Oct 232015
 
Yew berries showing the distinctive fleshy aril that surrounds a single seed.
Yew berries showing the distinctive fleshy aril that surrounds a single seed.

Yew berries showing the distinctive fleshy aril that surrounds a single seed.

The Fortingall Yew in Perthshire is a tree of international renown as potentially the oldest individual tree in Europe. It has been suggested that it could be as much as 5,000 years old, but this is not universally accepted and needs some qualification. For a start this is an estimate based on a girth measurement of 56 feet by Thomas Pennent in 1769. In old yews the definitive evidence in the form of annual growth rings has long since rotted away. The word ‘individual’ becomes important in qualifying this assertion as we now know certain trees clone themselves and can greatly exceed 5,000 years. Nevertheless, it is still a remarkable thought that this invidual yew tree could have been growing before the Great Pyramid of Giza or Stonehenge were even begun.

The first thing visitors notice about this ancient tree is that it is surrounded by a small enclosure built of stone with sections of iron railings giving glimpses of the ancient hulk within. The whole experience is reminiscent of a trip to the zoo. This unfortunate state of affairs came about as a way of putting off unscrupulous souvenir hunters who would help themselves to parts of the tree.

Male cones on the yew are small and spherical and shed copious pollen when they mature.

Male cones on the yew are small and spherical and shed copious pollen when they mature during winter and early spring.

Closer examination reveales the Fortingall Yew is a male tree. Yews are normally either male or female and in autumn and winter sexing yews is generally easy. Males have small spherical structures that release clouds of pollen when they mature. Females hold bright red berries from autumn into winter. It was, therefore, quite a surprise to me to find a group of three ripe red berries on the Fortingal yew this October when the rest of the tree was clearly male. Odd as it may seem, yews, and many other conifers that have separate sexes, have been observed to switch sex. Normally this switch occurs on part of the crown  rather than the entire tree changing sex. In the Fortingall Yew it seems that one small branch in the outer part of the crown has switched and now behaves as female.

Part of the conservation yew hedge that will ultimately encircle the Garden with over 2,000 individuals yews from ancient trees and wild collections.

Part of the conservation yew hedge that will ultimately encircle the Garden with over 2,000 individuals yews.

Three seeds have been collected and will be included in an ambitious project to conserve the genetic diversity of yew trees across their geographic range including Europe, the Caucasus, Western Asia and North Africa. The project will see the exisitng perimeter hedges at the Botanics replaced by a conservation yew hedge grown from cuttings and seed collections made from wild populations and significant ancient trees like the Fortingall Yew. The first phases of planting went in the ground in 2014 and on completion the hedge will encircle the Garden with a remarkable genetic resource of over 2,000 individual trees, each of which will have a story and can be traced back to their origins in Britain or beyond. This hedge could well be the largest conservation hedge of its kind anywhere in the world.

As it matures the hedge will display a range of characteristics reflecting the genetic diversity of the many individual trees involved and as such it will not look like any existing yew hedge. The Fortingall Yew itself will be represented in the hedge and so too, all being well, will its offspring via the curious ability of yew trees to change sex.

A visit to the Fortingall Yew on 12th October 2015 when three yew berries were collected from one discreet part of the canopy.

A visit to the Fortingall Yew on 12th October 2015 when three yew berries were collected from one discreet part of the canopy.

  20 Responses to “Oldest yew tree switches sex”

  1. I was most interested to hear you on the Canadian Broadcasting program, As It Happens the other night.
    Here on the British Columbia coast the native Yue is a relativaly rare wood. Historically used by first nations as arrow shafts. From about 1880 on and until fairly recently it was used as a boat building wood for bent frames where its remarkable duribility was second to none. It also bends well when green. It was used by owner builders due to its rarity as there was never a comercial market for it and it was harvested by the boat builder himself.
    We have one on our property in Victoria which I have revered since childhood.
    If I can be of any assistance to you in your ongoing study I would be delighted to collect and send you samples as you might require from local examples on Vancouver Island.
    Yours Truly,
    John West, Victoria, B.C.

