Talk:Treefall gap

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Talk:Treefall Gap I would just like to give everyone a heads up that follows this page that myself and two others will be working on this page as a project for our ecology class. Any and all suggestions are welcome. In particular, if anyone has any suggestions on major topics that we could expound on for this article, please let us know. Cjkeen (talk) 21:04, 2 October 2014 (UTC)

What is a treefall gap?

A usable definition (i.e. when determining turnover rate) of a treefall gap is a distinguishable “hole” in the forest with vertical sides extending through all levels down to an average height of two meters above ground (Brokaw). These “holes” occur as result of a falling tree or large limb. Tree falls are commonly caused by old age, natural disasters, parasites (i.e. epiphytes), etc.

Common sizes of treefall gaps

Show figure 1 (will be inserted if allowed from copyright owners) (Brokaw)

How small is too small to be considered a tree fall gap?

• According to Brokaw (Brokaw), the smallest gap must be easily distinguishable within the forest structure.

How large is too large to be considered a tree fall gap?

• There is no upper limit. However, it must be caused by a tree or a large limb (Brokaw). For example, a field would not be considered a tree fall gap (Jansen).

Length of gap-phase regeneration for a tree fall gap

Recovery time can be rapid (5-7 meters per year) or much slower depending upon the vegetation present in the gap (Carson). 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 (Carson).

Common pathways of regeneration in gaps

1. Seed: soil contains a number of seeds (seed bank (add wiki hyperlink)) that are ready to sprout once they receive sunlight as a result of the gap formed in the canopy (Carson).
2. Advanced regeneration: young plants that were originally present prior to the tree fall will quickly grow after being exposed to additional sunlight (Carson).
3. Vegetative reproduction: As the tree falls, it will pull other vegetation with it that can begin to reproduce within the newly formed gap (Carson). This is especially true for Lianas (see Alternative Pathway of Regeneration below).
4. Spreading laterally into the gap from the surrounding forest.

Alternative pathway of regeneration

Lianas (include wiki hyperlink) 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 (Schnitzer). 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. In a study conducted on Barro Colorado Island, Schnitzer et al found that lianas play a likely role in slowing gap-regeneration time (Schnitzer). Figure 1 (will be inserted if allowed) shows how Lianas have been able to keep gap in a low-canopy state, and this is especially true for gaps that are thirteen plus years old (Schnitzer). Figure 4 (will be inserted if allowed) demonstrates that as Lianas increase in density, species richness and pioneer tree (include wiki hyperlink) density decrease for all gaps (i.e. low and high canopy gaps) (Schnitzer). This data suggests that Lianas play a significant role in gap-regeneration time.

Intermediate disturbance theory and species abundance in treefall gaps

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 Theory, some disturbance is critical in order to allow the coexistence of multiple species in the same ecosystem (Kricher). The Intermediate Disturbance Theory 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 (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 due to gaps while others will not).

Seed dispersal in treefall gaps

Seed dispersal in treefall gaps is significantly reduced compared to normal understory areas (Puerta-Piñero). One study suggested that within the first three years after the gaps formed, the seed dispersal rate was 72% lower than the rates in its understory counter parts (Puerta-Piñero). The biggest decrease in seed dispersal is observed in seeds that are dispersed by animals, showing an 86% decline overall. However, while most forms of dispersion, such as wind and animal dispersal, show a reduction, explosive dispersion shows an increase (Puerta-Piñero). Explosive dispersion describes a mechanism by which the parent plant ejects its seeds using pressure, ensuring that they land far from the parent plant (Puerta-Piñero). 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 (Puerta-Piñero). These shrubs either survived the creation of the gap or migrated in soon after it was created (Puerta-Piñero). Due to 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 (Puerta-Piñero).

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 (Puerta-Piñero). 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 (Canham).

This light limitation often restricts a plant’s reproduction and growing ability (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 (Canham); however, the amount of light that is now able to penetrate depends on how big the gap actually is (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 (Canham). The increased amount of light that is now available to the understory community will release them from their previous light limitation (Canham).

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 (Carson). 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 (Carson). Lastly, the rate of regeneration of different species may differ based on the size of the gap (Carson).

1. Being in an area that now contains a high amount of light allows species that cannot grow in total or partial shade to develop rapidly. Due to the new levels of light, competition between the shade intolerant species and the species that prefer low levels of light would be eliminated (Carson). This release from competition would allow the number of shade intolerant species to increase.
2. 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 (Carson).
3. While species diversity may vary when the treefall gaps differ greatly in size, it has been argued that it is highly unlikely (Carson). This is due to the fact that the relationship between gap size and the microclimate is irregular because of the large spatial and temporal deviation in microclimate (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 (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. Some that have been studied in conjunction with treefall gaps are as follows:

Leaf Litter Ant Assemblages (Montane Cloud Forest)

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 actual densely wooded forests that surround them (Patrick). The formations of these gaps seem to have little to no effect on the ant species living there (Patrick).

Fruit-Feeding Nymphalid Butterfly Assemblages (Peruvian Rain Forest)

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 type of vegetation present in the gaps play a key role in determining which species of butterfly lived there (Pardonnet). This supports the hypothesis that in a natural forest, treefall gaps will promote species diversity (Pardonnet).

Spider Assemblages (Northeastern Brazil)

While there are a number of species that live in treefall gaps, the majority of them live in the outer-edge of the gap (Peres). Also, there is evidence that shows that the older a gap is the more species that are likely to live there (Peres). Microclimate plays a big role as well when it comes to species diversity (Peres). If the microclimate changes even slightly, it can change the number of spiders that live in the gaps.

Cjkeen (talk) 22:45, 20 October 2014 (UTC)