Dead trees play an under-appreciated role in climate change

Host: Shamini Bundell

Welcome back to the Nature Podcast. This week, dead trees, climate change, and the important role of insects.

Host: Nick Petrić Howe

And the UN’s upcoming biodiversity summit. I’m Nick Petrić Howe.

Host: Shamini Bundell

And I’m Shamini Bundell.

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Interviewer: Shamini Bundell

First up on the show, when it comes to modelling climate change – and in particular the effects of increasing levels of carbon dioxide in the atmosphere – a vital element is understanding the carbon cycle. That’s the way in which carbon atoms move between plants, animals, soil, rocks, oceans and the air. An expansive new research project has looked at just one element of this cycle – the carbon stored in and released from dead trees globally and how insects may be a key part of this. I spoke to one of the authors of the new study, forest ecologist Jörg Müller, and started by asking him why they set out to study dead trees.

Interviewee: Jörg Müller

It is well known that growing trees store carbon, and it's a huge debate on the globe – how to deal with forests – but dead trees are often forgotten.

Interviewer: Shamini Bundell

And how much of an impact does trees dying have on the carbon cycle, on the release of carbon dioxide and ultimately on climate change?

Interviewee: Jörg Müller

So, currently, 8% of the carbon stored in forests is stored in dead trees, so not in living trees. So, it matters and, in our analysis, we found out that the annual release of carbon from deadwood is about 115% of the carbon released by humans. So, it is a considerable part of the cycle.

Interviewer: Shamini Bundell

Wow, 115%, so it’s a decent chunk. And what are the different ways in which deadwood ends up decomposing back into carbon dioxide?

Interviewee: Jörg Müller

The very interesting thing in light of the carbon cycle is the speed and the contributors. So, all of these processes are managed by organisms, and this is about the interplay between fungi and saproxylic insects.

Interviewer: Shamini Bundell

So, when I think about wood decaying in a forest, I would definitely think about moulds and mushrooms and things like that breaking it down, but insects aren’t an obvious contributing factor. What kind of insects are we talking about?

Interviewee: Jörg Müller

Some people know those very fancy jewel beetles, the most beautiful beetles in the world with golden, shiny surfaces. So, some of the jewel beetles are important organisms breaking down wood inside, so the larvae that eat the wood. And another very famous group are the stag beetles with the big mandibles. Many others are much smaller. Some are known pests like bark beetles. So, the tiny organisms can have substantial effects in general on trees or even on the big composition. But so far it was unclear – is it a story about mushrooms only and the beetles are just some fancy, nice add on, or is it more?

Interviewer: Shamini Bundell

So, you wanted to be able to model and really properly understand the factors in this sort of particular part of the carbon cycle. But of course, there are different insect species in different parts of the world, different types of trees, different weather and climate. How did you make sure that you were getting a full view of how this worked?

Interviewee: Jörg Müller

We used basically two types of maps. One type was a map of forests in the world, so we were interested only in habitats with trees, of course. And the second was a climate map to see in which range of climates, from very cold to very warm, from very dry to very wet, trees are growing. And based on that, we asked ourselves, ‘Where do we know friends?’ And then we said, ‘Okay, we can cover this region with this friend and this and so on.’ And then we saw still some areas are missing. And then we searched for new groups or friends of friends to cover the full range.

Interviewer: Shamini Bundell

And it must have been quite an undertaking to manage all this data from sites all over the world like that.

Interviewee: Jörg Müller

Yeah, it was a huge collaborative act. The circumstances were not always so easy because some plots were destroyed by elephants, some plots were flooded. One plot was fully burnt so we lost all of our equipment there. So, it was quite a challenge to manage it.

Interviewer: Shamini Bundell

So, how exactly did you study how wood was decomposing in all of your different sites?

Interviewee: Jörg Müller

So, the aim was to have a highly standardised protocol because this was missing on such studies so far. So, what we did is each site had to select three native tree species from the site and cut pieces of 3 centimetres in diameter and 50 centimetres long.

