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 fores t, and humidity-senility 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.
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.
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