Mistletoe: the kiss of life for healthy forests

In many parts of the world, Christmas and mistletoe are inextricably intertwined. So, as December 25 draws nearer, now is a good time to remind ourselves about what mistletoe is and why we associate it with plum pudding and Santa.

There’s also been some recent discoveries about the role mistletoe plays in boosting biodiversity and improving ecosystem health.

The mistletoe Viscum album from Otto Wilhelm Tomé’s Flora von Deutschland Osterreich und der Schweiz 1885

The whole mistletoe/Christmas connection predates Christianity, with mistletoe featuring prominently in the Druid’s ancient winter solstice rituals.

With their bright green leaves and complete absence of roots, mistletoes are especially apparent on leafless hosts in the winter, and these sprigs of green in an otherwise lifeless forest inspired a rich folklore. Having harvested a mistletoe sprig from an oak with a golden sickle, the cutting was taken back to their temple where it was kept for three days.

On the fourth day (Christmas Day), the leaves were distributed to worshippers, signifying the rebirth of the sun and ensuring a bountiful harvest in the coming season.

Variations of these rites are still practised today. Mistletoe sprigs variously deter trolls from stables (Sweden), prevent nightmares (Austria), welcome loved ones home (Heathrow airport in London), or give a sharp-eyed colleague kissing privileges at the staff party.

The good parasite

In the natural world, mistletoe has long fascinated naturalists and scientists.

As canopy-dwelling parasitic plants, mistletoes are completely reliant on animals to disperse and “plant” their seeds on suitable hosts. Until recently, ecologists assumed that most dispersal was conducted by an exclusive group of fruit eaters–the mistletoe fruit specialists.

Eating little else (even feeding the sticky morsels to their chicks), members of eight groups of birds (including Australia’s Mistletoebird), these birds are now known to have very strict habitat preferences, only visiting areas with abundant mistletoe. A Mistletoebird carefully extracting a sticky mistletoe fruit from the tough outer skin. Fourteen minutes later, digestion is complete, the sticky seed is deposited on a perch and dispersal has been achieved Chris Tzaros

So, although important for spreading mistletoe to new hosts, these specialists rarely introduce mistletoe to new areas. That job is performed by a much larger group of occasional mistletoe-munchers, a group we’re only now starting to learn about.

The more you look, the more mistletoe mysteries you find. In 2001, I published a review of our current state of knowledge regarding mistletoe ecology, demonstrating that mistletoes represent ecological keystones in forests and woodlands worldwide.

As well as direct providers of food (fruit, nectar and succulent foliage), many animals prefer to nest in mistletoes, and the combined effects of these interactions was a positive effect on biodiversity: areas with more mistletoe have high numbers of animal species living in them.

We cherry pickers to access the canopy for the removal experiment. David Watson

It’s all in the leaf litter

To test this idea, and work out exactly why it is so, my team conducted a large-scale experiment. We removed all mistletoes from one set of woodlands, and left all the mistletoes alone in a second set (with a third set of woodlands naturally lacking mistletoe for reference).

The results were as rapid as they were striking. Within three years of removing mistletoe, the number of bird species dropped by over third! But, rather than being the fruit eaters or nectar feeders, it was the insect eaters that showed the clearest response.

In fact, once insect eaters were gone, there was no further effect of removing mistletoe on the reminder of the bird community. This response was especially true for ground-foraging insect-eaters: the robins, babblers, choughs and their ilk, a group of birds that has undergone widespread declines in south-eastern Australia. Grey Shrike-thrush, one of the ground-foraging insectivores that declined in woodlands after mistletoes were removed. Tom Rambaut

So, what’s the connection between a parasitic plant in the canopy and birds eating insects on the forest floor?

Through careful analysis of leaf litter and bird diets, I’ve demonstrated that the keystone effect of mistletoe is the result of bottom-up processes, driven by their high volumes of enriched leaf litter.

Unlike most plants that conserve their nutrients, withdrawing them prior to dropping their leaves, mistletoes shed their leaves as is, boosting availability of a wide range of nutrients and accelerating decomposition.

In turn, this leads to dramatically higher numbers of insects and spiders on the forest floor, in turn, providing food for insect eaters.

So, rather than the direct effects of food and shelter, this research suggests that the influence of mistletoe on biodiversity is driven by a different effect: the steady stream of leaf litter effectively fertilising the forest and increasing habitat quality for wildlife.

So, while some of you will be hoping for a kiss beneath the mistletoe, next time you see one in the bush, I encourage you to look down rather than up, to appreciate the effect these unsung heroes have on overall ecosystem health.