The trials and tribulations of trees

Communication, dessication, and reforestation...
18 June 2024
Presented by Will Tingle
Production by Will Tingle.


Trees in a forest


This week on The Naked Scientists, we're taking a look at trees. We look at how they communicate, the diseases they are fighting, and how beneficial to the climate planting trees really is...

In this episode

The tree trail

What is a tree?
Raffy Hull, Cambridge University Botanic Garden

Despite rampant deforestation in the last 10,000 years, there are still 400 trees for every one of the eight billion people on Earth. They occupy every continent on Earth, save for Antarctica. The tallest is almost the same height as the great pyramid of Giza. The smallest is about the size of a golf tee. But such a massive diversity in distribution and size begs the questions, ‘what do they all have in common? What makes a tree a tree?’ Well, what better place to find out than at the Tree Trail at Cambridge University Botanic Garden. I’m off to meet the oaks-pert who can help me separate the wood from the trees...

Raffy - Hi, I'm Raffy. I work in the learning team at Cambridge University Botanic Garden.

Will - And we are on the Tree Trail here. This is a beautiful tree trail and you've got redwoods and pines and even a few magnolias. Such a wide variety of trees. But how are you able to look at something and go, 'that is definitely a tree,' as opposed to looking at something which may be similar, but go, ' is definitely not a tree.'

Raffy - Well, a tree is a plant that's primarily characterised by its woody structure and its ability to put on growth in height and girth by secondary thickening. So that's growth at the stem or the trunk that allows it to get wider and thicker. Essentially, a tree is a plant with an elongated stem, branches, and leaves that are held up to the sun.

Will - How many trees do we estimate there are in the world?

Raffy - There are around 60,000 species of trees in the world that are known to science. So trees can adapt to nearly every niche in the world. They're found from tropical rainforests to boreal forests and everywhere in between.

Will - In terms of what we think about when we think of trees in terms of climate, probably the first thing that springs to mind is the fact that they can store carbon. They are a carbon sink, which is a phrase that's become quite a flashpoint in recent years. But what does that actually mean?

Raffy - Well, trees can store carbon by photosynthesis. So they fix carbon from the air, they take carbon dioxide and they turn it into energy and they store this energy as biomass in their trunks, their leaves, their roots, and their associated microbiota, so their mycorrhizal fungi in the soil.

Will - When do we first think trees came about?

Raffy - Tree-like plants first evolved 360 to 390 million years ago in the Devonian period. And these trees weren't like modern trees today. They resembled our ferns and our horsetails, very ancient tree-like plants. And they actually changed the earth radically. They helped contribute to the formation of the first soils, and they also changed the atmosphere. They pulled in carbon dioxide and they released oxygen paving the way for more complex ecosystems to develop.

Will - And in a fit of irony, that compaction that happened so many hundreds of millions of years ago is responsible for many of the fossil fuels that are putting them in so much trouble today.

Raffy - Exactly, yes. Those first trees created the coal forests, which we are using today.

Will - We have a gap in the clouds unbelievably on this beautiful June afternoon. Shall we go have a look at some trees?

Raffy - Yes. Let's walk the tree trail.

Will - We've got a gorgeous looking tree here. We've got pink and white petals.

Raffy - This is a horse chestnut. So this is on our tree trail. And it has these wonderful stumpy candle-like flowers, pink and yellow, and they actually change colour depending on whether they've been pollinated or not. So they're yellow when they haven't been pollinated to attract insects. And they turn red to show that they have been pollinated and there's no point of an insect visiting the flower. And that's actually quite common in this family in Sapindaceae. So you'll find that in species related to the horse chestnut as well.

Will - I just assumed it was a clumsy bee that stumbled out of the plant and just sort of rubbed pollen everywhere.

Raffy - No, it's a really good indication whether it's worth visiting or not. It's the same with forget-me-nots as well. They change colour when they've been pollinated. They're bright yellow and then they turn white when they've been pollinated.

Will - This is a remarkably tangled up knotted tree with sort of speckled mottled, almost camouflage-like bark. What are we looking at? We

Raffy - Are looking at the Persian ironwood. It's called Parrotia. It's a beautiful tree. It's quite old. It was planted in the late 1800s, but it's quite small. Still grows really slowly and its wood is very dense, actually sinks in water. It's so dense. The remarkable thing about this tree is the way that the branches have fused with one another. It's a process called inoculation, actually comes from the Latin to kiss, and they have, they fused at multiple points and formed this webbing. So it's a really gorgeous tree to stand under. And you're right, the pattern of the bark is just like camouflage.

Will - We're still fairly close to the road here, but having stepped under it, all sound is just dropped completely.

Raffy - I love that. It's like coming in, coming into a dome. Yeah, it's a really nice, quite private space.

Will - Well, thank you so much for taking me down this exquisite tree trail. I would be doing a disservice then if I didn't ask. How can people get involved?

