Wednesday, 11 December 2013

Mistletoe. So good, nature evolved it five times


Jonathan Briggs

There are hundreds of different mistletoes spread throughout every climate and continent other than Antarctica. They do not all look the same. What they all have in common is that they are all hemiparasites. Separately evolved in at least five distinct families of plants, they share  an evergreen habit, sticky berries, photosynthesizing green leaves that manufacture sugars and haustoria that penetrate woody hosts to anchor themselves and extract nutrients. In the UK our native species is Viscum album, my subject today.

Mistletoe is a parasite and one might assume that this is a bad thing. Not so. As a keystone species it brings rich ecological diversity to a wide range of habitats.



Mistletoe life cycle

In the UK, mistletoe’s significant vectors are the mistlethrush and the blackcap. Very few other birds tackle the viscous berries. Both birds spread the sticky seed which adheres to tree bark, germinates and produces haustoria which penetrate the host’s internal tissues. Haustoria are its only source of nutrients, mistletoe has no roots.

Co-evolution with  birds

Mistlethrush
Neither the mistlethrush nor the blackcap requires mistletoe for survival but there would be very little mistletoe without these birds! The mistlethrushes favourite foods are mistletoe, holly berries and juicy yew arils. They vigorously defend fruiting trees which they claim as their own! Berries are natures’s great invention to distribute seeds by enrolling animals and birds. For many berried seeds - although not mistletoe - the process of digestion within a bird’s gut is integral to their eventual successful germination. 
It would appear that because of the diverse feeding habits of the mistlethrush its excretion is a rather ‘hit or miss’ process - literarily so - and seed might find itself on the wrong tree, or on no tree at all. The sticky viscin that coats mistletoe seed enhances the seed’s chances if it does reach its target. The bird’s latin name, Turdus viscivorus says it all.

The Australian mistletoe bird does a much better job. Not only does the seed pass through its gut in as little as five minutes, it scrapes the sticky seed off its bottom with an inelegant shuffle. This links to a picture of this pretty bird.

The blackcap is a game changer. Since the nineteen eighties they have started to overwinter in England. A distinct genetic sub-race crosses from Germany. Spain’s loss has been our gain. The blackcap now resides here when  the juicy white mistletoe berries ripen.The bird precisely ‘sows’ the mistletoe seed as it scrapes its beak clean. Mistletoes are not  host specific, as is often stated, although not all tree species are able to act as a host. Blackcaps placing the seed are usually scraping the right tree! The gardener can pretend to be a bird and smear the seed on the bark of a suitable species such as apple, hawthorn and ash. Do not plant, just smear!


I love his black cap, his beak serves as the neb.
Mistletoe distribution

The recent ancestral home of Viscum album is the South West Midlands down into Somerset. The apple orchards in Somerset and Worcestershire are traditional havens. Sadly these orchards are in decline. The good news is that blackcaps are dispersing the mistletoe and moving the centre of distributional gravity to Cambridgeshire and Essex and generally south east. Mistletoe no longer is in retreat and private gardens and suburban habitats are starting to take on the mantle.

Good news, hot off the press, my friend Peter Williams has just returned from Cambridge where the mistletoe is profuse. He has sighted it on trees on the motorway embankments of South Yorkshire and growing strongly on poplars as far north as Goole. 


Mistletoe as a keystone species.

Mistletoe may be the coup-de-grace to an ancient tree weighed down by this invader, shaded by mistletoe leaves and depleted of nutrients. However to the general ecology of a site, mistletoe is a complete boon. We old age pensioners need tender loving care and potions to preserve us from our geriatric aches and pains. It is the same with trees. Ivy - which is not a parasite, but a semi-epiphytic scrambling plant - will also hasten the death of an old tree for similar reasons. The important biological principle is that mistletoe and ivy just like my own agues, are all signs, but not the fundamental cause, of eventual demise. It’s called ‘old age’.

Mistletoe makes it’s own sugars within its green leaves. It is incumbent on the leaves of a normal plant to constantly donate some of this nutrition back to the stems and the roots. In addition, deciduous trees at leaf-fall translocate break-down products back into the plant. Evergreen plants too withdraw resources before their leaves fall. No such responsibility falls on mistletoe and it is probably impossible for the haustoria to carry out this function. The consequence is that mistletoe leaves make very good fodder and support nectar rich flowers. Even better, when nutrient rich mistletoe leaves do fall to the ground, the deep litter provides the most magnificent resource for insects, invertebrates, small animals and birds.
Although the dense crowded ‘witches brooms’ of mistletoe provides nesting, cover and nutrition for numerous species of birds, it is the leaf litter under the mistletoe that makes the greatest ecological impact.
In an experiment reported in the New Scientist an area of land had it’s mistletoe pruned away and wild life was observed over a three year period. The ecological quality of the site was dramatically diminished and in particular animals and insect eating birds in the undercover below the trees were considerably depleted. 
One significant factor is that within an area of wooded land there is considerable variation in the distribution and density of parasitic mistletoe. In effect there are ‘hotspots’ of distinct biological activity. Separate micro-sites where different combinations of plants and animals prosper are key to ecological abundance.

