Saturday, August 21, 2004
A few years ago I began surrendering the yard's steeper slopes to nature. They're just not safe to mow. At least that's what I tell my wife. These unmowed patches have become my favorite parts of the yard.
This year my "natural gardens" are dominated by chicory, winged sunflowers, ironweed, and berry-laden pokeweed that towers over me.
Nature abhors naked soil. A plot abandoned in the spring sometimes produces more sheer biomass than a well-tended garden. Undisturbed for a hundred years, a vacant lot would eventually be tree-covered.
Such changes in vegetation over time are not random chance or coincidence-- the process is called plant succession.
The disturbance can be immediate and small scale, such as tilling and then ignoring a garden or abandoning a vacant lot. Or it can be major -- a forest fire, a clear cut, cultivation, or even a volcanic eruption.
Visiting Mount St. Helens 20 years after she blew, I was reminded how dramatic ecological recovery can be even after a catastrophic disturbance.
Sometimes ecological disturbances take place slowly. A retreating glacier, for example, moves only inches each year as it scours the earth beneath it.
Regardless of the nature of the disturbance, plants respond predictably. The species may vary, but the direction of change is virtually certain.
'Pioneers'
Ecologists call the first small plants that invade a disturbed area "pioneers." Their life cycle is short, and they produce many small seeds that are usually dispersed by the wind or by sticking to animals. Crabgrass, ragweed, daisies, burdock and chicory are common pioneers.
Most important, however, these seeds remain viable in the soil for many years. The pioneers' very presence shades the soil, making it less suitable for their own seeds subsequently. Long-lived seeds allow pioneers to be able to recolonize the area the next time it's disturbed.
Plants characteristic of mid-successional stages are more tolerant of shade and have larger root systems that make them better able to compete for moisture in the soil.
Blackberries, honeysuckle, eastern red cedar and sassafras commonly invade old fields and later yield to larger trees. Which plants occupy any successional stage is determined by soil quality, rainfall and temperature.
Final stage
Succession continues for decades, sometimes for centuries, until a stage is reached that is capable of replacing itself. This is called the climax.
In the East, ecologists recognize oak-hickory and beech-maple stands as two of the more common climax forests. In the Midwest and Great Plains, limited rainfall cannot sustain forests, so climax communities are grasslands.
Unlike pioneers, climax species are usually large plants that grow slowly and tolerate shade. After a lengthy juvenile period, they produce relatively few large seeds that are dispersed by gravity (acorns and walnuts, for example). The seeds are relatively short-lived -- they either quickly germinate and help perpetuate the climax or are eaten by animals.
In theory, a climax stand replaces itself until an outside force destroys it or at least sets it back. Then the successional process begins anew.
In reality, however, a climax is not a discrete unit, but rather a collection of ever-changing successional stages shaped by local changes in moisture, wind, frost, soil, topography, fire, and the activities of animals.
So don't be embarrassed by a weed-infested garden that hasn't gotten the attention it deserves. Just tell the neighbors that the yard is your child's science fair project in natural plant succession.
shalaway @aol.com