I though aspen groves are considered a single organism and among the largest. (I think this fungus was on an X-Files a decade ago.)

Four Interesting Facts about Mushrooms in Our National Forests – National Forest Foundation

Did you know that the biggest organism on earth is a fungus? One specific honey mushroom fungus ( Armillaria solidipes ) spans 2.4 miles across the Malhuer National Forests in the Blue Mountains of Oregon.

Four Interesting Facts about Mushrooms in Our National Forests – National Forest Foundation