General Sherman, the biggest tree in the world

The tree called "General Sherman" is not only the biggest giant sequoia, but it is also the biggest tree in the world. He is 83.8 m (274.9 feet) tall, his girth at breast height is 24,10 m (79 feet) (near the ground it is 31,3 m or 102,6 feet). The width of the crown is 33 m, and the first branch starts only at 40 m or 130 feet!  

How old is this tree?

For a long time it was thought that this tree had reached an age of 2000 to 3000 years. Others even spoke about ages of 6000 to 11000 years. Research from 2002 has shown that he is probably 'only' about 2000 years old. In the past sequoias have been cut down that appeared to be 3220 years old (by counting the tree rings) and it is assumed that there might be giant sequoias up to 4000 years old out there, because the biggest are not always the oldest ones. The image on the left is an attempt to show the entire tree in one image. Notice the people standing at the base of the tree (click on the image for a larger view). General Sherman is the biggest giant redwood, but he is not the one with the largest girth, nor is he the tallest. Also note that the tallest tree on earth is almost half of General Sherman's height taller.
He is often called the largest living thing on earth. Well, there might be some discussion about that.
Most of the tree consists of dead wood. Also the exact definition of a 'living thing' is not very clear. A strawberry plant for example can reproduce itself by shoots, that root themselves. Is a such a field of strawberry plants than one plant or are it multiple ones? In that sense entire woods of trees (like the quaking aspen, Populus tremuloides, for example) or fungi that reproduce for miles and miles beneath the ground can be seen as one living thing.
Anyway, whatever the case may be, it's a fact that no single standing tree was found that has a larger trunk volume than General Sherman.
  During a winter storm in 2006 the tree lost a large branch. When it fell, it smashed a part of the enclosing fence and cratered the walkway pavement surrounding the sequoia. The loss of the branch is seen as a natural defense mechanism against adverse weather conditions. Also the branch loss does not change the tree's status as the largest tree, as that honor was bestowed based on measurements of trunk volume, excluding branches.
The image on the left shows the fallen branch. The image on the right tries to depict the entire tree.
  Above are some typical views of General Sherman and his neighborhood during the touristic summer season.

Tree volume calculation

The General Sherman Tree is not the tallest tree in the world, nor the thickest at the base, but get its title "biggest tree in the world" from its total wood volume, or, more precisely, its total trunk volume.
More on the tallest trees on earth can be found below. The official number mentioned on the info panel near the tree is 52500 cubic feet, or, in decimal units, 1487 m³.
Here you can see the calculation of that number using a simple method, based on a number of girth measurements mentioned on the info panel: 31.3 m circumference near the ground, 5.3 m diameter at 60 ft etcetera.

The mathematic model used assumes the rate of girth change between measurement heights is constant, the tree is standing upright, and has circular cross sections at all heights. These are all conditions that are met sufficiently by this tree. The radius function r(x) with height x derived from these measurements is shown in the figure below. Also the 3D "cylinder" with radius r(x) is shown on the figure on the left. The volume I obtain like this is 1465 m³, almost identical to the "official" 1487 m³. For this I added 3 "measurements" to make the profile of the tree more natural. These are shown in red. By adding the first measurement the volume became somewhat smaller, by adding the other two it became larger. Without adding these the calculated trunk volume is 1494 m³, also nearly identical to the official value.
As a side: keep in mind that calculating the volume of a tree like this (which is nearly not as complex as other big trees like the coast redwood or live oak can get) with an accuracy of less than 1 m³ is hard and requires a lot of reliable measurements: an uncertainty on the height or girth measurements immediately influences the final tree volume result.
On the other hand, the order of magnitude can be obtained with confidence: we can conclude that "General Sherman" has a trunk volume of about 1400 to 1500 m³.
Here's a comparison to appreciate how large this is: while the largest trees in the U.S. in terms of wood volume are a few conifer tree species like the giant sequoia (1487 m³) and the coast redwood (1203 m³) on the western side of the Rocky Mountains, the biggest tree species on the eastern side of the Rockies, is the Southern live oak (136 m³) together with the bald cypress (146 m³).

Tallest, thickest, oldest?

