Old Tjikko is one of the internet's favorite trees. A Norway spruce growing near the treeline in Sweden, it is said to be the oldest tree in the world at 9,558 years. Over 26,000 web pages tell us about Old Tjikko, including the usually reliable National Geographic. As is common for such stories on the internet, the tale was immediately seized upon as fact. The truth is that Old Tjikko is a fairly ordinary Norway spruce of about 80 years of age.
Trees are the largest and longest lived organisms on the planet. They are also photogenic and inspiring. It is no surprise that the internet is filled with pages that trumpet the largest tree, the strangest tree, the oldest tree and all manner of tree stories. Some of them are even true.
In science, extraordinary claims require extraordinary evidence. It is certainly extraordinary to claim that a tree is almost 10,000 years old. Is there extraordinary evidence to support the claim?
Prof Leif Kullman and his student Lisa Öberg published a number of papers about treeline changes in boreal Sweden. Some of these include observations about Old Tjikko. Many of the papers are obscure and hard to find, and some are in Swedish. When I first read Kullman's papers, I was skeptical, particularly of the connection between very old wood in the soil and the tree (see below). However, I did not have access to all the papers. Fortunately, Dr. Gordon Mackenthun, General Secretary of the European Champion Tree Forum has analyzed all of Kullman's work and summarized it in a recent paper in the New Journal of Botany.
So, how old is Old Tjikko? Some trees are unitary, producing a single stem on a single root system. When a unitary tree dies, both the stem and the root system die. The age of a unitary tree can be determined by careful analysis of its annual rings. Other trees are modular (or clonal), with a long-lived root system supporting younger stems. When stems die, they are replaced but the root systems remains alive. Counting the rings of a modular tree only tells us the age of the stem, not the age of the entire clone.
Kullman acknowledged that the stem of Old Tjikko was not old, probably about 80 years. This is the stem seen in the well-known photo to the right. Around the stem, and visible in the picture, is krummholz (crooked wood), formed when branches low on the tree are pushed down by snow but continue to grow. Kullman and Öberg obtained wood from the soil beneath Old Tjikko and used radiocarbon dating to determine that the wood was about 9550 years old. The assumption was that the wood was from the same tree and that Old Tjikko is an ancient clone.
This result was only reported in a paper in Swedish, so until Mackenthun translated and analyzed the paper, it was difficult to assess the validity of the result. In several papers, Kullman and Öberg state that for this tree and several others, it was evident that the trees were reproducing clonally and that the wood pieces must be genetically identical to the stems.
There are several problems with this conclusion:
- The pieces of wood were collected under the tree, but it is never shown that they were connected to the tree. This could have been old wood from previous trees. In cold, wet boreal soil, fragments of wood can persist without decay for thousands of years.
- The conclusion that the tree is part of a clone and that therefore the wood fragments are genetically identical to the tree is unsupported by any evidence. When I read the Kullman papers which contained the assertion that the wood fragments were genetically identical to the tree, I assumed that the evidence was in one of the papers I could not obtain. Mackenthun shows that this is not the case. No genetic analysis on either the wood or the stem appears to have been conducted.
It is pretty clear that there is no solid evidence that a 9550 year old piece of wood in the soil bears any relationship to a nearby 80 year old tree.
Mackenthun also points out that there is evidence that the population of trees in the area of Old Tjikko had reproduced sexually during warm periods in the past. If trees could only reproduce clonally, then it might be reasonable to conclude that the wood and the stem were genetically the same. If sexual reproduction has been happening in the past, there is no reason to conclude that the tree and the wood bear any relationship to one another.
If the wood samples that Kullman collected still exist, it should be possible to do the kind of genetic analysis that would confirm whether the wood and the tree are a clone. Kullman declined Mackenthun's invitation to comment on his paper.
Old Tjikko is most like a fairly ordinary Norway spruce of around 80 years of age, with fragments of wood nearby that may be 9550 years old. No evidence exists that it is part of a clone. There is no evidence to support the conclusion that this tree is any older than 80 years.
Thanks to Henri Grisinno-Mayer of the University of Tennessee for bringing the Mackenthun paper to my attention and for providing a copy.