Gap (ecology)

A treefall gap is a distinguishable hole in a forest with vertical sides extending through all levels down to an average height of two meters above ground. These holes occur as result of a falling tree or large limb. The ecologist who developed this definition used two meters because believed that "a regrowth height of 2 m was sufficient" for a gap to be considered closed, but not all scientists agreed. For example, Runkle believed that regrowth should be 10–20 meters above the ground. Alternatively, a treefall gap as "the smallest gap [that must] be readily distinguishable amid the complexity of forest structure."

There is no upper limit in gap size. However, it 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 disasters, or parasites (i.e. epiphytes).

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:

This method does not necessarily take into account irregularity in gap shapes. Most gaps do not conform to one particular shape or another, 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 (i.e. not uniform) shaped and must be divided into smaller sections to be measured individually in order to obtain higher accuracy.

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.

Recovery time can be rapid (5–7 meters per year) or much slower depending upon the vegetation present in the gap. Some vegetation can impede growth (see Alternative Pathway of Regeneration below). Gap-phase regeneration is not completed until the intact canopy’s height is met by new growth.

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 due to 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 thirteen plus 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.

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