Plant succession is the natural progression of plant growth in a given area, over time. It is broken down into stages to better understand the activity occurring in the process. A classic example of plant succession occurred after the eruption of Mount St. Helens, which left behind a barren wasteland. Over time, plants began to re-emerge. The ecosystem has changed, as it has matured, from a few patches of small, annual plants to the forest that is beginning to form today.
The first stage of succession is known as the pioneer community. At this stage, a relatively few, very simple species exist within the ecosystem. These plants may be as simple as mosses and lichens and break down the underlying substrate to help establish an environment that other, more complex plants can grow in.
The second stage is characterized by the emergence of plants including annual herbs, grasses and small shrubs. These plants are fast growing and usually opportunistic. They are very prolific, producing millions of seeds which help to establish their dominant position in the plant community. These plants often die on an annual basis, providing more nutrient-rich organic material to improve and deepen the soil even more.
Perennial shrubs will eventually come to dominate the landscape, out-performing the grasses and herbs in the area. They are larger, with deeper root systems, making them more able to absorb sunlight and rainwater. Growth remains very rapid. Softwood saplings will also begin to grow during this stage.
Young Softwood Forest Stage
As the soil improves, small trees will begin to grow. These are generally fast growing conifers such as pines and firs, and deciduous softwoods, including aspen and beech. These trees will eventually take over the community forming a young forest. The soil will continue to be improved by leaf litter that will compost over time. The tree canopy will shield shrubs and grasses from the light, allowing only those species that can survive in shade to thrive.
Mature Old Growth Forest Stage
Fast growing conifers and deciduous trees will eventually die, producing gaps in the canopy. The soil will have been improved to the point where large hardwood trees, such as oak, maple, and black walnut can establish themselves. These will continue to thrive and eventually dominate the forest, pressing out less substantial softwood species.
Climax Community Stage
This stage occurs when the plants within the community achieve a state of equilibrium within the environment--as long as the environmental conditions continue unchanged. Plants intolerant of this community cannot compete and will inevitably fail. Those that are tolerant will continue to thrive.
This model is, understandably, oversimplified and relies on extreme consistency within the ecosystem. In real life, circumstances may change dramatically to alter the course of progression in these stages. Also, different environments--including desert, wetlands, tundra and jungle--all have significant variation in their development.