Communities are not stable. New species arrive and other species die off locally. Some species (like dandelion) are better at colonizing bare soil enabling others to live in their shade (and retained moisture). Some species (like maples) grow better as understory trees until they reach larger sizes. Trees sold in garden shops and plant nurseries often have their trunks wrapped to protect them from sunburn because they ordinarily grow in the shade of larger trees.
Around here, most succession that takes place is field succession. Bare ground is colonized by a pioneer community of annual weeds, dandelion, white clover,
plantain, and other colonizing plants. This eventually gives way to an old field community of weeds and grasses which includes plants such as goldenrod, Queen Annes Lace, Ox-eye Daisy, Red and Yellow Clover, yarrow, and milkweed. Then shrubs like sumac, poison ivy, brambles, Amur and Japanese
honeysuckles, and multiflora rose begin to grow. The first trees are typically weedy, trees such as cottonwood, mulberry, and juniper. This gradually gives way to coniferous woods containing species like juniper with a ground cover that includes Lycopodium.
About 50 to 100 years later, as other trees, such as wild cherry, green and white ash, boxelder, and sassafras,
begin to grow, the community is referred to as a
mixed mesophytic forest.
There is still a lot of light so there is much undergrowth (like the West Woods). Often, the next step is an Oak-Hickory forest which actually contains a number of species of
trees, but primarily oak and hickory. Finally, the community would reach the Beech-Maple forest stage, which is the climax community.
While much of the forest around campus was, long ago, cleared for farmland, and some of that, now abandoned, is in various successional stages, the remaining virgin forest around here is beech-maple. Examples of this include the area on the south side of the tributary of Five Falls creek, south of school, and Beech Knoll, the hillside to the west, near the road. In these areas, the trees (especially the beeches) are huge, with some over 2 ft. in diameter. These areas are composed primarily of very old, tall beeches and maples with a scant understory of beeches, maples, and pawpaw. Very little light penetrates so there is very little understory and it is very open underneath and easy to walk through. Spring wildflowers bloom before the canopy fills in, and parasitic beechdrops grow on the roots of the beech trees. Typically, there is no wind, and higher humidity and lower temperatures than nearby communities where more sun reaches the floor.
On the south shore of Lake Michigan is an area known as the Dunes which is noted for studies which have been conducted on its successional stages. The area closest to the water line is open beach, and from there, succession proceeds through young foredune (with some grasses) to older foredune (with cottonwoods, etc.) to pine dune to black oak forest to oak-hickory forest, and finally, to beech-maple forest.
Similar successional stages occur along the shores of the ocean, as in this photograph of open beach and sea oats growing on a foredune. These foredune areas are extremely susceptible to erosion. Thus, in many areas, public access is limited to designated trails or boardwalks.
In a typical floodplain area, succession would proceed from areas of ragweed to willow to red maples (which like wetter soil than sugar maples) to elm to oak-hickory, and to sugar maple.
In a pond, beginning with a bare bottom with little vegetation, the first plants to grow would be submerged vegetation, then floating and emergent vegetation, then as decomposition and silt fill in the pond, succession would proceed to various other stages ending up at beech-maple forest.
In areas which are disturbed (fire, plowing, etc.), secondary succession will occur. Many areas around campus are examples of this. There are several old, abandoned fields, areas where young, weedy trees are beginning to grow, West Woods which has reached the mixed-mesophytic stage, and the hillside on the east bank of Maple Creek where a number of sugar maples, in varying sizes, may be found.
It should also be mentioned that changes in the animal population generally follow along with succession in the plant community, but can also help cause it (birds carry seeds).
Causes of succession are related to changes within the community itself. Climate influences succession in a community, but does not cause succession. Changes in the make-up of a community can and do cause changes in the climate within that community.
Autogenic factors are those causes of succession which are due to the presence/growth of the plants, themselves. Plants change the ecosystem, making it unsuitable for themselves, and the changes they cause make the ecosystem more suitable for organisms whose requirements are more stringent/more like the current habitat than their own. For example, grasses stabilize the soil and add organic matter so cottonwood can grow. When pines/juniper eventually begin to grow, they create more shade so grasses can no longer grow, but hardwood trees like oak and maple can grow in their shade.
Allogenic factors are those causes of succession which are due to abiotic components of the ecosystem. For example, ponds fill up by an accumulation of decayed organic matter and silt washed in by erosion. This makes the environment less suitable for aquatic vegetation.
As succession takes place, the community becomes more complex in both number of species and interactions among species. Because of the greater species diversity, the community is, then, more stable.
A climax community is in equilibrium with its habitat--it has reached a steady state. However, depending on local variations in climate, etc. an area may never go all the way to beech-maple. For example, due to local conditions, a community might stop at Oak-Hickory. Theoretically, the climax (in eastern North American deciduous forest) is beech-maple, but if, for example, the forest is growing on an upland, well-drained slope where there is drier soil, the community may only go to oak-hickory. This is called an edaphic climax.
Many prairie species are also fire tolerant while most “invading” species are not, and it is thought that years ago, prairies were maintained by frequent fire during the drier seasons of the year. The people who manage the prairie preserves in Adams Co. use burning as a “natural” method of preventing trees, thistles, etc. from growing into and taking over the often-tiny prairie remnants that are left there.
Copyright © 1999 by J. Stein Carter. All rights reserved.
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