The most massive living thing on Earth (that we know of anyway) is a huge tree with multiple trunks. It's also described as a group of trees that are clones of each other. (Things in the real world don't always fit into simple categories.) The specific tree in question is an aspen often called Pando (Latin for "I spread"). As with redwoods, aspens reproduce by sprouting a clone from the root system, with the new trunk sharing use of the existing root. Pando is a collection of such trunks that covers over 100 acres in a national forest in central Utah.
The new sprouts are food to various foraging animals from rabbits to elk, but historically the number of foragers has been limited by predatory carnivores. Now the number of predators has declined because of human activities, and fewer predators means a larger population of prey species to eat the sprouts. If all the sprouts get eaten the older trunks eventually die without being replaced, and that's what's killing Pando. Humans are however trying to save Pando by building a fence around it.
There's more in this PBS NewHour report below: