Bristlecone Pine: Methuselah and Prometheus, the Oldest Trees in the World.

The oldest living tree in the world is a White Mountains, California, bristlecone pine (Pinus longaeva) named Methuselah [wiki], after the Biblical figure who lived to 969 years old. The Methuselah tree, found at 11,000 feet above sea level, is 4,838 years old - it is not only the oldest tree but also the oldest living non-clonal organism in the world.
Before Methuselah was identified as the world’s oldest tree by Edmund Schulman in 1957, people thought that the Giant Sequoias were the world’s oldest trees at about 2,000 years old. Schulman used a borer to obtain a core sample to count the growth rings of various bristlecone pines, and found over a dozen trees over 4,000 years old.
The story of Prometheus [wiki] is even more interesting: in 1964, Donald R. Currey [wiki], then a graduate student, was taking core samples from a tree named Prometheus. His boring tool broke inside the tree, so he asked for permission from the US Forest Service to cut it down and examine the full cross section of the wood. Surprisingly the Forest Service agreed! When they examined the tree, Prometheus turned out to be about 5,000 years old, which would have made it the world’s oldest tree when the scientist unwittingly killed it!