WASHINGTON (Reuters) - South America's vast Amazon region harbors one of the world's most diverse collection of tree species, but more than half may be at risk for extinction due to ongoing deforestation to clear land for farming, ranching and other purposes, scientists say.br clear='all'/img width='1' height='1' src='http://reuters.us.feedsportal.com/c/35217/f/654220/s/4bad599f/sc/32/mf.gif' border='0'/div class="feedflare"
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