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Habitat Fragmentation

Timber Harvesting near EdsonOne consequence of clear-cutting is habitat fragmentation. Fragmentation can be defined as the subdivision of once contiguous habitats and ecosystems into patches of various sizes and shapes. This type of fragmentation in a forested ecosystem is seen as a patchwork of treed areas and clearings. Timber harvesting may increase or decrease fragmentation, depending on the block size compared to the natural patch size. For example, harvesting within a large, even-aged stand increases fragmentation, whereas harvesting a large, patchy area may later produce a more uniform forest community.

Fragmentation decreases habitat effectiveness for many forest-dwelling species. Species requiring large tracts of undisturbed forest for food, cover and reproduction may be less successful in areas where this habitat type remains as patches only. On the other hand, other species of plants and animals will benefit from the large clearings and more forest edge.

During the past couple of decades, the forest industry has tended to use smaller-sized cutblocks in most harvest areas. More recently, a wider range of cutblock sizes and shapes have been employed. Although smaller blocks are more aesthetic and beneficial for regeneration of some tree species, they also effectively increase the degree of habitat fragmentation. Between 1966 and 1996, there were 51,700 coniferous and 4,500 deciduous blocks cut in Alberta over an area totalling one million hectares. These numbers represent a certain amount of habitat fragmentation in harvest areas with associated cutblocks and leave blocks. However, there are other factors that contribute to fragmentation in forested areas, including clearing for agriculture, energy exploration, building roads, urban expansion, and so on. Habitat fragmentation is a complex issue, and the forest industry is continuing to experiment with different types of cutting methods that will reduce the effects of fragmentation.

Department of the Environment. State of the Environment Report, Terrestrial Ecosystems. Edmonton: n.p., 2001. With permission from Alberta Environment.

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