So far, General Sherman, a giant sequoia, holds the title of being the world's most massive single tree.

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The World's Biggest Trees

­Tree size is often much more than just height. General Sherman, a giant sequoia -- one of two types of redwood found in California, the other being the taller, skinnier coast redwood we just mentioned --  is the world's heaviest and most voluminous tree at around 6,167 tons (5,595 metric tons) and 52,000 cubic feet (1,487 cubic meters) [sources: Britannica and NPS]. Located in California's Sequoia National Park, General Sherman is 274.9 feet (83.8 meters) high, but it's not the tallest tree [source: Britannica]. That honor belongs to a coast redwood named Hyperion, which clocks in at 379.1 feet (115.5 meters) tall [source: Leff]. General Sherman has a trunk circumference of 102.6 feet (31.1 meters), but it's narrower than a tree called El Arbol del Thule in Oaxaca, Mexico, that has a maximum circumference of 178 feet (54 meters) [sources: NPS and Barnett].

So there are many ways of looking at tree size, just as there are various ways of looking at what is the largest living organism. Some scientists consider massive reefs like Australia's Great Barrier Reef to be giant living organisms. There are also huge fungi out there, such as a 1,500-acre (607-hectare) fungus discovered in Washington, which was once considered the world's largest organism (at least in terms of area) [source: Grant].

But the world's most massive living organism is likely one group of quaking aspen trees located in Utah's Wasatch Mountains and nicknamed Pando (Latin for "I spread") [source: Grant]. Pando is considered one organism because the trees in the group share an identical genetic code and a common, interconnected root system.

The Pando trees are basically clones of one another. New trees are made as stems spread out as far as 100 feet (30 meters) from the base of the original tree and then periodically take root, creating new, genetically identical, connected trees. The process repeats itself with the clone trees, weather and other conditions permitting. The process, known as vegetative reproduction, is how strawberries and many other plants reproduce.

Pando is 47,000 trees spread across 106 acres (43 hectares) and may be up to 80,000 years old [source: Grant]. But in testament to the massive size of California redwoods, this batch of 47,000 trees weighs about 6,600 tons (5,987 metric tons), only a few hundred tons in total more than General Sherman [source: Sugarman].

Individual quaking aspens have narrow trunks and grow up to 100 feet tall [source: NPS]. Owing to their reproductive process and ability to thrive in harsh environments, quaking aspens are the most common tree in the United States.

Keep reading for more tree talk and links you might like.