World's Tallest Trees at Risk: Photos

With a nod to Arbor Day, here's a rundown of some of the Earth's biggest tree species -- and what ails them.

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As a recent article in the scientific journal Trends in Ecology & Evolution notes, the world's largest trees are among the longest-living organisms on Earth, and play important roles in ecosystems across the planet. But these majestic giants also face a range of serious threats, from deforestation, drought and invasive species to climate change. With a nod to Arbor Day, here 's a rundown of some of the Earth's biggest tree species -- and what ails them. The Giant Sequoia is found only in about 75 groves along the western slope of the Sierra Nevada mountains in California. The tallest specimen is around 274 feet in height. According to Yale's Environment 360 website, giant sequoias face a threat from climate change, because longer summers and insufficient snowpack from drought may make it difficult for young trees to survive.

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Named after its discoverer, 18th-Century Scottish biologist David Douglas, this species can grow to over 300 feet tall. It's found over a wide area ranging from British Columbia to Mexico. Once-vast forests of the trees have been decimated by logging.

This tree is also known as the English oak, though it's found across a wide area in Europe, northern Africa, and western Asia. It's also been imported to the northeastern and midwestern United States. One specimen in Poland grew to more than 140 feet tall. In Europe, this species and others are threatened by the prospect of increased forest fires and stronger winds, according to a 2008 European Union report.

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The Manna gum is found in cooler parts of Australia, where its leaves provide food for koalas. It can grow to nearly 300 feet in height.

This species is native to the central and northern California coast, and is closely related to trees that thrived during the Jurassic Era. Some specimens have lived for 2,000 years, and the tallest has grown to 367 feet. They're threatened by loss of habitat and possibly by climate change as well.

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This tree is found in the Australian states of Victoria and Tasmania, in areas with high rainfall and fertile soils. The tallest examples can stand nearly 300 feet tall. The're at risk from excessive logging. This image shows a clinometer used to measure the height of trees, in Mt. Field National Park, in Tasmania, Australia.

This tropical tree species is found in the southeastern Philippines. A massive specimen that is still growing near the outskirts of San Francisco in northeastern Mindanao reportedly is 288 feet tall. The species is threatened by logging and slash-and-burn deforestation for farming.

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