The 13,000-year old tree that survives by cloning itself

In California’s Jurupa Mountains, there is a very unusual group of tree – a Palmer’s oak. Unlike the mighty trees that usually bear the oak name, this one looks like little more than a collection of small bushes. But appearances can be deceiving. This apparently disparate group of plants are all clones of a single individual, and a very old one at that.

By repeatedly cloning itself, the Palmer’s oak has lived past the separation of Britain from continental Europe, the demise of the mammoths and saber-toothed cats, and the birth of human agriculture. It is among the oldest plants in existence, first sprouting from an acorn around 13,000 years ago. According to the creationist view of history, this tree was around 7,000 years old when the universe was created.

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Today, the Jurupa individual grows in a narrow gulch between two large granite boulders, and it’s the only one of its kind in the surrounding area. It seems distinctly out of place, living in a much drier climate and much lower altitude than others of its species. Palmer’s oaks usually like mountainous living accommodations with cool, wet climates.

The oak has around 70 clusters of stems, entangled in a dense thicket around 2.5 kilometres in width and one metre in height. Michael May, from the University of California, Davis, first suspected that these stems might all belong to the same tree because of their appearance. They all look very similar in terms of their leaves and growth patterns. Even though they were bursting with flowers (the sexual organs of plants), they showed no sign of sexual reproduction at all. The ground around them was littered with aborted acorns, but no seedlings at all.

An analysis of the plant’s proteins confirmed May’s suspicions. He found identical versions of nine different proteins from leaf samples taken from 32 of the 70 stem clusters. This staggeringly low genetic variation implies a set of clones. In some cases, a pattern like this could be a sign of extreme inbreeding caused by a limited population, but not here. May found two different versions of some of the proteins (representing two variants of the same gene), but the same two forms in every single sample he took. The odds of this happening because of inbreeding are less than one in a billion. The samples must be clones of one another.

The tree only expands once it is hit by fire, resprouting new shoots from scorched branches. This unusual strategy makes it relatively easy to work out how old the tree must be. May did this by measuring rings from various stems, estimating the tree’s growth rate and working out how long it would have needed to reach its current mighty size. For comparison, he took similar measurements from three other populations of Palmer’s oak to look at how fast the tree can grow under a range of different conditions.

Based on the Jurupa individual alone, May calculated that the tree is around 15,600 years old. The more realistic estimate, factoring in the growth rates of other trees, says that the oak is at least 13,000 years, with the stems growing at around one centimetre every year. There’s a lot of room for error in such estimates, but May says that his figure is based on a very conservative set of assumptions. If anything, it’s an underestimate for how old the Jurupa tree actually is.

It started growing during an Ice Age, and has survived through the subsequent warming and all the climate upheavals ever since. Few plants can compete with such a record-holder. Other possible contenders include a creosote bush in the Mojave Desert that’s around 12,000 years old, a box huckleberry plant that’s survived 13,000 years, and the oldest of all – a King’s holly from Tasmania that could be over 43,000 years old, and is the only example of its species. All of these green geriatrics are clones.

Reference: May, M., Provance, M., Sanders, A., Ellstrand, N., & Ross-Ibarra, J. (2009). A Pleistocene Clone of Palmer’s Oak Persisting in Southern California PLoS ONE, 4 (12) DOI: 10.1371/journal.pone.0008346

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