The largest organism found on Earth can be measured using a variety of methods. It could be defined as the largest by volume, mass, height or length. Some organisms group together to form a superorganism, though this cannot truly be classed as one large organism. (The Great Barrier Reef, the world's largest coral reef, stretching 2,000 km, is a collection of many organisms.)
The Aspen tree (Populus tremuloides) forms large stands of genetically identical trees (technically, stems) connected by a single underground root system. These trees form through root sprouts coming off an original parent tree, though the root system may not remain a single unit in all specimens. The largest known fully connected Aspen is a grove in Utah nicknamed Pando, and some experts call it the largest organism in the world, by mass or volume. It covers 0.43 km2 (106 acres) and is estimated to weigh 6,600 short tons (6,000 t).
A giant fungus of the species Armillaria solidipes (honey mushrooms) in the Malheur National Forest in Oregon was found to span 8.9 km2 (2,200 acres), which would make it the largest organism by area. Whether or not this is an actual individual organism, however, is disputed: some tests have indicated that they have the same genetic makeup, but unless its mycelia are fully connected, it is a clonal colony of numerous smaller individuals. Another clonal colony that rivals the Armillaria and the Populus colonies in size is a strand of the giant marine plant, Posidonia oceanica, discovered in the Mediterranean near the Balearic Islands. It covers a band roughly 8 km (5.0 mi) in length.
The world's largest single stem tree, by volume, is the General Sherman tree, a Giant Sequoia with a volume of 1,487 m3 (52,500 cu ft). This tree stands 83.8 m (275 ft) tall and the trunk alone is estimated to weigh over 2,000 short tons (1,800 t). The largest single-stem tree ever measured was the Lindsey creek tree, a Coast Redwood with a minimum trunk volume of over 2,500 m3 (88,000 cu ft) and a mass of over 3,600 short tons (3,300 t). It fell over during a storm in 1905.