View source for Treefall gap

{{Short description|Ecological feature}}
[[File:Small Forest Gap.JPG|thumb|Treefall Gap]]

A '''treefall gap''' is a distinguishable hole in the [[Canopy (biology)|canopy]] of a [[forest]] with vertical sides extending through all levels down to an average height of {{cvt|2|m|ft}} above ground.<ref name="brokaw">{{cite journal |last=Brokaw |first=N. |year=1982 |title=The Definition of Treefall Gap and Its Effect on Measures of Forest Dynamics |journal=Biotropica |volume=14 |issue=2 |pages=158–60 |doi=10.2307/2387750|jstor=2387750 }}</ref> These holes occur as result of a fallen tree or large limb. The ecologist who developed this definition used two meters because he believed that "a regrowth height of 2 m was sufficient" for a gap to be considered closed, but not all scientists agree.<ref name="sman" /> For example, Runkle believed that regrowth should be {{cvt|10–20|m|ft}} above the ground.<ref name="sman" /> Alternatively, a treefall gap is "the smallest gap [that must] be readily distinguishable amid the complexity of forest structure."<ref name="brokaw" />

There is no upper limit in gap size. However, it must be caused by a tree or a large limb.<ref name="brokaw" /> For example, a field would not be considered a treefall gap.<ref>{{cite journal |last=Jansen |first=D. H. |year=1990 |title=An abandoned field is not a tree fall gap |publisher=Vida Silvestre Neotropical |volume=2 |pages=64–67}}</ref> Tree falls are commonly caused by old age, [[natural hazard]]s, or [[parasitic plant]]s (or certain [[epiphyte]]s).

== 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.<ref name="sman">Schliemann, S., & Bockheim, J. (2011). Methods For Studying Treefall Gaps: A Review. Forest Ecology and Management,2011,1143-1151.</ref> Most gaps do not conform to one particular shape, so assumptions must be made.<ref name="sman" /> Gaps that are uniformly shaped (e.g. ellipse, triangle, etc.) can be quantified by measuring the length and width of each gap.<ref name="sman" /> Commonly, however, the gap is irregularly shaped and must be divided into smaller sections to be measured individually in order to obtain higher accuracy.<ref name="sman" />

;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.<ref name="sman" /> 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."<ref name="sman" /> 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 (5–7 meters per year) or much slower depending upon the vegetation present in the gap.<ref name="carson">Carson, W., Mascaro, J., & Schnitzer, S. (2008). TREEFALL GAPS AND THE MAINTENANCE OF PLANT SPECIES DIVERSITY IN TROPICAL FORESTS (CHAPTER 12). In Tropical forest community ecology. Chichester: Wiley-Blackwell Pub.</ref> Sometimes, vegetation can impede growth, as with [[liana]]s in [[tropical forest]]s. Gap-phase regeneration is not completed until the intact canopy’s height is met by new growth.<ref name="carson" />
# 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.<ref name="carson" />
# Advanced regeneration: young plants that were originally present prior to the tree fall will quickly grow after being exposed to additional sunlight.<ref name="carson" />
# Vegetative reproduction: As the tree falls, it will pull other vegetation with it that can begin to reproduce within the newly formed gap.<ref name="carson" /> This is especially true for [[Liana]]s (see Alternative Pathway of Regeneration below).
# Spreading laterally into the gap from the surrounding forest.<ref name="carson" />

=== Effects of liana in tropical forests ===
[[Liana]]s 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.<ref name="schnitzer">Schnitzer, S., Dalling, J., & Carson, W. (2000). The impact of lianas on tree regeneration in tropical forest canopy gaps: Evidence for an alternative pathway of gap-phase regeneration.Journal of Ecology, 88(4), 655-666.</ref> 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.{{cn|date=July 2023}}

A study conducted on [[Barro Colorado Island]] found that lianas play a likely role in slowing gap-regeneration time.<ref name="schnitzer" /> 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.<ref name="schnitzer" /> Further studies by Schnitzer et al. have shown that as lianas increase in density, species richness and [[Pioneer species|pioneer tree]] density decrease for all gaps (i.e. low and high canopy gaps).<ref name="schnitzer" /> 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.<ref name="autogenerated1">Connell, J. (1978). Diversity of Tropical Rainforests and Coral Reefs. Science,4335, 1302-1310.</ref> 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]].<ref name="carson" /> 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 ==
[[File:Treefall gap in Institute Woods.jpg|thumb|A felled tree allows direct sunlight to reach the forest floor.]]
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 [[shrub]]s and treelet species, which under normal circumstances never grow tall enough to reach the canopy.<ref name="puerta" /> 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.<ref name="canham">Canham, C., Denslow, J., Platt, W., Runkle, J., Spies, T., & White, P. (1990). Light regimes beneath closed canopies and tree-fall gaps in temperate and tropical forests. Canadian Journal of Forest Research-Revue Canadienne de Recherche Forestiere, 20(5), 620-631. {{doi|10.1139/x90-084}}</ref> This light limitation often restricts a plant’s reproduction and growing ability.<ref name="canham" />

