Disturbed and Fragmented Forests

When native vegetation is cleared for agriculture or other human activities, habitats that were once continuous become divided into separate fragments. Habitat fragmentation has a number of effects. In a forest, the canopy (the uppermost layer of vegetation) buffers the microclimate of the forest floor, keeping the forest floor relatively cool, moist, and shaded during the day, and reducing air movement and trapping heat during the night. When a forest is cut so that separate smaller forests remain with cleared areas in between, these effects are reduced. As formerly forested areas become exposed to direct sunlight, the ground becomes much hotter during the day; without the canopy to reduce heat and moisture loss, formerly forested areas are also much colder at night and generally less humid. In the remaining forest fragments these effects are strongest at the fragment edge and decrease toward the interior. In studies of Amazonian rainforest fragments, microclimate changes had strong effects up to 60 meters into the forest interior, and increased tree mortality could be detected within 100-300 meters of forest edges. Since species of plants and animals are often precisely adapted to temperature, humidity, and light levels, changes in these factors will eliminate many species from forest fragments. Shade-tolerant wildflower species of the temperate forest, late-successional tree species (those that dominate in mature forests) of the tropical forest, and humidity-sensitive animals, like amphibians, often are rapidly eliminated by habitat fragmentation because of altered environmental conditions, leading to a shift in the species composition of the community.

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Forest fragment edges may have very high daytime temperatures when the angle of the sun is low and very cold night temperatures due to the lack of buffering by other vegetation. However, dense tangle of vines and fast-growing pioneer species (the first to grow in a disturbed area) grow up at the forest edge in response to these altered conditions and often create an obstruction that reduces the effects of environmental disturbance on the interior of the fragment. In this sense, the forest edge plays an important role in preserving the composition of the forest fragment, but in the process, the species composition of the forest edge is dramatically altered, and the area occupied by forest interior species is dramatically reduced. Over time, the forest edge may be occupied by species of plants and animals different from those found in the forest interior. If a forest returns to the cleared area, either through natural growth of secondary forest or the establishment of a tree plantation the forest fragments might be protected from change.

Wind changes can have a significant effect in fragmented forest habitats. In an intact forest, wind velocity is substantially reduced by the tree canopies; the wind moves strongly over the forest but is reduced to a gentle breeze within the forest. When a habitat is fragmented, the wind is able to enter and move through the forest. The impact of wind will be greatest at the forest edge, which is subject to the full force of it, but the effects on air movement may be felt over a considerable distance as well, particularly in flat terrain. The increased wind and air turbulence directly damages vegetation, particularly at the forest edge. Trees that have grown up in the forest interior with minimal wind stress will have leaves and branches stripped off by the wind or may be blown down. Increased wind also leads to increased drying of the soil lower air humidity, and higher water loss from leaf surfaces. This increased water stress may kill many interior plant species and reduce the production of seeds. The overall result of wind effects may be to kill the trees along the forest edge,which will then be occupied by a new suite of species better adapted to the new conditions. Over time, with trees dying along the edge, the forest fragment will become smaller and might eventually cease to exist.Increased wind, lower humidity, and higher temperatures make fires more likely, especially when wood has accumulated on the edge of the forest where trees have died or have been blown down by the wind.