Giant Sequoia? Puny. Bristlecone pine? At that mawkish stage of youth.
When an underground fungus Armillaria bulbosa covering 37 acres was found in Michigan in 1992, it was declared the world's largest organism. But it was quickly surpassed.
An even bigger Armillaria ostovae, aka the "honey mushroom",
was found the same year near Mount Adams in Washington, covering 1500
acres (600 hectares). It was noticed because of a huge tree die-off
-- A. ostovae kills the trees whose roots it invades. Of course
this one's been killing for 1500 years.
Quaking Aspen (Populus tremuloides) trees have evolved to clone themselves when conditions are severe enough to require it. 106 acres of aspen trees in the Wasatch Mountains of Utah were discovered (in 1992 as well) to be genetically identical. 10,000 years old, and over 6 million
metric tons in weight.
The current record for humongoust fungus is the 2,200 acre (880 hectare)
A. ostovae wreaking a swath of destruction through the Malheur
National Forest in eastern Oregon. Discovered in the year 2000, it is
somewhere between 400 and 1000 years old.
The creosote bush (Larea divaricata) is a hardy shrub that grows
in the Basin and Range country of the American Southwest. An original
bush will split apart as it ages and the pieces will continue to grow
radially outward. What appears to be a ring of bushes is really all
one bush. The largest known creosote bush ring is 50 feet (15m)
in diameter. Not large by these standards, but the bush is 13,000 years
Many of you will suggest that coral reefs are the largest and oldest
organisms on Earth, having grown over millions of years. The
largest one of these is Australia's Great Barrier Reef. Of course
the individual coral polyps are all genetically different -- The GBR is
an "organism" the way Gaia is an organism.
I read a brief article in Omni magazine almost 20 years ago telling
of a single-celled organism that floats on the surface of the sea.
The interesting thing about it was that although the organism was distributed
throughout the Earth's oceans they all appeared to be genetically identical.
I can't find anything to corroborate the story -- if it is true, would
be the unassailable winner.
Honorable mention goes to the box huckleberry (Gaylussacia brachycera), a shrub which grows in a few scattered places in eight Mid-Atlantic states in the United States. It's an extremely rare plant, as each plant sreads 6 inches a year at most, and cannot fertilize itself. In fact, box huckleberry seeds have never been observed in the wild. Box huckleberries survive by sending out underground rhizomes to colonize nearby areas. One colony of box huckleberry clones in Perry County, Pennsylvania is believed to be 12,000 years old. One scientist has conjectured that all box huckleberries are one plant.