What is the heaviest known living organism?

Pando, a stand of quaking aspen in Utah - about 47,000 stems that look like a forest and are one male tree, connected by a single root system and genetically identical. Estimated at around six million kilograms, which makes it the heaviest known organism, and possibly thousands of years old.

Aspen clone like this routinely; Pando is remarkable for scale rather than method. Each stem lives perhaps 130 years and is replaced from the roots, so the organism persists by continuous replacement - which raises the same question the Ship of Theseus does, since no part of Pando is ancient even if Pando is.

It is also dying, and slowly enough that it took a long time to notice. Deer and elk eat the new shoots, their predators were removed, and grazing pressure has meant almost no stems have successfully replaced the ageing ones for decades - so the population is old with no next generation. Fenced areas regenerate well, which identifies the cause. The heaviest living thing on Earth is being eaten to death by deer.