Succession

Secondary succession occurs after disturbances that are not intense enough to kill all plants, so that regeneration can occur by re-sprouting and growth of surviving individuals, and by the germination of pre-existing seeds to establish new plants. This regeneration by surviving plants and seeds is supplemented by an aggressive invasion of plant seeds from elsewhere. Another characteristic of secondary succession is that the soil still retains much of its former character, including previous biological and climatic influences on its development.

Secondary successions are much more common than primary successions, because disturbances are rarely intense enough to obliterate previous ecological influences. Most natural disturbances, such as windstorms, wildfires, and insect defoliations, are followed by ecological recovery through secondary succession. The same is true of most disturbances associated with human activities, such as the abandonment of agricultural lands, and the harvesting of forests.

Secondary succession can be illustrated by an example involving natural regeneration after the clear-cutting of a mature, mixed-species forest in northeastern North America. In this case, the original forest was dominated by a mixture of angiosperm and coniferous tree species, plus various plants that are tolerant of the stressful, shaded conditions beneath a closed forest canopy. Some of the plants of the original community survive the disturbance of clear-cutting, and they immediately begin to regenerate. For example, each cut stump of red maple (Acer rubrum) rapidly issues hundreds of new sprouts, which grow rapidly, and eventually self-thin to only 1-3 mature stems after about 50 years. Other species regenerate from a long-lived seed bank, buried in the forest floor and stimulated to germinate by the environmental conditions occurring after disturbance of the forest. Pin cherry (Prunus pensylvanica) and red raspberry (Rubus strigosus) are especially effective at this type of regeneration, and these species are prominent during the first several decades of the secondary succession. Some of the original species do not survive the clear-cutting in large numbers, and they must re-invade the developing habitat. This is often the case of coniferous trees, such as red spruce (Picea rubens). Another group of species is not even present in the community prior to its disturbance, but they quickly invade the site to take advantage of the temporary conditions of resource availability and little competition immediately after disturbance. Examples of these socalled ruderal species are woody plants such as alders (in this case, Alnus rugosa) and white birch (Betula papyrifera), and a great richness of herbaceous perennial plants, especially species in the aster family (Asteraceae), such as goldenrod (Solidago canadensis) and aster (Aster umbellatus), along with various species of grasses, sedges, and other monocotyledonous plants.

This secondary succession of plant communities is accompanied by a succession of animal communities. For example, the mature forest is dominated by various species of warblers, vireos, thrushes, woodpeckers, flycatchers, and others. After clear-cutting, this avian community is replaced by a community made up of other native species of birds, which specialize in utilizing the young habitats that are available after disturbance. Eventually, as the regenerating forest matures, the bird species of mature forest re-invade the stand, and their community re-assembles after about 30-40 years has passed.