Interviewer: Shamini Bundell

And how did you manage to sort of separate out the impact of insects versus other things affecting the decomposition of your wood?

Interviewee: Jörg Müller

So, we conducted an exclusion experiment where we used cages and some metal mesh on the ground, which does not allow termites to have access to the wood. And the second treatment was an open access for everyone.

Interviewer: Shamini Bundell

So, it was basically like trapping your piece of wood in a cage so that none of the insects can get in?

Interviewee: Jörg Müller

Exactly.

Interviewer: Shamini Bundell

And how did you know that the cage itself wasn’t sort of changing the decomposition of the wood?

Interviewee: Jörg Müller

We had a second cage type, which was an open cage which has almost the same microclimate as a closed cage, but bigger holes which allow insects to walk in or to fly in.

Interviewer: Shamini Bundell

And once you’d collected all of your data, what did you find about how much difference insects were making in different places to how fast that wood was decomposing?

Interviewee: Jörg Müller

Yeah, the major finding was that on the global perspective, on the annual release, insects are about almost 30%.

Interviewer: Shamini Bundell

So, we already know that the release of carbon from deadwood has a huge impact, more than that which is released by humans, and now you’ve found that 30% of that sort of decomposition is being caused or aided by insects.

Interviewee: Jörg Müller

Exactly. And the interesting thing is this is always annual value for the whole globe. But what we see is that the variation on the globe is massive. The insects can play from a tiny negative role to release to a high positive role of carbon release. And this is very, on the first glance, fully surprising. How can insects reduce the release of carbon from deadwood? And the interesting thing is that a number of insects bring their own fungi to the wood and they are like farmers, so they are interested to raise mushrooms and to eat the mushrooms, and these mushrooms outcompete the principal decomposer of deadwood so that you slow down the decomposition process. And the next interesting question is, how is the influence of insect decline on the globe? We probably are faced with, at the moment, how this will affect all of these processes. So, this is beyond our study. We just showed in our study it matters, and we can show it matters, particularly the tropics. From the whole carbon released from deadwood annually, it is more than 90% from the tropics.

Interviewer: Shamini Bundell

Now that you know that in the tropics there’s a huge amount of carbon being released from deadwood each year, insects play a really big role in that, does that impact policy at all?

Interviewee: Jörg Müller

Because the tropical forests are so significant on the global carbon cycle, even with our study now, it’s not the first time that such a result comes out that the tropics are important for the global climate. Even from the deadwood perspective they are, and this means we have to take care of the tropical forests much more than we currently do. And when we see the pressure on them in countries like Brazil, it’s a clear motivation to be more careful with this important terrestrial habitat.

Interviewer: Shamini Bundell

That was Jörg Müller, and you can find his paper, out in Nature this week, linked to in the show notes.

Host: Nick Petrić Howe

In a moment, we’re going to be hearing about COP15 – the UN’s convention on biological diversity. Before that, though, we’ve got Dan Fox here with this week’s Research Highlights.

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Dan Fox

More massive stars seem to host bigger planets, and new research may have an explanation. Of the more than 4,000 planets that have been spotted outside of the Solar System, bigger ones seem to circle around more massive stars. Previously, this fact puzzled astronomers, but there may be answers in the planets’ atmospheres. Planets form from the dust and gas that surrounds baby stars. As material collects, gravity pulls more matter into place, until eventually you have a planet. According to the new work, the planets around more massive stars have high proportions of hydrogen and helium in their atmosphere. The authors suggest that when larger stars are forming, there are bigger disks of material which allows the hydrogen and helium to be collected by the planets more efficiently. This efficiency translates to bigger planets. Gather up that research in Astronomy and Astrophysics.