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;<ref name="canham" /> however, the amount of light that is now able to penetrate depends on how big the gap actually is.<ref name="canham" /> 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.<ref name="canham" /> The increased amount of light that is now available to the understory community will release them from their previous light limitation.<ref name="canham" />

== Seed dispersal ==
[[Seed dispersal]] in treefall gaps is significantly reduced compared to normal understory areas.<ref name="puerta">Puerta-Piñero, C., Muller-Landau, H. C., Calderón, O., & Wright, S. J. (2013). Seed arrival in tropical forest tree fall gaps. Ecology, 94(7), 1552-1562. {{doi|10.1890/12-1012.1}}</ref> One study suggests that within the first three years after the gaps formed, the seed dispersal rate was 72% lower than the rates in nearby understory.<ref name="puerta" /> Most forms of dispersal, such as wind and animal dispersal, show a reduction in the number of seeds dispersed.<ref name="puerta" />  However, explosive dispersion shows an increase <ref name="puerta" />

Explosive dispersion describes a mechanism by which the parent plant ejects its seeds using pressure, ensuring that they land far from the parent plant.<ref name="puerta" /> This type of dispersal is used in several shrub species. The increase shows that treefall gaps have a positive effect on the seed dispersal of shrubs.<ref name="puerta" /> These shrubs either survived the creation of the gap or migrated in soon after it was created.<ref name="puerta" /> Because of their unique method of dispersal, these plants have an advantage when colonizing the gap. As the treefall gap ages and the canopy layer begins to return to normal, other forms of seed dispersal begin to increase in frequency, eventually returning to their pre-gap values years after its formation.<ref name="puerta" />

== Species diversity ==

=== Flora ===

It is proposed that treefall gaps maintain plant species diversity in tropical forests in three main ways. First, they create habitats that have a lot of light.<ref name="carson" /> Being in an area that contains a high amount of light allows species that cannot grow in total or partial shade to develop rapidly. The increased levels of light creates competition between the shade intolerant species, and the species that prefer low levels of light are eliminated.<ref name="carson" /> This release from competition would allow the number of shade intolerant species to increase.

Second, species may be able to survive on resources that spread from the middle of the gap all the way to the surrounding denser forest.<ref name="carson" /> Tree or plant diversity may be maintained by the distribution and sharing of resources over a gradient if species are uniquely adapted to the resources available in a specific treefall gap.<ref name="carson" />

Lastly, the rate of regeneration of different species may differ based on the size of the gap. While species diversity may vary when the treefall gaps differ greatly in size, it has been argued that it is highly unlikely,<ref name="carson" /> since the relationship between gap size and the microclimate is irregular because of the large spatial and temporal deviation in microclimate.<ref name="carson" />

Support for these three hypotheses is mixed, but there is evidence that supports the fact that some species of plants benefit from being in gaps more than others. In tropical forests, gaps maintain the diversity of some plant groups, which could possibly contain much of the vascular plant community in these areas.<ref name="carson" />

=== Insects ===

Treefall gaps, like the rest of tropical forests, contain an abundant number of animal species. As with all tropical habitats, insects make up the majority of the animal biomass.

It has been thought that the development of treefall gaps would cause harm to the development of leaf litter ant assemblages, but that is not the case according to one study. Species of ants that are found in treefall gaps are the same species that are found in the densely wooded forests that surround them.<ref name="patrick">Patrick, M., Fowler, D., Dunn, R., & Sanders, N. (n.d.). Effects of Treefall Gap Disturbances on Ant Assemblages in a Tropical Montane Cloud Forest. Biotropica, N/a-N/a.</ref> The formations of these gaps seem to have little to no effect on the ant species living there.<ref name="patrick" />

There have been studies that show a noticeable difference in the species diversity of butterflies in treefall gaps and those in the surrounding understory. The types of vegetation present in the gaps play a key role in determining which species of butterfly live there.<ref name="pardonnet">Pardonnet, S., Beck, H., Milberg, P., & Bergman, K. (2013). Effect of Tree-Fall Gaps on Fruit-Feeding Nymphalid Butterfly Assemblages in a Peruvian Rain Forest. Biotropica, 612-619.</ref> This supports the hypothesis that in a natural forest, treefall gaps will promote species diversity.<ref name="pardonnet"/>