    • Dear John, thank you for the information about yew in Canada. Really interesting. Thank you also for the kind offer of seed samples. I will pass the information on and we may get back in touch. The Canadian yew is a distinct species from the yew in Britain. There are in fact around 12 species of yew globally and the Canadian yew is one of the few that routinely has male and female parts on a single tree. For this reason it has been assumed that having the sexes on separate trees is the ancestral condition and that Canadian yew has evolved from an ancestor that had separate sexes. Although our native yew (Taxus baccata) does not normally have both sexes on one tree there are rare observations of this and more work is needed to fully explain what is going on.

      • Max I heard you talking on the BBC radio news today, promoting theories as to why the Yew tree in question has apparently changed sex. One obvious (to me anyway) reason is that the branch in question is perhaps not of the same tree, but is a tree growing out of a tree rather than out of the ground. It could have somehow become naturally grafted to the existing male tree by a bird dropping in a nook of the bark and this having naturally taken root. Can you test whether the female tree is genetically different from the existing tree?

        • Dear George,
          Many thanks for your observation that opposite sex branches on yew trees might result from seedlings that have germinated up in the canopy of an existing tree. This is a perfectly plausible suggestion and in damp climates tree seedlings are known to do this, particularly on the trunks of fallen trees. With advances in DNA techniques it would be a routine test to verify whether or not the opposite sex branches matched the DNA ‘fingerprint’ of the main tree. I believe that verification in this way has shown that such branches are not seedlings, although I’m not aware that this particular test has been carried out on the Fortingall Yew. I do think that at least some of these rare yews with both sexes will turn out to be seedlings that have grown in such a way that they appear to be part of what is infact a separate individual.

  2. I was reading about this in The Guardian and found you via the link.
    How amazing to think about what this amazing tree has lived through…if only it could talk!
    A great article and I’m delighted to have found your blog.

  3. Dear Coleman,

    I have certain queries regarding the findings –
    1. Is it the only Yew tree in that locality?
    2. If yes, then could the change in sex be a survival strategy in response to the absence of same species partner?
    3. If ans to Q1 is no, then are those trees are of the same age and show the same behavior of sex change?

    • To answer your questions the Fortingall Yew is not the only yew in the area and a female tree grows quite nearby so both sexes are present as mature trees. Your line of reasoning assumes that an individual tree can somehow know if the opposite sex is in close proximity. As far as I know plants can’t do this, but you are right in thinking that this could be a survival strategy. It could, for example, be the only way an isolated male could reproduce.

  4. We have a large Yew tree that has for years had a sprout (about ten inches high) where the leaves are without green chlorophyl (leaves being light yellow). Is this another known trait of the Yew tree, and might a cutting grow into a yellow Yew tree?

    • Dear Rupert,
      Your tree has produced what is called a ‘sport’ in the differently coloured sprout. What this means is that a genetic mutation and/or a change in gene expression has resulted in an alteration of the appearance that is quite possibly stable over time. This sort of change may well be behind the sex switching of indiviual branches, but we just don’t know quite what is triggering the sex changes at present. In fact, we don’t understand sex determination in yew tree at the most basic level. It sounds like the change in your tree has had some impact on the production of the green pigment that is responsible for photosynthesis – chlorophyll. In some cases there is no chlorophyll produced in the mutant twig and here it is effectively a parasite on the rest of the tree as it cannot produce any food. Such sprouts would not root if cuttings were taken. However, assuming chlorophyll is present, the answer to your question is yes you could produce a tree that was entirely like the unusual shoot. Indeed, some cultivated versions of popular garden plants have arisen in just this way.