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Dan Fox

Climate change is making more ice melt, and this melt seems to be moving continents. It’s well known that when ice melts, landmasses underneath are lifted up, as they no longer have that weighty burden. But new research suggests that continents are not only moving vertically but also horizontally. By combining satellite data on ice with models on how the Earth’s crust responds to changes in mass, the research showed that between 2003 and 2018 the ground has shifted horizontally across much of the Northern Hemisphere. Canada and the United States have moved as much as 0.3 millimetres a year. Pretty slow for a snail but rather speedy for a continent, and the research suggests that in some places the horizontal movement outpaced the vertical. Move more than a millimetre to find that research in Geophysical Research Letters.

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Host: Nick Petrić Howe

Now, in case the Research Highlights weren’t enough for you, we’ve got even more Dan Fox in this next piece! Dan has been talking to Ehsan Masood, Nature’s Africa and Middle East Bureau Chief about the upcoming UN Biodiversity Conference COP15. He started by asking Ehsan what the conference was all about.

Interviewee: Ehsan Masood

So, alongside climate change, in 1992, nearly 30 years ago, the international community also created an international agreement, a global agreement, to conserve biological diversity and to equitably share the benefits of biodiversity from things like medicinal plants. And this all came together in a United Nations agreement. It was the same time as the climate convention and, like the climate convention, every few years there’s a big gathering of all of the people involved, ministers, the heads of states, the campaigners, the businesses and so on, and they meet to agree things. This time around, it’s a really important meeting. It’s going to be in Kunming in China. It’s been delayed. It will be almost delayed for two years by the time it happens in May 2022. And they need to agree a new set of rules, if you like, new targets, a new timetable to protect biodiversity.

Interviewer: Dan Fox

So, broadly, where are we in terms of biodiversity going into this conference? How are we doing?

Interviewee: Ehsan Masood

How are we doing, or how are we not doing is probably a better way of putting it. It’s bad, is all I can say. Since the 90s, into the early 2000s and now, there have been repeated pledges to, first, slow down the rate of biodiversity loss, which is by many estimates the most severe since the last mass extinction. That doesn’t mean to say there’s going to be another mass extinction tomorrow, but we’re sort of beginning to turn the curve in the wrong direction. There have been these promises that the international community will do what it can to slow down and then eventually reverse the loss of biological diversity that’s happening, the level of species in terms of extinctions, ecosystems degradation, and all the other ways in which we’re losing the natural world. But unfortunately, every time we, as in the world and all of those who govern us, all of our representatives, every time we make these agreements and we give deadlines – at the last meeting the deadline was ten years and they agreed that in ten years they would make progress – and it hasn’t happened. And before that, there was another plan, and they promised they would make progress, and it hasn’t happened. So now, it’s sort of like the third time, and there’s a lot of frustration, particularly among scientists who, like in climate change, have been sending out warning signals for a while, and they just feel they’re not being listened to.

Interviewer: Dan Fox

So, are the reasons that biodiversity policy has perhaps struggled to get traction the same as with climate policies?

Interviewee: Ehsan Masood

Some of the reasons are similar. Of course, the big one being that biodiversity loss shares some of the same causes as climate change, particularly unchecked industrialisation, the kind of industrialisation that happens without asking questions. What are the consequences of pumping fossil fuels into the atmosphere? What are the consequences of industrial-scale agriculture? What are the consequences of razing forest land and so on? And so, some of those things are similar. I think biodiversity has always had this additional hurdle, which is one of language and terminology. Even the word ‘biodiversity’ is like ‘bio-what’? Biodiversity is shorthand for biological diversity, so what is that? Immediately, there’s this sort of barrier to understanding. And then on top of that, with climate we kind of understand there’s a threat and we know that we need to decarbonise because the planet is heating, the planet is warming. What about losing biodiversity? It doesn’t quite have the same urgency. What is the nature of the risk and the threat? It’s not easy to explain, and I think that’s been a barrier to policymaking.

Interviewer: Dan Fox

So, does biodiversity have the equivalent of net zero?

