Succession is the change in either species composition, structure, or architecture of vegetation through time. Vegetation has three important aspects that are subject to change.
Species composition includes the roster of the species. There can be many or few species in vegetation.
Structure refers to the absolute and relative abundances and other relationships of the species constituting the community. The total number of individuals of each species, or the rank order of abundance of the different species are examples of vegetation structure.
Architecture refers to the arrangement in three dimensional space of any of the elements of a community. Architecture can reflect the spatial distribution and clustering of species, of life forms of plants, or patches of vegetation.
The term "succession" was coined when ecologists assumed that one community of plants gave way to another. Hence, communities succeeded one another. However, we now understand that communities may not always be discrete and that they may blend into one another gradually through time. In addition, there may not be a fixed sequence of species that necessarily has to occur in a region. General tendencies in succession may appear, although the trends are often probabilistic and flexible.
Succession occurs in all sorts of vegetation. Indeed in some cases, succession coverts one vegetation type to another. In some cases, succession takes place on new substrate that has few available nutrients or seeds and other sources of plants. For example, ponds may undergo succession through bogs to closed canopy forests. Or an area denuded by the eruption of a volcano may develop a plant cover that changes into a grassland or a forest.
In other cases, the site may have previously supported vegetation, and have soils, nutrients, and seeds or surviving plant parts available for vegetative propagation. The post-agricultural succession at the Hutcheson Memorial Forest Center is an example of such secondary successions.
Similarly, gaps formed by windthrown trees go through compositional and structural changes through time. Floods and fires may kill existing dominant plants and the altered or obliterated community become a site for succession. In these cases, low-statured vegetation dominated by herbaceous plants or shrubs will usually be replaced by trees, ultimately forming a multi-layered forest.
Here are some general sources on succession that provide a good introduction to the topic.
Barbour, M. G., J. H. Burk, and W. D. Pitts 1987. Terrestrial Plant Ecology. The Benjamin Cummings Publishing Company Inc. Menlo Park, California. 634 pages.
Glenn-Lewin, D. C., R. K. Peet, and T. T. Veblen 1992. Plant Succession: Theory and Prediction. Chapman and Hall, London. 352 pages.
Luken,J.O. 1990. Directing ecological succession. Chapman and Hall, New York. 251 pages.
Morin, P. J. 1999. Community Ecology. Blackwell Science, Massachusetts. 424 pages.
Niering,W. A. 1987. Vegetation dynamics (succession and climax) in relation to plant community management. Conserv. Biol. 1, 287-295.
Ricklefs, R. E. 1990. Ecology. W. H. Freeman and Company, New York. 896 pages.