== Treefall gaps, blow-downs, and the carbon cycle ==
Large-scale blow-downs of trees are generally caused by [[squall line]]s or [[tornado]]es generating strong winds.<ref name="Negron">Negrón‐Juárez, R. I., Chambers, J. Q., Guimaraes, G., Zeng, H., Raupp, C. F. M., Marra, D. M., Ribeiro, G. H. P. M., et al.(2010). Widespread Amazon forest tree mortality from a single cross‐basin squall line event. Geophysical Research Letters. {{doi|10.1029/2010GL043733}}</ref> Some studies have measured the downburst speed of these winds to be between 26 and 41&nbsp;m/s, which is enough force to cause large-scale blow-downs.<ref name="Negron" /> Studies have shown that small-scale disturbances, such as treefall gaps forming after tree deaths, have a higher reoccurrence rate than do large-scale disturbances, such as blow-downs.<ref name="Santo">{{cite journal|last1=Espírito-Santo|first1=Fernando D.B.|last2=Gloor|first2=Manuel|last3=Keller|first3=Michael|last4=Malhi|first4=Yadvinder|authorlink4=Yadvinder Malhi|last5=Saatchi|first5=Sassan|last6=Nelson|first6=Bruce|last7=Junior|first7=Raimundo C. Oliveira|last8=Pereira|first8=Cleuton|last9=Lloyd|first9=Jon|last10=Frolking|first10=Steve|last11=Palace|first11=Michael|last12=Shimabukuro|first12=Yosio E.|last13=Duarte|first13=Valdete|last14=Mendoza|first14=Abel Monteagudo|last15=López-González|first15=Gabriela|last16=Baker|first16=Tim R.|last17=Feldpausch|first17=Ted R.|last18=Brienen|first18=Roel J.W.|last19=Asner|first19=Gregory P.|last20=Boyd|first20=Doreen S.|last21=Phillips|first21=Oliver L.|title=Size and frequency of natural forest disturbances and the Amazon forest carbon balance|journal=[[Nature Communications]]|year=2014|volume=5 |page=3434 |issn=2041-1723|doi=10.1038/ncomms4434|pmid=24643258 |pmc=4273466 }} {{open access}}</ref> Since small-scale disturbances are more frequent, they account for about 88% of carbon emissions from the [[Amazon rainforest]] and for more biomass loss then do large-scale disturbances.<ref name="Santo" /> These small disturbances add carbon into the environment and back into the [[carbon cycle]]; however, it is not a significant amount when compared to how much carbon is being sequestered in the above ground biomass. Because of this, the Amazon is considered to be a [[carbon sink]].<ref name="Santo" />

== Use of satellite technology in research ==

When researchers attempt to locate treefall gaps on a broad scale, it can be difficult to do so on the ground because of the large scale of the forests that they are generally focusing on. Satellite imagery has proven quite useful for large-scale disturbances such as blow-downs but remains difficult for small-scale disturbances such as treefall gaps. In some instances [[Global Positioning System]]s (GPS) are used to map the centers of treefall gaps in order to determine their frequency.<ref name="Santo2">Espírito-Santo, F., Keller, M., Linder, E., Oliveira Junior, R., Pereira, C.,
& Oliveira, C. (2013). Gap formation and carbon cycling in the Brazilian Amazon: Measurement using high-resolution optical remote sensing and studies in large forest plots. Plant Ecology & Diversity, 2013.</ref> Pictures taken by these satellites are radiometrically changed from numbers into physical units.<ref name="Santo2" /> Light Detection and Ranging ([[Lidar|LiDAR]]) is used to distinguish canopy gaps on spatial scales for separating the different types of gaps.<ref>Ashner, G., Kellner, J., Kennedy-Bowdoin, T., Knapp, D., & Anderson, C. (2013).
Forest Canopy Gap Distributions in the Southern Peruvian Amazon.</ref> With satellite imaging, steps have to be implemented to remove extraneous details such as seas, rivers, clouds, etc.<ref>Thevand, A., Gond, V., & De Alcantara, E. (n.d.). Detection
Of Gaps In An Atlantic Mangrove Forest Using Optical Satellite Imagery</ref> These can interfere with the results. Overall, the mapping of treefall gaps has progressed greatly since the development of satellite technology.

== References ==
{{reflist|2}}

[[Category:Forest ecology]]