Interviewee: Ehsan Masood

It doesn’t, and that is what some scientific groups would very much like it to have. They see that part of the problem is that the agreements are made and there isn’t sort of one or two things that can really capture international attention, the attention of young people, the attention of businesses and NGOs. And so there is talk of trying to capture biodiversity perhaps using an index where you collapse several things, species and ecosystem, and tell biodiversity’s benefits to people into one kind of scorecard-type number. That’s an idea. Another idea is just to choose one thing, extinction, rate of extinctions, let’s just monitor that and see how we’re doing, and that could be like a thermometer. As with climate change, as net zero has done, as the 1.5 degree target has done, biodiversity needs something that’s relatable and which people can then understand why it’s so important.

Interviewer: Dan Fox

So, what do you think is likely to come out of this event?

Interviewee: Ehsan Masood

I’m not a betting man, well, mostly. But if I were, I would say, based on previous COPs, there will be an agreement because these people who organise these events, they are like expert at getting thousands of people into a room and then forging some kind of consensus around what needs to be done. So, there will be an agreement. It will not be what the conservation groups and scientists want to happen. So, in a sense, it will be sort of similar in terms of climate change. And I think what’s quite sad and potentially problematic, but probably one for a future COP, is that there won’t be a lot of discussion or if you like review or evaluation as to why previous agreements have not worked. It’s as if we’re on a kind of treadmill or a conveyor belt. ‘Oh, the last one didn’t work. Well, we’ll just have some new plans and some new targets and we’ll just give ourselves another ten years.’ No one is, in a sense, doing the social science of international environmental policy, like why did the last one not work? Who were the players? What were the pinch points? What were the blockers? What worked well? What didn’t work well? These agreements are not very good at introspection and self-reflection, and so it’s very likely that we’re going to agree a plan and there’ll be some great big event and there’ll be lots of TV and buzz and media around it, and few people will actually ask, ‘Why are we failing?’ Now, there is a body. There’s an IPCC-style body for biodiversity. It’s less well known. It’s called IPBES – everyone here loves their acronyms – and this stands for Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services. It is still new and it’s kind of finding its feet, but it needs to be playing more of a role, like the IPCC does in bringing together all of the different points of view, particularly from the scientific community, and then presenting that to policymakers, and I’m hoping that some of that will happen too.

Host: Nick Petrić Howe

That was Ehsan Masood talking to Dan Fox. And if you want to learn more, Nature has an editorial about the biodiversity summit due to take place in Kunming, China in this week’s issue.

Host: Shamini Bundell

Finally on the show, it’s time for this week’s Briefing chat, where we discuss some intriguing science articles from the Nature Briefing. Nick, what’s sparked your interest today?

Host: Nick Petrić Howe

Well, I’ve got something that I hope will spark your interest too, and I’m going to try and do my best sort of clickbait-y voice. The story I’ve been looking at is about cannibal cane toads.

Host: Shamini Bundell

Oh, I mean, anything with cannibals or toads or cane sounds great to me. So, is this a horrible wildlife story where we thought the animals were really cute? Because I saw a video on Twitter this week of a really nice, friendly looking tortoise eating a cute little chick, so are the cane toads just horribly eating each other? Is that what’s happening?

Host: Nick Petrić Howe

I mean, they are horribly eating each other, but it’s not something that we thought was cute and then has turned out not to be. The cane toads that I’ve been reading about are ones in Australia and these are very, very invasive species. In some ways, they’re almost a quintessential invasive species that were introduced into Australia in 1935 and have since run rampant across the country because they have no natural predators.

Host: Shamini Bundell

Oh my gosh, yeah, I have actually heard of this. I feel like they spoofed it in The Simpsons once and there were just toads everywhere. So, have they been causing huge problems in Australia?

Host: Nick Petrić Howe

They’ve caused huge problems for a long time, but this story is about the fact that they’re so numerous and they have so few natural predators that the biggest evolutionary pressure comes from themselves. So, some of these toads have started eating each other when they’re very young.

