Disturbance in plant communities has been defined 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, fire, 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 finite 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 simplified 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. [...]
[...] Conclusion This discussion has demonstrated that disturbance to natural forests varies greatly in scale and effect in different forest types and for different disturbance agencies, such that robust generalizations are difficult to construct. A disturbance regime has components describing the intensity, frequency, and extent of its effects, although these are rarely quantified. These properties are important because they may influence emergent properties of the forest community, such as species composition and tree diversity. For example Connell's intermediate disturbance hypothesis, relates the intensity, frequency, or timing of disturbance to community diversity and has recently been tested in forests (see Biodiversity: Plant Diversity in Forests). [...]
[...] Studies of forest regeneration on landslides in the Caribbean have suggested that old landslides provide a habitat for some species that do not occur elsewhere in the surrounding forest matrix. Active volcanoes have the potential to destroy forests over a large area by the direct effects of lava and ashfall and indirect effects caused by tsunamis and changes to atmospheric conditions. For example, the 1883 and subsequent eruptions of Krakatau, in the Sunda Straits between Java and Sumatra, are still evident in the contemporary flora of the Krakatau island group, which is dominated by a small group of well-dispersed tree species. [...]
[...] Fires are a natural and inherent component of the disturbance regime in most natural forests, including those in the wet tropics that were formerly considered not to burn. However, recent evidence derived from dating of charcoal fragments extracted from soil profiles in tropical rainforest areas of South America and Africa have demonstrated a history of recurrent fires on millennial timescales even in sites that are currently very wet. Fires are an even more important feature of the disturbance regime for dry tropical forests and woodland, Mediterranean vegetation, and boreal forest, for which frequent and intense fires may be an important component of ecosystem functioning. [...]
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