As we're talking of long-lived trees: Many years ago, I heard a report about someone investigating the fungi in a wood. Much to their surprise, they found it constituted a single plant, which was basically wearing the wood like a wig. It was thought to be the world's largest-known organism, but they were expecting to find bigger ones, as it wasn't a particularly large wood.

.... If anyone can find the article, please do paste the link here. My google fu has deserted me this morning.