reductions in species numbers. Once, when the landscape was less altered by human impact, that was not likely to cause alterations in species numbers. But now, with nature preserves more and more isolated from each other, conservation biologists talk about them as "islands" of natural areas surrounded by "seas" of developed land. And just as the ocean is not a good source of tree seedlings, urban or agricultural landscapes are not good sources of seeds for natural areas.

In response to this line of thinking, some ecologists have started talking about setting aside "equilibrium landscapes" which would be too large to be destroyed by a single disturbance. An equilibrium landscape would not be some kind of steady state, Lorimer explains, but it would contain fairly constant areas in each stage of ecological succession. "This landscape would help maintain biodiversity," Lorimer explains, "because habitat for each species would always be present."

But he points out that the enormous 1988 fires in 2.22 million-acre Yellowstone National Park show that some equilibrium landscapes would have to be extremely large. "Some people feel it would be desirable, but it may not be practical at Yellowstone," he adds.

However, in the northern hardwoods forest of northern Michigan, a more manageable 20,000-acre preserve might comprise an equilibrium landscape.

At last! you're ready to take the famous twisted quiz.

For a look at some assembled wisdom (reading and surfing) on twisters...