Near Fish Lake Forest, in Utah, there's a massive 106-acre forest that is actually made up of one massive tree. The tree is known as Pando, and it's a colony comprised of a single male Quaking Aspen. It is a single living organism, because it has identical genetic markers and one massive underground root system.
Pando estimated to weigh 6,600 tons. This would make it the heaviest living organism and it is thought to be 80,000 years old. In 2006, the US Postal Service called it one of the 'wonders of America' when it printed a stamp with Pando on it.
Forest fires have kept other trees from growing in the area where Pando is. Pando was able to survive the fires through its underground root system. Basically whenever there was a forest fire, even if the stems were burnt to the ground, the roots would stay alive, and it's massive size would give it the advantage over other organisms.