Mistletoe’s mystery, magic and myths

No, not the whimsical Christmas stuff but the biological story. Everything I thought I knew about mistletoe is wrong.
  • It is a harmful parasite. No its not, its an ecological desirable parasite.
  • The seed needs to pass through the gut of a bird to germinate. Wrong.
  • It is host specific. Seed needs to be transferred from apple to apple, poplar to poplar or ashes to ashes. No it does not.
  • Seed needs to be pushed under the bark. No, it needs to be daubed.
  • British mistletoe is in decline. It is increasing its geographical distribution

Jonathan’s wonderful blog


Jonathan Briggs

In my research for this post I came across Jonathan Briggs’ fascinating Mistletoe Blog, which I have brazenly plundered. In the link I give he has some very interesting things to say about one of our heroes. Jonathan has given me permission to copy some of his lovely pictures.



28 comments:

  1. I used to think the mistletoe/tree relationship was symbiotic but from what you say it is of no benefit to the tree

    ReplyDelete
    Replies
    1. Nothing would surprise me Sue, but I am unaware of any advantage to the host plant although fertile leaf litter might help water uptake to the tree if the soil below had otherwise been compacted. If anyone has any ideas please let us know.

      Delete
  2. I have always know mistletoe as a parasitic plant rather than as a symbiotic one, as there is no obvious benefit to the host. If its leaf litter is fertile relative to that of the host then that is only as a result of the mistletoe extracting the nutrients from the host in the first place and concentrating them. The mistletoe will lose water to the atmosphere at a greater rate than the host - this is the way by which it "sucks" nutrients out of the tree - so it won't help when water is scarce.
    Dave
    [a research scientist in a former existence who used to work on another parasitic plant, Striga: http://en.wikipedia.org/wiki/Striga_hermonthica]

    ReplyDelete
    Replies
    1. Thanks anon. for your helpful comment with which I completely agree, mistletoe will increase transpiration from the overall plant surface. I was just thrashing around in response to Sue's comment (she is often so right in her comments) and in speculative mode.
      Of course as a no dig gardener I am extremely interested on the effects on the soil of leaf litter and it's organic content as well as the nutrients which you rightly point out have been extracted from the plant.
      I wonder if their is a word for a relationship not directly beneficial to one of the partners but beneficial to the environment in which it grows
      I am off to look up striga!

      Delete
    2. Sorry Dave, I have just spotted you are not anon!
      Striga looks fascinating

      Delete
  3. I had a black cap male in the garden last year for the first time which has now reappeared, there could have possibly been a female but they are not easy to spot when amongst the sparrows and such like having a brown cap. As an OAP euthanasia by mistletoe or ivy...............interesting?!

    ReplyDelete
    Replies
    1. I like to draw a parallel between the cycle of lifetime human health with that of a tree, Rick. I would not bring the axe, or more likely chainsaw into the equation!

      Delete
  4. If anyone is interested in ecosystems, the Open University have a free course (along with many other free courses), the next one starts in May https://www.futurelearn.com/courses

    It would be interesting to know if mistletoe has been tested for symbiotic relationship? (Dave, are you still reading? You might know) In the OU course they show lab tests on tree and fungi symbiosis, which use a seedling and fungi grown in lab. If my memory is correct (bit lazy, didn't write any notes!) they introduce radioactive carbon to the plant via carbon dioxide, and then can track the progress of the carbon through to the fungi. The same test for mistletoe might be a bit more difficult, and a bit less important, so I wonder if it's ever happened. The fact that mistletoe is evergreen, and is not shaded during winter, does offer an opportunity for the tree to benefit, especially if it could get off to a strong start in the spring by getting carbs from the mistletoe. Interesting!!!

    Meanwhile I'll look forward to the mistletoe invasion in East Sussex :-)

    Sarah

    ReplyDelete
    Replies
    1. That Sue has started a very interesting thread and your thoughtful comments are certainly food for thought. It is a very attractive idea that a photosynthesising entity attached to a deciduous tree might have some spare sugars to donate! As suggested in my post I am unaware that this is possible but I am always suspicious that when a plant seems to succumb to a pest or disease or parasite too easily that there might be more in it for the plant than we realise.
      I wonder if Jonathan Briggs reads this post whether he has an insight into this. He might of course have blogged about it already.

      Delete
    2. Had a quick look around Jonathan's blog, but couldn't pick out any posts that might discuss the idea. But even if he has, without research providing hard evidence, it's still just an idea. An exchange of energy stores, one organism using them to fruit during the winter, the other during the summer, would be a neat bit of evolutionary team work (or, as I've learned from my OU course, a neat bit of mutualism) so I'm hoping the mistletoe does pay it's rent.

      Delete
  5. I think so more interesting thoughts about this come in. I hope so

    ReplyDelete
  6. Thank you for a very interesting article. We don't have mistletoe in North America or if we do have any they do not look like the European kind. The black cap comments are also interesting. We also have birds that love berries that are poisonous to humans (poison ivy and poke weed berries).



    ReplyDelete
    Replies
    1. Thanks for the comment and the reminder that mistletoe is poisonous. Interesting you do not have your own mistletoe and presumably no mistletoe Christmas tradition. Do you still get your Christmas kisses?

      Delete
    2. This comment has been removed by the author.

      Delete
    3. I think Alain may be worried that Ruthie may become too affectionate if she got hold of some mistletoe!

      Previous comment deleted as there was an important typo.

      Delete
    4. People will be wondering what on earth you deleted Sue
      (she is getting pernickety she has only changed 'to' to 'too'!)

      Delete
  7. I enjoyed your post on mistletoe. It is not a plant I am familiar. I love the closeup photo of it too. Roger, I always have to Google your blog because you don't leave a working link when you comment. People can't click to get to you which is helpful in getting more readers. Since I always answer comments, I went to the extra trouble.

    ReplyDelete
    Replies
    1. Thanks Donna
      and there I was thinking I was! (in the same way clicking on to your identity gerdenwaltalk takes one through the ether to your lovely pictures) Next time I comment I will watch what I am doing.

      Delete
    2. When I click on your name above your comment I go directly to your Google+ page Roger with all your posts set out - this is what Google does if you have a + account. So you are doing things correctly. I also tried from the comment you left on Donna's blog and the link came straight to your blog so unless you have changed things could it be something not working on Donna's blog. Sorry to interfere but it's hard to check such things yourself isn't it?

      Delete
    3. Thanks for your work on my behalf Sue. it was my impression too that was how google + worked -so I don't know what happened on Donna's blog. Thanks to Donna it looks as if she tweaked my identity to come through to my blog direct. I wish I knew how to do these things!
      At the moment my blog search box does not work and I am waiting for Cathi to help.I am completely dependant on you ladies!

      Delete
    4. The search box isn't working on any Blogger blogs, Roger. Blogger are aware of the problem and are supposed to be working on it - whatever that means!

      Delete
    5. Should add that I think if you leave a comment on a Blogger blog of someone with a Google+ account the name link goes to G+. As Donna's is a Word Press account it will come back directly to this blog. So it isn't a skill known to everyone but yourself.

      Delete
  8. Roger, I changed it in the comment section, that is why it worked for Sue. I usually don't do that either, but I do like when my readers can visit blogs they might not know. I made the correction.

    ReplyDelete
  9. The fruits are very lovely, almost like pearls. Interesting about the coevolution with birds. I've assumed that all poison berries are eaten by at least some species of birds - or the plants would not survive. Doll's Eyes and Red Baneberry would be North American examples.

    ReplyDelete
    Replies
    1. Interesting points Jason. I grow Red Baneberry but do not know Dolls eyes. I think there are other vectors of berry seed as well as birds.
      and of course not all berries are poisonous. Tomato is a berry and it's evolutionary strategy is to pass through the gut of an animal- including us!

      Delete
  10. I have an old apple tree in my garden that really should be cut down - it's something my husband and I disagree about. The tree is unhealthy, but gives both us and our neighbours a little privacy from each other. Over Christmas, I acquired some Mistletoe from a relative in London. (Ironically, we live in Bucks and I didn't come across any for sale this year). It is fairly wrinkly looking now, but I have decided to smear some of the berries on my apple tree as per your instructions. Hopefully this will take, and we will get beautiful mistletoe, and perhaps see the tree move towards its natural demise a little more quickly. I assume, btw that mistletoe is a slow grower? Can you advise? Also happy New Year - looking forward here to another year's worth of your interesting blogs.

    ReplyDelete
    Replies
    1. I expect it to be a rather hit or miss process. I googled 'Jonathan Briggs mistletoe growing' and found How to grow your own mistletoe. He says that after about three years it starts to make quite vigorous growth.

      Delete
  11. I fear tree may be gone by then, (!) but we'll try anyway. I might have to rub it on the underside of the branches as we are currently getting daily deluges! Many thanks for the information.

    ReplyDelete

Related Posts Plugin for WordPress, Blogger...