When a habitat suddenly has room for life--perhaps over land created by volcanic action or because fire swept out what had lived in an area--succession can take place; primary succession if the habitat is brand new and secondary succession if the habitat is one open to colonization because of damage or disturbance. New life moves in, creating what is called a pioneer community. Plant forms like lichen and mosses can live in places with little soil and, indeed, form it. This paves the way for the next group of plants.
Pioneering plants change the nature of the habitat. The change in the environment creates new opportunities for different kinds of plants, along with creatures that can live amid and because of the new plants. Plants in this new stage of succession are low-growing, tend to sprout quickly and have seeds that blow in on the wind. These include plants that are opportunists, like weeds, for instance. Other plants that take root during this stage include grasses and wildflowers.
The grassland creates soil and increases that soil's richness so that shrubs can grow. Shrubs are woody plants that include low-growing plants, bushes and small trees that grow up to about 20 feet tall. These plants can be evergreen or deciduous and are perennial. They exist with annuals and biennials.
While shrubs take hold, fast-growing trees are also rooting. Often these are predominantly pine forests. As needled trees like pines grow tall, broadleaf trees--dogwoods and gum trees, for instance--begin forming an understory.
Slow-growing, hardwood trees like oaks eventually take hold in the young forest. When these mature, the eco-system has reached a peak diversity, with plants and animals occupying what seem to be stable niches. It might take hundreds of years for this so-called climax community to be achieved. It's important to understand that the kinds of plants and the time it takes to reach the climax stage depend on the location in which the successive stages occur. Stages overlap, as well, and the number of stages can vary.