  5. Dear Max,
    it was a pleasure for me to hear about you and yew 🙂 on the italian radio some days ago.
    It was a great surprise for me to listen your discovered in Edinburgh! Let’s say, that I have particular affection for yew : I went to the international yew conferences in Europe because I studied yew behaviour for 3 years (during my past PhD at University of Tuscia-Viterbo, Italy).
    With our team we recorded informations on yew sex reversal (that, as you told, it is a rare phenomenon but it is not unknown in Taxus baccata), our group discovered it also in Italy.
    We performed some researches that attest the occurrence of the ‘sex-reverse’ behaviour in yew in some critical condition of stress or due to particular association with other trees (a collegue of mine, from university of Tuscia,performed his PhD thesis about sex behaviour in yew population across Italy).
    Following this line of research, my PhD thesis actually had the aim to explore from “inside ” Taxus baccata , trying to track the cause of its sex reversal behaviour starting from its DNA.
    Thus I started first of all updating the karyotype of this specie and then we tried to find sexual chromosomes in yew (or at least some differences in chromosomes of male and female individuals).
    Unfortunately I moved suddenly from Viterbo to Naples in the end of 2013 and because of lack of foundings we didn’t go forward in a deeper research. Actually it is a pitty, considering the great team effort and my enthusiasm for this species and this topic.
    However I am finishing to write the paper about my PhD work (hoping to submit it till the end of this year or maximum in January).
    Anyway I would like to thank you for your work of dissemination about this beautiful tree (that is monumental, beautiful but we know also precious for us and for human health), hoping that will be usefull to preserve its important and its genetic diversity.
    If you want we can keep in touch, would be great for me to have connection with yew again.
    cheers
    Maria Paola

    • Dear Maria,
      Many thanks for your very interesting comment about the search for the mechanism behind sex reversal and initial sex determination in yew. Theory would suggest that stressed female trees might switch to being male as a way of saving resources. Is that what you have observed in response to stress? I have just written a second blog on the subject as the large amount of interest in this story has generated enough new information to justify it. The particularly interesting thing is that we seem to have an earlier sighting of a female branch on the Fortingall Yew that may well be the same branch I saw. The sighting was in 1996!
      Regards,

      Max

  6. Dear Max and all,
    it a pleasure for me to speak about yew. It would be great and useful to share all our knowledge about its fascinating behavior.
    Yes, with my past group of University of Tuscia (Viterbo, Italy) we also found rare cases of yew trees with both sexes on the same individual. Moreover we found that females yew individuals are more subjected to stress, and they would be let’s say “totipotent”, so able to carry out what is called sex-shifting (in particular environmental conditions) from female to male.

    We keep in touch
    regards
    Maria Paola

    • Dear Maria,
      Many thanks for confirming that female yews are more prone to the effects of stress, due to the hight level of energy investment in fruiting, and that this can lead (rarely) to sex switching behaviour.

      Best wishes,

      Max

  7. […] Coleman of the Royal Botanic Garden recently posted in his blog that a small branch with berries on the male Fortingall Yew tree sparked his […]

  8. […] Coleman of the Royal Botanic Garden recently posted in his blog that a small branch with berries on the male Fortingall Yew tree sparked his […]

  9. […] Coleman of the Royal Botanic Garden recently posted in his blog that a small branch with berries on the male Fortingall Yew tree sparked his […]

  10. […] was very surprised to see a few berries on what was clearly otherwise a male tree,” Dr. Max Coleman, a science communicator at the Royal Botanic Garden Edinburgh told The […]

  11. […] Read more at the Royal Botanic Gardens Edinburgh blog […]

  12. Max Coleman
    The news is very curious about many things longevity of the tree, preservation, but more interestingly, a dioecious species being a male individual producing female flowers on a branch.
    I think it’s an extraordinary case (but not only) and wonder if they are studying the genes involved in sex determination in gymnosperms and if these genes may help explain how you can produce the evolution of flowering plants. It may help to understand the origin of the hermaphrodite, monoecious and dioecious flowers.Why in the genus Taxus there monoecious and dioecious espedies?

    I await your response. Thanks.

    Alfonso

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