Which is the world's most massive tree, and arguably the largest living organism in the world ?

The giant sequoia is the world's most massive tree, and arguably the largest living organism on Earth.It is neither the tallest extant species of tree (that distinction belongs to the coast redwood),nor is it the widest (that distinction belongs to the baobab tree or Montezuma Cypresses), nor is it the longest-lived (that distinction belongs to the Great Basin bristlecone pine). However, with a height of 286 feet (87 m) or more, a circumference of 113 feet (34 m) or more, an estimated bole volume of up to 52,500 cubic feet (1,487 m3), and an estimated age of 1800–2700 years, the giant sequoia is among the tallest, widest and longest-lived of all organisms on Earth.

Giant sequoias grow in well-defined groves in California mixed evergreen forests, along with other old-growth species such as California incense cedar (Calocedrus decurrens). Because most of the neighboring trees are also quite large, it can be difficult to appreciate the size of an individual giant sequoia. The largest giant sequoias are as tall as a 26-story building, and the width of their bases can exceed that of a city street. They grow at such a rate as to produce roughly 40 cubic feet (1.1 m3) of wood each year, approximately equal to the volume of a 50-foot-tall tree one foot in diameter.This makes them among the fastest growing organisms on Earth, in terms of annual increase in mass.

Giant sequoias occur naturally in only one place on Earth—the western slope of the Sierra Nevada mountain range in California, on moist, unglaciated ridges and valleys at an altitude of 5,000 to 8,000 feet (1,524 to 2,438 m) above mean sea level. There are 65–75 groves of giant sequoias in the Sierra Nevada, depending upon the criteria used to define a grove. The northernmost of these groves is Placer County Grove in the Tahoe National Forest, Placer County, California,[10] while the southernmost grove is Deer Creek Grove in the Giant Sequoia National Monument, Tulare County, California. The combined total area of all groves of giant sequoias is approximately 35,600 acres (14,407 ha).

Giant sequoias are in many ways adapted to forest fires. Their bark is unusually fire resistant, and their cones will normally open immediately after a fire.However, fire is also the most serious damaging agent of giant sequoias. Seedlings and saplings are highly susceptible to death or serious injury by fire. Larger giant sequoias are more resistant to fire damage, due to their thick protective layer of nonresinous bark and elevated crowns. However, repeated fires over many centuries may penetrate the bark and destroy the vascular cambium. Nearly all of the larger trees have fire scars, many of which cover a large area of the base of the tree. Older trees are rarely killed by fire alone, but the resulting structural damage may predispose a tree to collapse and fire scars also provide entry for fungi which may cause root disease and heart rot. The resulting decayed wood is then more easily consumed by subsequent fires. The result of this cycle is further structural weakening of the tree, which may eventually lead to its collapse.

Fire scars are thought to be the main cause of dead tops. Although lightning strikes rarely kill mature trees, lightning sometimes knocks out large portions of crowns or ignites dead tops. The most common cause of death in mature giant sequoias is toppling, due to weakening of the roots and lower trunk by fire and decay. The extreme weight of the trees coupled with their shallow root systems contributes to this weakening. Other causative factors include wind, water-softened soils, undercutting by streams, and heavy snow loads in the crowns

The Washington tree, located in the Giant Forest Grove in Sequoia National Park provides a good example of the aforementioned phenomena. This tree was the second largest tree in the world (only the General Sherman tree was larger) until just a few years ago. In September 2003, the tree lost a portion of its crown as a result of a fire caused by a lightning strike. This reduced its height from nearly 255 feet (78 m) to about 229 feet (70 m). The structurally weakened tree partially collapsed in January 2005, as the result of a heavy snow load in the remaining portion of its crown; it is now approximately 115 feet (35 m) tall.