Many plant communities are not self-sustaining. A field in the temperate deciduous forest biome will remain a field only as long as it is grazed by animals or mowed regularly. If these factors are eliminated, the balance tips in favor of other species. The newcomers will, in turn, establish conditions that no longer favor them but promote the growth of still other species. The revered naturalist and writer, Henry David Thoreau, called this process succession.
Primary Plant Succession
The process of plant succession begins just as soon as a land area capable of supporting plant life is formed. Some examples:
- accumulation of sand dunes at the edge of the ocean or a lake
- cooling of a lava flow
- exposure of rock by a retreating glacier
Bare rock succession in a temperate deciduous forest biome
- The first colonizers are lichens and certain mosses. Acids secreted by the lichens attack the rock and provide bits of soil. Additional soil particles may be formed by weathering or be blown in from elsewhere. Damage and decay to the lichens supplies some humus, and eventually enough soil is formed to support other mosses.
- The growth, death, and decay of mosses produces more humus, and soon there is enough to support the growth of
- grasses and
- shrubby growth such as low-bush blueberries and huckleberries. These, in turn, provide the conditions for such sun-loving, fast-growing species as
- gray birch trees and poplars (quaking aspens).
- In time, white pines replace these. In the dense shade of mature white pines, only shade-tolerant maples and beech seedlings can gain a foothold. If the shallow-rooted white pines are removed by a hurricane or by lumbering, the maples and beeches can take over.
Another example of plant succession occurs as shallow ponds gradually fill in with soil washed in from the surrounding terrain and organic matter produced by underwater plants.
As we walk from the edge of a poorly-drained, boggy pond back into a temperate deciduous forest, we pass through a series of zones that recreate in space the plant succession that has been occurring in time.
one passes concentric zones, each representing a later stage of plant succession as the soil has become firmer and the shade denser.
- From the swamp loosestrife at the waters edge past
- sphagnum moss, leatherleaf, sheep laurel, and pitcher plants, then
- swamp azaleas, high-bush blueberries and poison sumac, followed by
- black spruce and American larch and, finally,
- swamp maples and alders
Secondary Plant Succession
Lumbering, farming, fires, and hurricanes interrupt the process of succession by removing the dominant plants in the community. Their elimination sets the stage for a new succession to begin.
The many abandoned farms in New England (I live on one) illustrate this. People often wonder why our pioneers built stone walls through the woods. The answer is that they did not. The walls in the woods today once marked the boundaries of fields and pastures, but when cultivation and grazing ceased, a secondary succession began. Where I live,
In general, plant succession is a reflection of the increasing efficiency of the community at intercepting the energy of the sun and converting it into chemical energy. As one stage of succession follows another,
- the grass of abandoned fields soon became invaded by low-growing, herbaceous species like plantains, and golden rods.
- These were quickly followed by woody shrubs like the common juniper, blueberries and gray-stemmed dogwood
- Soon sun-loving gray birch, poplars, and Eastern red cedar ("old-field cedar") became established.
- White pines or, in sandy well-drained locations, oaks have followed.
- These will persist until some disturbance such as fire, hurricanes, or lumbering open up the land, and the process of secondary succession begins again.
- the biomass of the community increases. This is the outcome of the increasing amount of
- net productivity — calories stored by the plant community.
- This, in turn, provides calories for a larger community of consumers.
- As succession continues, the diversity of species in the community increases — at least for a time.
- When the system approaches its climax, the rate of increase in net productivity of the plants is consumed by its own heterotrophs.
- The system comes into equilibrium and reaches peak efficiency at channeling the energy of the sun into the food web of the community.
The graph (from Whittaker, R. H., Communities and Ecosystems, Macmillan, 1970) shows the changes in number of species, biomass, and net productivity during secondary succession in a temperate deciduous forest over a period of 160 years.
6 May 2011