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Natural disturbances in forest environments

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  1. Introduction.
  2. Small-scale disturbance: gap phase dynamics.
  3. Changes in microclimate and resource availability following gap creation.
  4. Forest regeneration in gaps.
    1. The changes in microclimate and resource availability.
    2. Photoblastic germination.
    3. Germination in gaps.
  5. Importance of chance effects.
  6. Forest growth cycle.
  7. Large-scale disturbances.
    1. Disturbances that result in primary succession.
    2. Disturbances that result in secondary succession or recovery.
  8. Conclusion.

Disturbance in plant communities has been de?ned as consisting of ?the mechanisms which limit the plant biomass by causing its partial or total destruction.' In forests, disturbance arises from the agencies of tree damage or death. At small spatial scales, individual trees die standing or fall over, but in both cases a gap in the canopy is created and this initiates a successional process known as the forest growth cycle. The agencies of natural disturbance at larger spatial scales include windstorms, ?re, and landslides and these factors vary in their impacts on forests and the ensuing mechanisms of forest recovery. Natural disturbance regimes in forests are important because they impact on tree population dynamics, the relative abundance of different species and functional groups, the biomass and carbon content of vegetation, and interactions with other components of the biotic community. Community ecologists have highlighted the importance of disturbance among mechanisms proposed for the maintenance of tree species richness, particularly in species-rich tropical forest communities. Small-scale natural disturbances are an inherent component of all plant communities because plants have a ?nite lifespan. In forests, the size of the individual tree at the time of its death and the mode of death determine the scale of the disturbance created.

[...] However, the threshold for a natural disturbance event is usually regarded as the death of an individual large canopy or emergent tree, which results in the creation of a hole through all layers of the forest down to 2m above the ground surface canopy gap). The size of a canopy gap varies according to the height of the tree that died, its architecture (height: canopy width), and its neighborhood. The fall of a large tree will inevitably lead to damage or death of surrounding trees, particularly if their crowns are connected by lianas. [...]

[...] However, it must be recognized that this description is an over simpli?ed caricature of many highly complex processes that collectively reduce the predictability of forest regeneration path- ways in a particular site. For example, it is highly unlikely that all species that have the ecological potential for regeneration in any particular site will actually get there, because of constraints on dispersal. Forest Growth Cycle The processes of tree death and regeneration described above are intrinsic to all natural forest communities. [...]

[...] Forest Regeneration in Gaps The changes in microclimate and resource availability that are induced by gap creation contribute to the mechanisms of forest regeneration, which describes the processes of recovery following disturbance. Regeneration proceeds via processes of both sexual and vegetative modes of reproduction. It is initiated by the germination of seeds, which either emerge from the buried soil seed bank or arrive in the gap after it has been created, or by the release of seedlings and saplings that were present at the time of gap creation (advanced regeneration). [...]

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