Q:

When does a secondary succession occur in an ecosystem?

A:

A secondary ecological succession will follow the destruction of the vegetation in an ecosystem, even if it has been completely destroyed in a major forest fire, provided that the soil has been left intact. Seeds brought into the area by wind or by animals, or seeds that were left below the soil before the disturbance, will take root and become the pioneer plants of the secondary succession. Herbaceous plant growth, such as grass, will begin to grow first, followed by bushes and the first-emerging trees, and lead ultimately to a restoration of the original ecosystem.

A variety of mechanisms can trigger the secondary succession and affect the length of time of the restoration. Factors include the condition of the soil prior to the disturbance, trophic interactions and trade-offs between competitor and colonizer species.

A secondary succession occurring in a forest recovering from a major disturbance, such as a fire, will progress through four phases. The first is an establishment phase in which new seedlings from the first-appearance succession trees germinate in the soil left after the disturbance. In the thinning phase that comes next, these trees begin to compete for water, nutrients and light. The weaker trees are thinned out during this phase and new hardier shade-tolerant tree species begin to grow. The first-appearance trees begin to die off during the transition phase which follows. They are not able to reproduce because of the shaded areas they created, and the later-appearing shade-tolerant trees start to take over the forest. In the final steady state phase, the later shade-tolerant trees reproduce and grow under the forest canopy and the first-appearance trees, which were unable to reproduce and grow in the shade, are replaced.


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