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Smaller tropical forest fragments vanish faster than larger forest blocks

March 11, 2020

In one of the first studies to explicitly account for fragmentation in tropical forests, researchers report that smaller fragments of old-growth forests and protected areas experienced greater losses than larger fragments, between 2001 and 2018. The results suggest tropical forests are likely to continue shrinking if large-scale efforts to protect blocks of natural forest are not swiftly implemented. Matthew Hansen et al. emphasize that distinct conservation strategies are needed to combat deforestation in large forest blocks as compared to in smaller, more fragmented regions. In the latter case, efforts should focus on connecting small forest fragments, the authors say, to reestablish wide-ranging tree cover. This would be important in Central America, West Africa, and Southeast Asia, where contiguous natural forest blocks are absent. By contrast, an approach aimed at conserving large existing forests is required for the Amazon Basin, Congo Basin, Indonesian Borneo, and the island of New Guinea, say Hansen and colleagues. While many previous studies have documented tropical forest fragmentation, none have incorporated measures of fragmentation with data explicitly documenting the spatial extent of forest loss, since changes in tree cover are not typically included in monitoring protocol. To better understand how fragmentation affects forest cover loss, Hansen and colleagues first analyzed maps of tropical tree cover for the year 2000, identifying fragments at least 10 square kilometers large. The researchers then calculated forest loss per fragment over the following 18 years, calculating the loss as a percentage of fragment size. Smaller chunks of forest experienced proportionally higher losses, with the 36,282 smallest fragments (under 675 square kilometers) losing 11.5% of their tree cover each year, while the 22 largest stretches (greater than 75,000 square kilometers) lost 2% each year. Hansen and colleagues note that forested areas largely managed for economic purposes did not experience declines related to fragment size, since resources were invested to protect the lands.
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American Association for the Advancement of Science

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