Tallest tree in the world?
So the giant sequoia is the biggest tree on the planet according to volume, but he is not the tallest (tallest giant sequoia: 94.9 m, Redwood Mountain Grove, Kings Canyon National Park).
Three other North American conifer reach staggering heights near the wet west coast. The tallest measured specimen of the coast redwood (Sequoia sempervirens) is 115.55 m (379.1 feet) tall and is the tallest tree in the world. The tallest known Douglas-fir (Pseudotsuga menziesii), the so called Doerner Fir (used to be known as the Brummit fir), is 100.3 m tall and the tallest known Sitka spruce (Picea sitchensis) has a height of 96.7 m.
    The two images on the left show Sequoia sempervirens: on the left a typical foggy coast redwood grove, in the center the "Screaming Titans", one of the biggest, but not the tallest coast redwood.
The Australian mountain ash (Eucalyptus regnans) reaches 100 to 101 m (328 to 331 ft) and is the tallest hardwood tree on earth. The image on the right shows "Icarus Dream", one of the tallest measured mountain ash (97 m). Just like the coast redwood (Sequoia sempervirens) the mountain ash stands are under big pressure of logging companies and already logged trees would purportedly have reached bigger heights.
More about the world's tallest trees.
Largest girth?
The giant sequoia is also not the tree with the largest girth (thickest giant sequoia: 8.98 m diameter, Boole Tree). Some African baobabs (Adansonia digitata), see the image on the right, have similar girths at the end of the wet season. During the heavy rainfall of the wet season, these trees try to store as much water as possible in their thick trunks to survive the hot dry season so their girth changes constantly during the year.
  If you take trees with multiple stems into account, there is a thicker Montezuma cypress (Taxodium mucronatum). A strangely shaped specimen, the famous árbol del Tule in Mexico, has a "diameter" of 11.42 m (image on the left).
Oldest tree in the world?
The giant sequoia definately is one of the species that can become very old, but it has to leave the first place to the bristlecone pine (Pinus longaeva), a small Californian type of pine (4000 to 5000 years) and the south American alerce, Fitzroya cupressoides.
The oldest Pinus longaeva of which the age was determined exactly based on tree ring countings, was 4844 years old in 1965 (the tree was cut).
  The images above show such pines. They grow at high altitudes in quite poor conditions, so these tough trees grow rather slowly and are gnarly and weathered. The oldest living Pinus longaeva is about 4789 years old and is the world's oldest tree.
Take a moment to think about how old this is. America was not yet discovered, there was no Christianity or Islam. Rome and Athens were not founded yet, even the pyramids in Egypt were not yet built when this tree was already growing and was facing all weather.
Then the Western men comes and cuts them down. Such a lack of respect, that's just beyond my comprehension.
Comes in second on the list of old trees: Fitzroya cupressoides. The age of a certain specimen in Chile appeared to be 3622 years old in 1993.
The giant sequoia comes in third: a specimen was 3266 years old when it was cut down.
A list with old trees in the world: the oldlist database.
  Also in Europe trees that are centuries old can be found. In Western Europe, these are without any doubt the yew trees (Taxus baccata) of which the oldest ones can be found on ancient graveyards in Normandy (France) and Great-Britain. Because the tree's status in Germanic-Celtic mythology, yew trees were planted on the sacred ground of the graveyards. They were never cut down because of their location, not even when during the middle ages yew wood was very wanted to make superior longbows.
The images above show two such old yew trees in the south of England (on the left the old yew of Tandridge, on the right the old yew of Crowhurst). And I who was thinking - until I saw these trees - that here in Western Europe, yew trees were only planted to make hedges...
When we here talk about the age of trees, we only take individual non-clones into consideration, because a tree can die off en reshoot from its roots over and over again. That way, in Western Europe, oaks and limes that are hundreds or maybe thousands years old can be found.
Nevertheless, it may be clear that the giant sequoia, the tree species with the largest trunk volume is one of the oldest, thickest, and tallest trees on the planet. A second remark you might make is, that the tallest, biggest as well as the oldest tree grow in California.
Other tree records
For the ones who like to read about tree records, I'll throw in another one. The tree with the largest crown is a banyan tree (Ficus benghalensis) in Howrah, India. The crown of this tree covers an area of about 3.7 acres (1.5 hectares). But for me, this tropical tree doesn't play a fair game. It reproduces using strangling, supporting roots.
The following photo probably makes things more clear.
On this image I pose near an Australian Moreton Bay fig (Ficus macrophylla), a close relative of the banyan. From the branches this tree continuously produces roots, so that after some time, a complex network of roots is formed that indeed can cover a large area. It is even possible that the original stem dies off and that the tree can "go on a walk".
This photograph was taken on a hot Italian summer day in the botanical garden of Palermo, Sicily. The tree had already crossed the walking path near the tree. According to the Italian Corpo Forestale this tree has a girth of 13.5 m and a height of 30 m, although you can ask some questions about the point of talking about a girth here. A five minutes walk away from there, on Piazza Marina, there's an at least equally impressing tree of the same kind with a "girth" of 20 m.