Host: Shamini Bundell

Oh, so this is like a new development?

Host: Nick Petrić Howe

Yes, that’s right. So, there was a study this week, and I was reading about it in a news story in Nature, where they’ve sort of observed that cane toads have been eating each other when they’re in their very early stages. So, tadpoles will eat hatchlings, so when they’ve just hatched from the egg, those are hatchlings, and the tadpoles will eat them. And this seems to be a thing that only happens with the Australian version of the cane toads. They’re originally from South America, and in the study they looked at South American cane toads and they compared them with Australian cane toads, and it seemed like the Australian cane toads were just way, way, way, way more likely to eat each other than the South American versions. So, it may be some sort of adaptation to living in Australia with just too many cane toads.

Host: Shamini Bundell

And were they looking at the South American cane toads and the Australian cane toads in the same circumstances or were they looking at them in their natural environments so it could be a sort of environmental pressure rather than an inherent change that’s pushing them towards it?

Host: Nick Petrić Howe

So, this was a laboratory study, so they had them in the lab and they bred them in the lab, so they bred them a couple of times so they could get some tadpoles and they could get some hatchlings, and then compared them in identical conditions. And it seemed that there was something intrinsic about the Australian cane toads which meant that the tadpoles were 2.6 times more likely to cannibalise the hatchlings than the South American counterparts.

Host: Shamini Bundell

So, natural selection has acted relatively quickly and produced maybe some sort of genetic or epigenetic change.

Host: Nick Petrić Howe

Yeah, and it seems like, actually, the Australian tadpoles are literally sniffing out the hatchlings because they’re attracted to the scent of them. So, they did another experiment where there was a cage full of hatchlings and an empty cage, and the Australian cane toads were 30 times more likely to go to the one full of the hatchlings, whereas the South American ones had no preference for either cage. So, there is something about them, and they think it’s the scent from their poisonous skin, that is attracting the tadpoles.

Host: Shamini Bundell

Oh, that is slightly creepy. Cannibal cane toads – I don’t know why I didn’t expect it to be creepy. But yes, it has lived up to the excellent headline there.

Host: Nick Petrić Howe

Yeah, it kind of says it in the name. But it gets even weirder because the hatchlings are also quickly developing their own defence to this. So, it seems like the Australian hatchlings, when there are tadpoles present, will more quickly develop than they would otherwise, so they’ll skip through the developmental process so they can become tadpoles more quickly because the tadpoles won’t eat other tadpoles. They’ll only eat things that are younger than them. So, they just try and quickly get through this so they can avoid being eaten, I guess.

Host: Shamini Bundell

And dare I hope that this is good news for the sort of battle against invasive cane toads?

Host: Nick Petrić Howe

It may not be good news for them, although maybe they’ll eat each other and that’ll solve that problem, but this is an opportunity for scientists to watch an evolutionary process in real time. Cannibalism is relatively rare and seeing it evolve is even rarer, and this is something that’s happened very quickly. Like I said, the cane toads have only been in Australia for 86 years, so almost in real time, or as real time as you can get with evolution, you can actually watch it happening. And they’re going to start working out if there’s some sort of genetic underpinning to this because, like I said, it seems like there’s something intrinsic about the Australian cane toads which means they’re more likely to perform this type of cannibalism.

Host: Shamini Bundell

Well, I’ve got a sort of evolution story as well this week. Although, rather than watching evolution in progress, it is a snapshot from the distant past. A new fossil of a very cool and unusual pterosaur has been discovered. Now, this was a Nature Research Highlight that the Briefing linked to, and there is also a very cool National Geographic article that went into slightly more detail. And one of the exciting things, aside from cool pterosaur bones, about this is that the fossil was seized from criminals just before it was exported from where it was collected in Brazil.

