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
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.|
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
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.