Living on the edge

Lately, I've been thinking about tension zones and ecotones.

If you're still with me, you might want to keep reading because these topics are fascinating--especially for gardeners and plant geeks.

We learned about tension zones in my master naturalist courses earlier this summer. An ecological tension zone, anywhere in the world, is where two broad ecological regions meet. Any tension zone is a diverse area--where representative plant and animal species from both zones on either side overlap.

For example, here's a map from the Biota of North America Program (BONAP), showing floristic tension zones throughout the U.S.:

Source: bonap.org

My state, Wisconsin, is split by a diagonal line running from the northwest through the southeast. Two major ecological zones fall on either side, and where they meet, the plant and animal species are more diverse. You can click here to see more information and maps explaining this concept, and to read about the floristic zones in other U.S. regions. (I couldn't find a simple, nonacademic online resource for worldwide tension zones, but if you research your specific country or region, hopefully a similar resource is available.)

I don't live along Wisconsin's tension zone--I'm firmly in the ecological region shared with most of Illinois and parts of Iowa, Minnesota, and Missouri. This region is known for its fertile soil, deciduous forests and Oak savannas.

And then, of course, there are edges on a much smaller scale--for example, where a sandy soil region meets a clay soil region.

Or even smaller areas that you can observe just standing in one spot. For example, ecotones are defined by the Encyclopedia of Earth as transitional areas between two different ecosystems, such as a forest and a grassland. Click here to read about this fascinating topic.

This also ties in with the concepts of permaculture. Appalachian Feet did an excellent post a while back titled, "How to Learn About Permaculture." If you missed the post, it's definitely worth reading.

All of these concepts have significance for gardeners, botanists, biologists, naturalists, and anyone else who studies or simply enjoys nature. Ecotones, for example, are thought to be important ecological indicators of global change. They're thought to be sensitive to changing climates, and many scientists advocate monitoring ecotones to detect patterns of global change.

My garden resides along a small ecotone. It's a space between an open suburban area and a small forest along a glacial drumlin. The biodiversity is especially evident when the seasons change--when the migrating birds travel into, out of, and through the area, and when the deciduous trees either leaf out or lose their leaves. Back in the days before large-scale agriculture and settlement, this plot of land was probably part of an Oak opening, with less than 50% tree canopy coverage and frequent fires. The part at the top of the drumlin might have had more trees (as it does now) because of its placement on higher ground.

These concepts of tension zones, ecotones, and Oak openings were becoming common parlance for plant specialists like John Muir at the turn of the 20th century. He describes the uniqueness of Oak openings in "The Story of My Boyhood and Youth":

"When an acorn or hickory-nut [grub] had sent up its first season's sprout, a few inches long, it was burned off in the autumn grass fires; but the root continued to hold on to life, formed a callus over the wound and sent up one or more shoots the next spring. Next autumn, these new shoots were burned off, but the root and calloused head, about level with the surface of the ground, continued to grow and send up more new shoots; and so on, almost every year until very old, probably far more than a century, while the tops, which would naturally have become tall broad-headed trees, were only mere sprouts seldom more than two years old. Thus the ground was kept open like a prairie, with only five or six trees to the acre, which had escaped the fire by having the good fortune to grow on a bare spot at the door of a fox or badger den, or between straggling grass-tufts wide apart on the poorest sandy soil.

The uniformly rich soil of the Illinois and Wisconsin prairies produced so close and tall a growth of grasses for fires that no tree could live on it. Had there been no fires, these fine prairies, so marked a feature of the country, would have been covered by the heaviest forests. As soon as the oak openings in our neighborhood were settled, and the farmers had prevented running grass-fires, the grubs grew up into trees and formed tall thickets so dense that it was difficult to walk through them, and every trace of the sunny 'openings' vanished."

While the "Oak opening" ecotone in my garden is now partially cultivated, it's a fascinating little place teaming with life. Understanding what it was, what it is, and what it can be is helpful to me, as its gardener and caretaker.