A treefall gap is a distinguishable hole in the canopy of a forest with vertical sides extending through all levels down to an average height of 2 m (6.6 ft) above ground. These holes occur as result of a fallen tree or large limb. The initial definition used two metres as it was proposed that "a regrowth height of 2m was sufficient" for a gap to be considered closed, but not all scientists agree. For example, Runkle proposed that regrowth should be 10–20 m (33–66 ft) above the ground. Alternatively, a treefall gap is "the smallest gap [that must] be readily distinguishable amid the complexity of forest structure." There is no upper limit in gap size. However, the creation of the space must be caused by a tree or a large limb. For example, a field would not be considered a treefall gap. Tree falls are commonly caused by old age, natural hazards, or parasitic plants (or certain epiphytes).
Measurement Scientists have not been able to come to a conclusion on one absolute method for measuring a treefall gap. However, there are two types of measurements that are commonly used:
Two-dimensional This method does not necessarily take into account irregularity in gap shapes. Most gaps do not conform to one particular shape, so assumptions must be made. Gaps that are uniformly shaped (e.g. ellipse, triangle, etc.) can be quantified by measuring the length and width of each gap. Commonly, however, the gap is irregularly shaped and must be divided into smaller sections to be measured individually in order to obtain higher accuracy.
Three-dimensional This method provides a more accurate measurement as it takes into account differences in structure of the gap descending from the canopy to the forest floor. One of the most recent strategies, pioneered by K. and S.I. Yamamoto, "utilized two photographs of the canopy gap, taken at different heights, and a series of geometric calculations to calculate the gap area." By doing this, the scientists were able to obtain a more accurate measurement as they are now accounting for differences in structure around the periphery of the gap.
Gap regeneration Recovery time can be rapid (5–7meters per year) or much slower depending upon the vegetation present in the gap. Sometimes, vegetation can impede growth, as with lianas in tropical forests. Gap-phase regeneration is not completed until the intact canopy's height is met by new growth.
Seed: soil contains a number of seeds (seed bank) that are ready to sprout once they receive sunlight as a result of the gap formed in the canopy. Advanced regeneration: young plants that were originally present prior to the tree fall will quickly grow after being exposed to additional sunlight. Vegetative reproduction: As the tree falls, it will pull other vegetation with it that can begin to reproduce within the newly formed gap. This is especially true for Lianas (see Alternative Pathway of Regeneration below). Spreading laterally into the gap from the surrounding forest.
Effects of liana in tropical forests Lianas are a common woody vine found in tropical forests. These vines utilize trees to venture into the canopy in search of sunlight and nutrients. Thus, when a tree falls, it brings all the liana growth with it. Following a tree fall, lianas have a high survival rate (~90%) and they can quickly begin sprouting. This causes potential problems as new trees begin to grow but are unable or are limited by the presence of lianas. Many gaps have been found to enter a state where growth has been halted because of lianas. Therefore, scientists have begun looking into their effects on gap regeneration. A study conducted on Barro Colorado Island found that lianas play a likely role in slowing gap-regeneration time. Lianas have been able to keep a gap in a low-canopy state, and this is especially true for gaps that are at least 13 years old. Further studies by Schnitzer et al. have shown that as lianas increase in density, species richness and pioneer tree density decrease for all gaps (i.e. low and high canopy gaps). This data suggests that lianas play a significant role in gap-regeneration time.
Intermediate disturbance hypothesis and species abundance Treefall gaps are important in maintenance of some plant species diversity. Disturbance is important in tropics as a mechanism for maintaining diversity. According to the intermediate disturbance hypothesis (IDH), some disturbance is critical and the maximum number of species will be found where the "frequency and intensity" of disturbances are at an intermediate scale. IDH helps explain the Gap Hypothesis which postulates that more light and more diverse resources caused by the falling of a canopy tree may aid in more species abundance. Although treefall gaps have been shown to promote species diversity among a variety of species, a gap's effect on different species is likely to produce mixed results (i.e. some species will experience more diversity because of gaps while others will not).
Light penetration
The creation of a treefall gap causes a break in the canopy to form, allowing light to penetrate through to the understory. This light can now reach shrubs and treelet species, which under normal circumstances never grow tall enough to reach the canopy. Under a normal canopy (i.e. one where there is no treefall gap), there is very little light that filters through, placing a light limitation on the understory plant community. This light limitation often restricts a plant's reproduction and growing ability. When a treefall gap forms, there is a distinguishable difference in the amount of light that penetrates through to the lower levels of the forest; however, the amount of light that is now able to penetrate depends on how big the gap actually is. A treefall gap that is only 5 meters in diameter will allow less light to reach the understory than a larger gap that is 10 meters wide. In addition, a smaller gap will receive more direct radiation from the sun, whereas a larger gap will receive high amounts of diffuse radiation. The increased amount of light that is now available to the understory community will release them from their previous light limitation.
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