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Pando (Latin for "I spread"), also known as The Trembling Giant, is a clonal colony of a single male Quaking Aspen (Populus tremuloides) determined to be a single living organism by identical genetic markers and one massive underground root system. The plant is estimated to weigh collectively 6,000,000 kg (6,600 short tons), making it the heaviest known organism. The root system of Pando, at an estimated 80,000 years old, is among the oldest known living organisms.
Pando is thought to have grown for much of its lifetime under ideal circumstances: frequent forest fires have prevented its main competitor, conifers, from colonizing the area, and a climate shift from wet and humid to semi-arid has obstructed seedling establishment and the accompanying rivalry from younger aspens.
During intense fires, the organism survived underground, with its root system sending up new stems in the aftermath of each wildfire. If its postulated age is correct, the climate into which Pando was born was markedly different from that of today, and it may be as many as 10,000 years since Pando's last successful flowering. According to an Organization for Economic Co-operation and Development (OECD) report:
Clonal groups of P. tremuloides in eastern North America are very common, but generally less than 0.1 ha in size, while in areas of Utah, groups as large as 80 ha have been observed (Kemperman and Barnes 1976). In the semi-arid western United States, some argue that widespread seedling establishment has not occurred since the last glaciation, some 10,000 years ago (Einspahr and Winton 1976, McDonough 1985). Indeed, some biologists feel that western clones could be as old as 1 million years (Barnes 1966, 1975).—Organisation for Economic Co-operation and Development, Consensus Document on the Biology of Populus L. (Poplars)
Pando was discovered by Burton V. Barnes of the University of Michigan in the 1970s. Barnes was widely considered an expert on North American aspen at the time, having been one of the first to describe the clonal growth of aspen from an extensive root system as part of his dissertation at Michigan in the late 1950s. Barnes had described Pando as a single organism based on its morphological characteristics. Building on Barnes's earlier work, Michael Grant of the University of Colorado at Boulder re-examined Pando and claimed it to be the world's most massive organism in 1992.
The clonal colony encompasses 43 hectares (106 acres) and has around 47,000 stems, which continually die and are renewed by its roots. Many of the stems are connected by its root system. The average age of Pando's trunks, or stems, is 130 years, as deciphered by tree rings. Michael Grant in BioScience said:
...quaking aspen regularly reproduces via a process called suckering. An individual stem can send out lateral roots that, under the right conditions, send up other erect stems; from all above-ground appearances the new stems look just like individual trees. The process is repeated until a whole stand, of what appear to be individual trees, forms. This collection of multiple stems, called ramets, all form one, single, genetic individual, usually termed a clone.
Some experts[who?] speculate that Pando's reign since 1992 as the heaviest-known organism may be short lived. Less well-studied Quaking Aspens in Utah may be 80 hectares in extent and one million years old. Other large colonies could exist elsewhere. A clonal colony of at least seven Coastal Redwoods could weigh more, though no such stand has been systematically sought and identified yet. Other scientists think that portions of Pando's root system may be dead and might have led the plant to split into separate groups and it therefore would not be one organism.
Tree experts also note that the organism's age cannot be determined with the level of precision found in tree rings; some claim Pando's age is closer to 1 million years. Its current 80,000 year designation is based on a complex set of factors including the history of its local environment such as: The evidence indicating that there are few if any naturally occurring new aspens in most of the western United States since a climate shift took place 10,000 years ago and eliminated favorable soil conditions for seedlings; the rate of growth (including the differences of rates in distinct climates when accounting for its local-climate history, that males grow slower than females, and that aspens grow slower at higher elevations – Pando is at 2,697 m, or 8,848 ft, above sea level); its size; and its genetic code in comparison to the mutations found among aspens born in the modern era. Michael Grant summed it thus:
Despite enormous crops of viable seeds, successful seedling establishment appears to be a rare event in the semi-arid West, but the establishment of new trees from seeds appears to be common in the moist, humid forests of New England... aspen establishment from seeds probably has not occurred in the western United States since the last glaciation, some 10,000 years ago... Part of the rationale behind current age estimates for aspen clones is that sexual reproduction is effectively frustrated by the rarity of a favorable suite of conditions in semi-arid environments... High levels of genetic variation and excesses of heterozygotes are found in [the aspen of] semi-arid environments... Clonal reproduction is more common in arid environments... Heterozygotes often exhibit superior longevity in forest trees [across many species]... growth rate of aspen decline with elevation, steepness of slope, age of the ramet, and exposure to wind... growth rate decreased dramatically with elevation... The researchers reported that the area of the female clones was 41% greater than males, the number of female ramets 52% greater, and the basal area of females 56% greater [when compared at the same age and environment]...
Conversely, other observations in the region show that seedling establishment of new clones is regular, and often abundant on sites exposed by wildfire. J. L. Howard (U.S. Forest Service Fire Effects Information System) states:
Kay documented post-fire quaking aspen seedling establishment following 1986 and 1988 fires in Grand Teton and Yellowstone National Parks, respectively. He found seedlings were concentrated in kettles and other topographic depressions, seeps, springs, lake margins, and burnt-out riparian zones. A few seedlings were widely scattered throughout the burns. In Grand Teton National Park, establishment was greatest (950-2,700 seedlings/ha) in 1989, a wet year, but hundreds to thousands of seedlings established each year despite drought conditions in 1986-1988 and 1990-1991. Seedlings surviving past one season occurred almost exclusively on severely burned surfaces.
Other candidates for oldest or heaviest living organisms include the possibly larger fungal mats in Oregon, the ancient clonal Creosote bushes, and strands of the clonal marine plant Posidonia oceanica in the Mediterranean Sea.