March 11th

SUCCESSION

Succession occurs when the species found in a community change over time.

I. Terms.

Primary Succession occurs as communities develop from 'new' land areas which have no prior biological presence. Examples include development of communities on newly erupted lava flows, volcanic islands or on land exposed when glaciers melt.

Secondary Succession occurs when after a disturbance removes some, but not all, life from an area. Because of this, communities begin their succession with some life present. Examples include recovery of forests following fires or floods, or the development of communities following the abandonment of farm fields.

II. Mechanisms underlying succession.

Why does community composition change over time? Three primary processes have been advocated:

A. Facilitation occurs when species modify the environment to make it appropriate for other species. This can be seen with Aspen trees which grow up following fire or clear-cutting of northern Wisconsin forests. These trees like to live in full sun. However, they grow so thickly that they make deep shade under their canopy. This changes the environment, and makes it possible for woodland species to recolonize these sites.

B. Tolerance occurs as the most competitively fit species persist in a community while the least competitively fit species are eliminated. Thus, the species which can best tolerate the conditions on a site will persist there the longest. This can be seen with oak trees which can also colonize recently cut or burned Wisconsin forests. Unlike Aspens, however, the oak tress are good competitors, will continue to live on these sites, and will eventually become more common.

C. Inhibition occurs when some species eliminate competitors by releasing toxins into the environment which will not effect them but which will make other species less healthy. This can be seen with Black Walnut trees, which release the chemical Juglone into the soil as their dead leaves decay. This chemical prevents germination of many seeds, and decreases the health of other plants already living in the area.

III. Stages of Forest Secondary Succession

Forests progress through four general successional states as they recover from a disturbance:

A. Establishment phase occurs immediately following a disturbance, as seedlings from early successional trees (like Aspens) germinate in the recently cleared land.

B. Thinning phase happens when these early successional trees seedlings begin to grow up, and start competing with each other for light, water, and nutrients. This intense intra-specific competition will thin (eliminate) the weakest individuals from the area. During this time, the shade-tolerant, highly competitive late-successional trees (e.g. maples, oaks, hemlock) begin moving in under the early-successional trees.

C. Transition phase happens when the early successional trees begin dying from old and are replaced by individuals of late-successional species. This happens because the early successional trees could not reproduce in the moist, shaded environment which they created, leaving only late successional trees to replace them when they die.

D. Steady State phase happens as late successional trees take over the forest canopy. Because their seedlings can live in the shaded, moist environment which their parents have created, when the adult trees die, they are replaced by their offspring, continuing their presence in the forest.

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