Host: Nick Petrić Howe

Well, I mean, that is very intriguing. But maybe before we get into that, I’m imagining a pterosaur as sort of like a flying dinosaur. Is that right? What sort of dinosaur is this?

Host: Shamini Bundell

Nick, right, okay, let’s go back to some basics here. Now, important fact, Nick – pterosaurs are not dinosaurs.

Host: Nick Petrić Howe

Oh, sorry.

Host: Shamini Bundell

It’s okay. They are a related kind of reptile, and they were around for a lot of the same period and they went extinct in the same sort of mass extinction event. So, yes, in the scenes of dinosaurs, the dinosaurs ruled the land but the pterosaurs ruled the skies. So, there were a huge variety of flappy reptiles with sort of membranous wings. This is the technical description. And several of them, this newly discovered fossil included, have very cool sort of head crests – weird, large shapes sticking out from their skulls.

Host: Nick Petrić Howe

Oh, okay, so this new fossil, it was seizes in a police raid, which sounds fascinating, but what else is sort of interesting about this new fossil in particular?

Host: Shamini Bundell

It is an amazingly well preserved piece, so I think a lot of people are excited about it. And it’s a species that was previously known, I think, only from some skulls, so they were now able to get a sort of pretty much fully preserved and really well-articulated skeleton with soft tissue. And if you look up the pictures of this specimen, this crest, as I mentioned, on the head, a sort of big, round sail thing off the top of its skull, you can see the shape of that embedded in the rock, which is very cool. And it looked very large and somewhat unwieldy.

Host: Nick Petrić Howe

So, is this new fossil able to shed any sort of insights of why it had this sort of funny crest on top of its head?

Host: Shamini Bundell

Well, palaeontologists have their theories as to this sort of headgear, and I think a very popular one is that it was some sort of signalling, maybe something do with competition for mates, maybe they were showing off to each other. But an interesting thing that now we have more than just the skull that comes off of this skeleton is that, while they’ve confirmed that this creature could definitely fly, looking at it, it looks like this headgear probably made flight kind of unwieldy. Long-distance flying for this kind of pterosaur – Tupandactylus, it’s called – was probably somewhat tricky. So, the new idea is that these animals probably spent most of their time foraging on land.

Host: Nick Petrić Howe

Oh, wow, that’s really interesting. And well, I feel like I’ve asked all my science questions as is my job, so I’m interested about this police raid. What was going on? Why was there a fossil that had to be raided by the police?

Host: Shamini Bundell

Well, no, this is a big problem. So, Brazil has a lot of sort of valuable fossils. The rules are very strict over there. You are not allowed to, I think, even search for fossils without some kind of a permit, and definitely not sell them. So, criminals go and dig them up and export them, and they end up in private collections all over the world, and there are fossils in private collections that have never been studied, so it’s really, really good that this one was rescued and is now freely available, and the scientists at the University of Sao Paolo have been able to study it in detail.

Host: Nick Petrić Howe

Oh, wow, so this is a scientific discovery we may not have known about had it not been for police intervention. That is absolutely fascinating. But I think that’s probably all we have time for on the Briefing chat this week. Thank you so much for speaking to me, Shamini. And we found both those stories in the Nature Briefing, a daily email newsletter with a handpicked selection of top science news. And you, listener, can subscribe for free by clicking the link in the show notes.

Host: Shamini Bundell

Before we go, we’ve got some excellent videos out this week, including a documentary on new trials that are using CRISPR gene editing to treat sickle cell disease and an animation on how diabetes works, so you can check out both of those on our YouTube channel, and as usual, we’ll include the link at the end of the show notes.

Host: Nick Petrić Howe

That’s all for this week. As always, if you want to comment on anything you’ve heard on the show then we’re on Twitter – @NaturePodcast. Or we’d love to hear from you via our email address – podcast@nature.com. I’m Nick Petrić Howe.

Host: Shamini Bundell

And I’m Shamini Bundell. Thanks for listening.