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She had more than just a Christmas kissing attraction; plays an extremely important role in ecosystems. It is a parasitic plant – it takes water and nutrients from the host plant to survive – but it also gives back to the environment around it, providing a food source for animals, insects and birds. Research has shown that when you pull mistletoe from trees, the number of bird species living in the area can drop by more than 25 percent.
And mistletoe plays an important, under-recognized and paradoxical role in dealing with climate change.
As ecosystems degrade in a warming world, many animals and birds increasingly rely on common berry-bearing parasitic plants like mistletoe. Mistletoe also provides a cool refuge for nesting birds and welcome shade for animals resting below. They can even help cool cities. But mistletoe is also extremely sensitive to bouts of extreme weather conditions such as drought. Climate change is taking a heavy toll on them, just as animals are becoming more reliant on them.
David Watson knows these things well. In both academia and the media, he has come to be known as the “Mistletoe Man”—ever since his student research project stumbled upon the fact that certain desert birds were only found in places with mistletoe on trees. Since then, he has tracked mistletoe around the world and co-authored a paper in the journal’s 2022 issue. Annual Review of Ecology, Evolution and Systematics on the role of parasitic plants in a warming world. Watson, a community ecologist at Charles Sturt University in Albury-Wodonga, spoke to Novable Magazine about the latest results. This interview has been edited for length and clarity.
In your review, you write that parasitic plants like mistletoe are particularly sensitive to environmental stress, such as drought or frost. Why’s that?
The basic reason is just basic physiology. They have no storage organs, no way to store carbohydrates. They have no root system, bulbs, rhizomes. So when they lose their leaves, that’s it, they’re screwed. This could happen through herbivory, for example, if a lot of caterpillars come across, or a low-level fire that comes through and disperses the canopy. If the host plant is stressed by drought and begins to wilt, the mistletoe will just curl up and die. They are strangely sensitive to many of these disorders. That’s why the world isn’t full of them, because they’re actually quite picky. They are difficult to grow.
Do you have something in your yard?
I work. But all the mistletoe we have are just locals. I tried to plant all sorts of exotics from my travels, and none of them worked.
How do these plants cope with climate change?
We see time and again that when climate changes, food and the things that depend on that food can often be out of sync – a shift to earlier springs, for example, can mean most berries are produced too early for animals that need them later in the season. We see mistletoe becoming disproportionately important in many systems because other things go wrong, but mistletoe is still there — it’s reliable. In any month of any year, you can find mistletoe either fruiting or flowering in most parts of the world. They are just good at what they do. Thus, reliance on mistletoe as a resource has increased.
But then, we’re also seeing a die-off of mistletoe. We are seeing increased vulnerability to disruptions, whether heat waves, drought or fire. So, on the same page, you have animal communities that lean more and more toward this group of plants, but those plants are struggling to survive.
Can you give an example of where those changes are happening now?
My colleague, Francisco Fonturbel, works in southern Chile. Where there is mistletoe, because it is a reliable source of nectar, the southernmost hummingbird (Sephanoides sephaniodes) becomes a resident. They pollinate mistletoe, but they also pollinate all kinds of other plants. After the drought, the mistletoe dies, and those hummingbirds become migratory: they pack up, follow the nectar further north. One study found that mistletoe mortality doubled in the dry year of 2015, and hummingbird visits declined.
When hummingbirds leave, local plants no longer have pollinators. This is predicted to trigger a community-wide extinction cascade, although this has not yet been documented.
In Australia, large-scale research shows that mistletoe is extremely important during drought as a sort of last-ditch nectar resource. But then, that same paper shows that drought kills many mistletoe: In the summer of 2009, for example, there was an extended heat wave in Melbourne, including the hottest day on record—and nearly 90 percent of the monitored mistletoe population died. This caused a decline in the number of birds and insect-eating animals.
It’s not everything. Some tropical systems, some temperate forest systems, don’t show those early warnings of system failure, these mistletoe deaths. But in many arid zones, and in some southern forests at higher latitudes, we are already seeing food webs break down. We don’t want to sound the alarm and say the sky is falling, but it doesn’t look good.
Are there any other models to show where this could lead in the future?
