There are two types of ecological succession; primary and secondary. If a field that has been used to grow agricultural crops is abandoned, it will soon become overgrown with grasses, goldenrod, and other herbaceous plants. If this field was once a woodland, the weed field will be invaded in a few years by shrubs such as blackberry, sumac, and hawthorn. In time the field will support a woodland of hickory, maple, oaks or pines. This is not a haphazard process; it is an orderly sequence of change from simple to more complex communities, from a few species of grasses and herbs to a forest of trees, shrubs, and a variety of ground plants. For each geographic region, with its particular conditions of climate and soil, there is a characteristic sequence of communities. Because this succession takes place where vegetation was formerly established, and proceeds from a state in which a prior community of plants and animals was present, it is called secondary succession. Thus, secondary succession occurs in areas that are disturbed by men or animals or by natural forces such as fires and wind storms.
Primary succession begins on surfaces that have never before been colonized by plants or animals. Bare rocks on exposed sites are common locations for primary terrestrial succession. These rocks are first colonized by lichens which spread over the surface of the rock. When organic debris and soil begin to accumulate in crevices in the rock, other plants can take root. Mosses and grasses become established and, as wind and water and ice create deeper crevices with greater accumulations of soil, woody plants begin to take hold. Eventually deep soil may be formed, allowing the development of shrub communities and finally forests.