Shades of Green: Earth's Forests -- Ecological Succession

There are always disturbances in the forest landscape, whether or not they are directly caused by humans. Forests might be cleared, burned or flooded, but eventually if conditions become suitable once again, the bare land will begin to change back into a forest.

Grasses are the first plants that grow on bare ground, beginning the process of succession that eventually leads to the growth of trees. Photo by Maya Walters.

This happens very slowly and gradually however. Before trees can move in, the area must first be colonized by grasses and shrubs. These first plants to move in are called pioneer plants, and they need to be tough and grow quickly in order to survive in the often harsh conditions of recently disturbed areas.

Above and right: Grasses are abundant and spread quickly with the many seeds produced each year. Photos by Maya Walters.

The pioneer plants are the first step in changing a disturbed area back into a forest. Gradually they are replaced by larger shrubs and trees that take longer to grow -- the process known as ecological succession. The patterns of succession are relatively predictable in most areas. Grasses always move in first, followed by a series of vegetation which eventually leads to the "climax forest". Any particular region has its own set of climax species, which are the plants that are best adapted for the area and will persist after succession has finished, until another disturbance clears the area.

Pioneer species grow quickly when an area is disturbed. Each square meter of healthy soil could contain as many as 1000 dormant seeds. When the vegetation is cleared, many of these seeds sprout immediately. If the vegetation is cleared and the topsoil is also disturbed or relocated, the area remains bare and is susceptible to severe erosion.

Flowers which provide food for insects are common in early stages of succession. Photo by Maya Walters.

The grasses that move in as pioneer species are often thought of as weeds, the subsequent growth of shrubs are considered undesirable "brush". But without these intermediate stages, the disturbed habitat can't return to a natural forest. For example, in temperate forests, if the shrubs are not allowed to grow, insect pests turn to feed on young trees instead. This has happened in many places where trees are replanted after an area is clearcut. Large quantities of pesticides are then brought in, polluting the soil and water and altering the natural ecosystem even further.

The term jungle actually refers to a tropical forest at a certain stage in the succession process. Because a disturbed area is exposed to sun, plants grow especially quickly. The bushes and vines grow thickly, and do not reach as high as the surrounding forest. In many areas, since the rainforest soil is so poor, repeated clearing of the forest will lead to severe soil depletion, and the normal patterns of succession will no longer occur. Instead of returning to forest, the area will become a savanna because there aren't enough nutrients remaining in the soil to support trees.

Jungles typically contain very thick vegetation with numerous vines and low bushes, but without trees reaching as high as ones in mature rainforest. Photo by Maya Walters.

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