Plant communities can change through time. An important kind of community change is succession. Succession is the gradual replacement of one plant community with another, sometimes of very different composition, on the same site. Succession can begin on completely barren sites (such as on lava flows following a volcanic eruption, as happened at Mt. St. Helens). Succession begins commonly when an existing plant community is disrupted by some kind of disturbance. The disturbance can be natural, like a forest fire or a flood, or it can be human-caused, like logging or farming. In either case, after the disturbance, the plant community begins the process of succession. First, colonizing plant species enter the site. These are usually easily dispersed, fast-growing plants that can produce many offspring quickly, such as fireweed (Epilobium). Over several years in an environment conducive to forest formation, plants that are slower-growing and require longer periods to produce offspring-such as red alder (Alnus rubra) in western Washington forests-become established. Finally, the "permanent" members of the community, which can replace themselves if they are undisturbed, will become established. These include the longer-lived trees that grow slowly and have shade tolerant offspring. Succession occurs over tens to hundreds of years and ends when the composition of the plant community stabilizes (that is, when the members of the community replace themselves rather than being succeeded by another kind of plant) to form a so-called climax community. The climax community remains until there is another disturbance, and then the cycle begins over again.

Lava from Mt. St. Helens created a barren landscape, as shown here near Spirit Lake, on which succession has begun.	In the volcanic blast around Mt. St. Helens, succession has begun to replace destroyed forests.

Colonizers, such as this spotted cat's-ear (Hypochaeris radiata) of the sunflower family, usually produce high numbers of seeds that are easily dispersed over great distances.	Fireweed is a common colonizing plant found in disturbed areas of Washington.