Classifying the world’s tallest, thickest, biggest, and oldest trees can be an extreme challenge. For the National Geographic "Extreme Trees" poster, featured in several of the magazine’s international editions, we took a look at what counts when measuring. Trees grow, limbs fall, ways of measuring change, and new trees are discovered, so figuring out which trees to highlight wasn’t easy.

To accurately measure these behemoths, scientists need knowledgeable tree climbers and high-tech-equipment. Redwoods have long been known as the tallest trees on Earth, yet the current champion, 379.1 feet (115.6 meters) tall, was found just four years ago in California’s Redwood National Park. In 2008, a new tallest flowering plant was discovered in Tasmania—a 327-foot (99.6 meter) mountain ash tree.

It’s just as hard to decide which is the world’s thickest tree. Bald cypresses, African baobabs, and giant sequoias vie for the title. Should you just take the measurement of the diameter at the base? Or is that measurement skewed by the fluted nature of the trunk? Robert Van Pelt, a renowned tree expert at the University of Washington, calculated that the diameter of the bald cypress, which holds the current record, in Oaxaca, Mexico, was about 34 feet (more than 10 meters). Others give an even greater figure. Van Pelt based his on a cross-sectional survey of the tree, rather than just its widest diameter.

The biggest tree in the world is the giant sequoia, though this title needs a bit of qualifying too. It’s actually the world’s largest single-stemmed tree in terms of total wood volume. The 274-foot-tall (83.5 meters) tree in California’s Sequoia National Park, with 52,584-cubic-feet (1,489 cubic meters) in the main trunk alone, has held the size record for almost eight decades.

Even getting information about the oldest living tree in the world is difficult—the location of the Great Basin bristlecone pine, the oldest known individual tree, is a carefully guarded secret. Scientists say it’s at least 4,800 years old. Oldest certainly doesn’t mean biggest. Bristlecones grow only about 30 feet (9 meters) high, with strips of living stem surrounding mostly dead trunks.

—Christy Ullrich

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