Not. It is so complex in terms of interactions between the natural enemies of the mistletoe plant, pollinators and host seed dispersal mechanisms. We have no control over those interactions. We can make really quick and dirty models, but that’s just a guess. They are not nearly detailed enough to make meaningful predictions.
This seems to be a big problem I hear from many scientists: with biodiversity loss and climate change, there are so many unknowns and so many interactions, we just don’t know how badly things can go wrong or how quickly.
Yes that’s right.
Are there ways mistletoe could help species cope with climate change?
There is now a growing body of work showing that parasitic plants, because they have a very high water content in their tissues, are cooler. They create small cold spots in the canopies of their host plants compared to the surrounding vegetation. You can feel it: you can walk up to a mistletoe, just grab it on a hot day and it’s cool to the touch. Birds know this: they nest and rely on these things when it’s hot. A nice study showed that kangaroos prefer to rest in the shade under mistletoe trees, because that shade is several degrees cooler in the heat of the day.
But those involved in climate change modeling do not necessarily think about the microclimate and characteristics of individual plants or plant groups. A system with parasitic plants has a much greater diversity of climatic microsites than one without. So that will be key to persistence for many different groups. It is worldwide.
Is there anything we can do to help mistletoe?
The first thing, as with everything in the environment, is to stop destroying things. Mistletoe is routinely removed from trees by arborists and tree surgeons: They have been taught that it is bad, that it is a parasite. The work on mistletoe and other parasitic plants that affect host mortality is solid: as a general rule, parasitic plants do not kill their hosts. Let’s start by leaving it alone.
Second thing: Actively return it. We do it. In Melbourne, the council wanted to be seen as proactive about nurturing wildlife habitat, even as new roads were being built, new rail infrastructure was being installed and many trees were being felled. So they reached out to me and said, “Hey, can we get mistletoe on one of our street trees?” We put mistletoe up there. They had little babies that are maybe the size of footballs now after four or five years. We assumed there would be some public backlash, but it was the opposite. People say, “This is great.” I think the tide of opinion is starting to turn.
It’s a little early to tell how it’s going to change things, but we’re really excited about it. There is growing concern about heat island effects in cities: all those hard surfaces trap heat during the day and then radiate it out at night. Some colleagues have shown quite nicely that mistletoe affects the entire water balance of the tree: the entire canopy is cooler when it’s really hot and really dry. So adding mistletoe to street trees could actually help cool cities.
The season in which many people traditionally celebrate mistletoe is approaching. Any thoughts on this custom?
The kissing thing comes from a Druidic ceremony that guarantees a bountiful harvest for the following spring. Think about it: if you’re walking through a European woodland in winter, there’s snow on the ground, all the trees have lost their leaves, and you see a green plant on a tree. I mean, it has to be divine, it has to be magic. Not much of a stretch.
So it was removed with a golden sickle and then passed on to all farmers to guarantee a successful harvest. It was all about fertility, about renewal. Originally, when the mistletoe was hanging on the door, you could take a berry and that was one kiss.
Many First Nations groups have stories surrounding mistletoe. The Noongar people of southwestern Australia place enormous importance on the world’s largest species of mistletoe (Nuitsia floribunda). It’s called the Western Australian Christmas tree. It’s a huge thing: a tree 10 meters tall with bright orange, yellow flowers at Christmas time. It is a spirit tree, where the ancestors stop and rest before leaving this world. You are not allowed to mess with those plants. They are holy.
It seems that societies have long recognized the importance of these plants. But maybe today ecosystem or climate scientists don’t get the attention they deserve?
If you’re not including parasitic plants in your studies — if you’re working on herbivory, looking at squirrel homesteads or bud break times — and you’re not paying attention to parasitic plants, you’re probably missing a trick. Every terrestrial system has parasitic plants — no aquatic parasitic plants, which is very cool. There is now mounting evidence that in every system we’ve studied, parasitic plants are disproportionately important in explaining what’s going on.
Just watch out for those guys, because they’ll tell you things about the system that you probably want to know.
This article originally appeared in Novable Magazine, an independent journalistic venture from Annual Reviews. Sign up for the newsletter.
10.1146/knowable-121222-1
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