When the vegetation in an area is destroyed, it is usually replaced. Often the new vegetation is different from what was destroyed. And in most cases, this new vegetation is gradually replaced with another and then another. The sequence for any particular area is always the same. Eventually the vegetation becomes stabilized.
This permanent, stable vegetation is said to be the climax vegetation for that particular region. The replacement of one vegetation, and then another, in a predictable sequence is known as succession. The fact that successional stages and climax vegetations differ from place to place results from differences both in the environment and in the history of the vegetation present.
Obviously, a particular species can appear in successional vegetation only if there is a seed source. The seed may be present in the ground or provided by patches of successional vegetation. Because climax vegetation is susceptible to destruction by fire, flood, or some other catastrophe, no area is long without successional growth.
Most human societies continually disrupt natural vegetation. Consequently, the vegetation people see daily is successional rather than climax vegetation. Examples may be found in roadsides, fence rows, neglected yards and fields, and cut-over land.
When first visited by Europeans, much of the Americas supported climax vegetations. Because the forests seemed untouched by humans, they came to be known as virgin forests. Usually what the layperson calls a virgin forest the biologist recognizes as the local climax vegetation. The common term second growth most often refers to a successional forest.
The factors controlling successional patterns differ from place to place, and they are often poorly understood. Most studies have involved land that was cleared for agricultural use and then, later, abandoned. Whenever bare ground is left undisturbed, rapidly growing annuals (plants that set seed the first year and then die) may appear.
Although there is a characteristic first-year vegetation in each plant community, in many places these plants are replaced by introduced annuals that everyone calls "weeds." This is one example of the fact that the history of an area affects the pattern of succession.
During the first year, biennial (two-year) and perennial (many-year) plants also get started. The second spring these plants draw on reserves, usually stored in their roots, and make such rapid growth that they crowd out the young annuals. Many biennial species have their early leaves in a whorl close against the ground.
These leaves create dense shade around the base of the plant and contribute to suppression of nearby annuals. The dandelion is a biennial with this characteristic growth pattern.
Although the details vary for reason or another, one stage of succession leads to another. The growth habits of annuals and biennials may explain which are dominant the first and second year in certain biomes, but many other factors can be involved. Shade tolerance at various stages in the life cycle differs among tree species and may determine the pattern of forest succession.
In much of the deciduous forest biome, pines precede the climax vegetation. Most pines are particularly vulnerable to shading. Lower limbs die as the pine forest grows dense, and pine seedlings never survive in deep forest shade. The monks, maples, beeches, and other members of the climax forest are more shade-tolerant. Small trees become tell and thin in heavy shade but survive and eventually grow taller than the pines.
Soon shade from the deciduous species kills the pines. The seedlings of the deciduous trees also survive in the shade of larger ones, so there are always small trees in the understory. When a tree is blown over, others quickly replace it in the canopy. Thus shade tolerance helps explain why the deciduous trees replace the pines and also why the deciduous trees are the climax vegetation. For details of succession in a well-studied part of the deciduous forest biome.
The series of vegetational stages that follow destruction of a climax vegetation, as we have just described, is often called secondary succession.
Note that secondary succession begins when soil is available. Primary succession is a series of events whereby plants colonize an area of barren rock, such as fresh lava. Presumably this process has occurred at least once wherever there is now vegetation.
In many instances primary successions have been associated with the evolution of species and have required millions of years. However, we see examples of the early stages of primary succession today on rock outcrops. The original colonizing plants are usually lichens that produce acids.
The acids dissolve certain minerals in the rocks. This process roughens the surface, so the lichens can attach better, and it also makes minerals available for lichen growth. Over the years, mineral grains released by the acids and decaying lichen components create a small amount of soil. Other plants, particularly mosses, may grow in the lichen soil and so constitute the second stage of primary succession. Later events depend on the environment and the species present, just as we noted with secondary succession.