How mistletoe is expected to fare on a warming planet
Mistletoe is more than just a Christmas kissing attraction; plays a very important role in ecosystems. It is a parasitic plant, it needs water and nutrients from a host plant to survive, but bring back also to the environment that surrounds it, providing a food resource for animals, insects and birds. Studies have shown that when you remove mistletoe from trees, the number of bird species living in that area may decrease. more than 25 percent.
And the mistletoe plays an important, little recognized and paradoxical role in the face of climate change.
As ecosystems degrade in our warming world, many animals and birds are increasingly dependent on common berry-bearing parasitic plants, such as mistletoe. Mistletoe trees also provide cool shelters for nesting birds and pleasant shade for animals resting below. They can even help cool cities. But mistletoe are also exceptionally vulnerable to extreme weather events like droughts. Climate change is taking a heavy toll on them, just as animals begin to rely more on them.
David Watson knows these things well. In both academia and the media, he’s been known as “the mistletoe boy” ever since his student’s research project stumbled upon the fact that certain desert birds were found only in places with mistletoe. in the trees. Since then, he has tracked mistletoe around the world and is co-author of a article in the 2022 edition of 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 with well-known magazine about the latest results. This interview has been edited for length and clarity.
You write in your review that parasitic plants like mistletoe are particularly sensitive to environmental stress such as drought or frost. Why is that?
The rationale is just basic physiology. They have no storage organs, there is no way to store carbohydrates. They have no root system, no bulbs, no rhizomes. So when they lose their leaves, that’s it, they’re screwed. That could happen through herbivory, like lots of caterpillars coming in, or a low level fire going through and burning the canopy. If the host plant is stressed by drought and begins to wilt, the mistletoe will simply curl up and die. They are strangely sensitive to many of these disturbances. That’s why the world isn’t full of them, because they’re actually quite picky. They are difficult to cultivate.
Do you have some in your backyard?
Do. But all the mistletoe we have is local. I have tried planting all kinds of exotics from my travels, and none have worked.
How are these plants faring in the face of climate change?
We see time and time again that when climates change, foods and the things that depend on those foods are often out of sync: a shift to earlier springs, for example, could mean most berries are produced too early to animals that need them later. season. We’re seeing mistletoe become disproportionately important in many systems because other things are out of control, but mistletoe is still there, it’s reliable. In any month of any year, you can find a mistletoe in most parts of the world that is either in fruiting or blooming. They are just good at what they do. So there is a greater reliance on mistletoe as a resource.
But then, we’re also seeing mistletoe die-offs. We are seeing increased sensitivity to shocks, whether it be heat waves, droughts, or fires. So on the same page, you have animal communities that are increasingly leaning towards this group of plants, but those plants are struggling to persist.
Can you give an example of where these changes are happening now?
My colleague, Francisco Fontúrbel, works in southern Chile. Where there is mistletoe, because it is a reliable source of nectar, the southernmost hummingbird (Sephanoides sephaniodes) becomes resident. They pollinate mistletoe, but they also pollinate all sorts of other plants. After the drought, the mistletoe dies off and those hummingbirds become migratory: we pack up, they follow the nectar farther and farther north. A study showed that mistletoe deaths doubled in the dry year of 2015, and hummingbird visits decreased.
When the hummingbirds leave, the local plants no longer have pollinators. This is predicted to trigger a community-wide cascade of extinctions, though that has yet to be documented.
In Australia, large-scale research shows that mistletoe is super important during drought as a kind of last minute nectar resource. But then that same work shows that drought kills a lot of mistletoe: in the summer of 2009, for example, there was a prolonged heatwave in Melbourne, including the hottest day ever recorded, and nearly 90 percent of a whole monitored of mistletoe died. That caused a drop in the number of birds and animals that eat insects.
It is not general. Some tropical systems, some temperate forest systems, don’t show those early warnings of system failure, these mistletoe deaths. But in many arid areas and in some southern forests at higher latitudes, we are already seeing food webs break down. We don’t want to sound alarm bells and say the sky is falling, but it’s not looking good.
Are there models that show where this can lead in the future?
No. There is so much complexity in terms of the interplay between the mistletoe plant’s natural enemies, pollinators, and the host’s seed dispersal mechanisms. We have no control over those interactions. We can make really quick and dirty models, but it’s just guesswork. They are not detailed enough to generate meaningful predictions.
This seems to be a big problem that 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 bad things can go or how fast.
Yes it’s correct.
Are there ways mistletoe could help species weather 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 cool spots on the crowns 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 depend on this material in hot weather. A good study showed that kangaroos prefer to rest in the shade under trees with mistletoebecause that shade is a few degrees cooler in the heat of the day.
But climate change modelers don’t necessarily think about the microclimate and characteristics of individual plants or groups of plants. A system with parasitic plants has a much greater diversity of climatic microsites than one without. So it’s going to be key to the persistence of many different groups. That is worldwide.
Is there anything we can do to help the mistletoe?
The first thing, as with everything in the environment, is to stop destroying things. Arborists and tree surgeons routinely remove mistletoe: 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 strong: as a rule, parasitic plants do not kill their hosts. Let’s start by leaving him alone.
Second thing: actively give it back. We are doing that. in melbourne, the council wanted to be seen to be proactive in promoting wildlife habitats, even as it built new roads, installed new rail infrastructure, and felled many trees. So they came up to me and said, “Hey, can we put some mistletoe on some of our street trees?” We put mistletoe there. They have had little babies who are now maybe the size of a football after four or five years. We assumed there was going to be some public pushback, but it was quite the opposite. People say, “This is great.” I think the tide of opinion is starting to turn.
It’s a little early to say how that’s going to change things, but we’re very excited about it. There is growing concern about the effects of heat islands in cities – all those hard surfaces that trap heat during the day and then radiate it off at night. Some colleagues have shown very well that mistletoe affects the entire water balance of the tree: all the the canopy is cooler when it is very hot and very dry. So adding mistletoe to street trees could help cool cities.
We are coming to the season where many people traditionally celebrate mistletoe. Do you have any thoughts on this custom?
The kissing comes from a druidic ceremony to guarantee a bountiful harvest for the following spring. Think about it: if you’re walking through a European wooded area 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, that has to be divine, it has to be magical. It’s not much of a stretch.
So it was harvested with a golden sickle and then distributed to all the farmers to ensure a prosperous harvest. It was about fertility, renewal. Originally, when a mistletoe ball was hung on a door, you could take a berry and that was a kiss.
Many First Nations groups have stories about the mistletoe. The Noongar people of southwestern Australia place great importance on the world’s largest species of mistletoe (floribunda nuytsia). It is called the Western Australian Christmas tree. It’s a giant thing: a 10-meter-tall tree with bright orange and yellow flowers at Christmas. That’s it spirit tree, where the ancestors pause and rest before leaving this world. You are not allowed to play with those plants. They are sacred.
It seems that societies have long recognized the importance of these plants. But maybe ecosystem or climate scientists aren’t giving mistletoe the attention it deserves today?
If you’re not including parasitic plants in your studies, if you’re working on herbivory, observing squirrel ranges or budding timing, and not paying attention to parasitic plants, you’re probably missing a trick. All terrestrial systems have parasitic plants; there are no aquatic parasitic plants, which is great. There is now mounting evidence that in all the systems we have studied, parasitic plants are disproportionately important in explaining what is going on.
Just keep an eye on those guys, because they’ll tell you things about the system that you probably want to know.
This article originally appeared on well-known magazine, an independent journalism effort of Annual Reviews. Sign up for the Newsletter.