Over periods of years or decades, the plants that grow in any given place change. New species take the place of those that came before. This process is called plant succession or, more broadly, ecological succession, because as the plants change so do the microorganisms and animals.
Succession is different in each ecosystem. In the eastern United States for example, there is an abundance of broad-leaved trees such as oak and hickory, rare and usually diminutive in the Rockies. Here pines, spruces, and firs predominate, with occasional groves of aspen or cottonwood.
There are patterns to these changes from one species to another. For example, aspen (Populus tremuloides) grows well in sunny openings. Therefore, a slope that has been cleared of evergreens by an avalanche is a perfect spot for a grove of aspen to get started. Similarly, lodgepole pine is very good at populating areas that have been burned.
Eventually, though, these trees put themselves out of business by creating so much shade that their seedlings do not survive as well as competing species like spruce, fir, and ponderosa pine (Pinus ponderosa). Thus the forest type succeeds from aspen or lodgepole to ponderosa pine, Engelmann spruce (Picea engelmannii), Douglas-fir (Pseudotsuga menziesii), or subalpine fir (Abies lasiocarpa).
Usually it is a mix of these which form the climax vegetation, a group of species that will stand over time unless disrupted by natural catastrophe or climate change. Pondersosa pine and Engelmann spruce tend to forest the lower elevations, below nine or ten thousand feet; Engelmann spruce and fir grow in the higher elevations, up to tree line.
It would be wrong, though, to leave the impression that succession begins with trees. In places bare of vegetation, smaller plants must begin the process--mosses, grasses, soft flowering plants (forbs), and shrubs. Even on bare rock, lichens can take advantage of microscopic holds and begin the process that leads to succeeding generations.
Eventually, most plant communities reach a concluding phase, called the climax community. Here in Rocky Mountain National Park, the climax communities are some form of conifer. However, many factors can affect the process of succession, halting it in an early stage or obliterating a climax stage. Elk, for example, may browse aspen seedlings and prevent a grassland from succeeding to aspen groves. Avalanches, wind storms, and floods can wipe out even the largest evergreens.
Perhaps the most electrifying and devastating example of this occurred in 1982, when a high altitude dam burst open and sent much of its lake rushing down the Roaring River. The flood devastated much of the town of Estes Park and left an alluvial fan of soil, sand, and rock at the base of the river. Scientists have been studying succession on the alluvial fan with much interest