Raffy - The great thing about the tree trail and many of our other trails is that they're available all year round. So you can pick up a leaflet from the ticket office or you can see it online. Both offer free for you to enjoy a curated route through the garden. So we have a selection of trails, um, along with the tree trail. We've got medicines from plants, dyes from plants, plants that have inspired, um, design and technology, real breadth of subjects. So you can take your pick. And summer is a great time to come and do them. I said that they're there all year. Summer's the best time.

Tree rings indicate the age of a tree in years

How tree rings reveal our climate history
Ulf Büntgen, University of Cambridge

Whilst we have much to learn about how trees affect climate, the trees themselves are also extremely useful for examining our recent climate history. By looking at how much a tree’s rings grow year on year, you can get a good idea of how hot the summer periods were in that part of the world in that year. Cambridge University’s Ulf Büntgen has been tapping into just such a resource, and I went to meet him at his lab…

Ulf - There are different treeing parameters, there are different tree species, there are also different sites. All this has to be taken into account and then depending on our question, we have to basically adapt the methods, the archive, and the approach. So let's start from a simple example. We are interested in reconstructing past temperature variations. So all these trees, they record something that happens during their growing season, which is roughly the summer from spring into autumn. We have to go to trees that grow under conditions where summer temperatures or changes in summer temperatures are becoming most relevant for the growth. So the northern treeline in Siberia or in Scandinavia where small changes in summer temperature are already reflected in the annual growth rings. These trees would function almost like a meteorological thermometer for us. If it's a little bit warmer during these summer months or weeks, we would expect a wider ring. We also would expect the higher wood density. So how dense is the wood in a certain year? And with density we actually mean the ratio between cell size, that is a lumen, and cell wall thickness. And then another parameter beside ring widths and wood density would be wood anatomical parameter. So we can find really fine grain, go into a single year and then look at the cells and they maybe show us some anatomical features that also refer to a frost event or an insect affiliation. So things that are really happening within one of these growing seasons and are reflected or basically are reflected in anatomical anomalies.

Will - When you get this information, when you have a look at the rings that have grown on a tree year on year, and you see the difference in width. And if you get, say, a wider one after a narrow one, you go, 'we must have had higher temperatures this summer.' Are you able to discern the difference in degrees? How good is your resolution?

Ulf - The first important thing is that it's never the absolute width, but it's a relative change from year to year. So we would measure these different parameters, ring width, density and so on for hundreds of trees for hundreds or thousands of years, right? And then we build tree ring chronologies. Each of the trees that we consider will show slightly different growth variations because each tree is an individual and influenced by different things. If we merge the information, if we average the information from many hundreds of trees, we cancel out the individual 'noise,' so these individual growth variations. And we emphasise the common signal. So then we already get a very strong time series of what this forest basically tells us and what we need. Now that is the answer to your question. We need an overlapping period to instrumental measurements of temperature. So we know, for instance, for the last a hundred or 150 years, in some places, like in England, we even have longer meteorological records, right? And we need the overlap between the proxy and the measured temperatures. And in this period of overlap, which is usually the 20th century, we are building a model. And then we can transfer wide narrow, wide narrow into degree Celsius or millimetre precipitation or a drought index. And then we actually see what is worse that we are taking out of the trees.

Will - A massive advantage, as you've already said, is the fact that you have access to so many samples so close to each other. Because say for instance, you could only look at one tree and it may have had a blight one year, that affected growth. The ability to then look at 400 other trees and go, 'no, that was actually pretty good growth that year' seemingly gives you a big leg up over say drilling a hole in the Antarctic ice sheet.

Ulf - Yes, that's correct. And obviously the different climate proxy archives or records like ice cores, tree rings, they all have their advantages and disadvantages. So ideally the paleoclimatologist at the end wants to create something that we call a multiproxy record or approach, where we basically take the benefits from each of the records and disregard where they are less good.


How trees 'talk' to each other
Tom Crowther, ETH Zurich

Trees have been around for hundreds of millions of years and cover about 30% of the Earth’s surface. That level of ubiquity and proximity implies that trees can, to an extent, communicate with one another. To do this, trees have formed a symbiotic relationship with another group of organisms: the fungi. These vast and complex underground networks have remained unknown to us until relatively recently, but are the source of some remarkable interactions between trees. Here’s Tom Crowther at ETH Zurich…

Tom - We all know that forests are these incredible ecosystems that we all experience where we go exploring the wilderness. But what we often don't know is that there is an immense network of fungi that exists below the soil. These invisible little filaments of fungi spread all throughout the soil, and they actually grow in association with those trees, allowing those trees to access the nutrients and water that they need to survive. And there are millions of them in every single gram of soil. They spread throughout the entire ecosystem. And because they're so thin, they're able to really tap into the phosphorus and nitrogen and all the things that plants need to access. And what they do is they then grow in association with the plant roots. And so they can exchange that nitrogen and phosphorus for the carbon, which the trees are accessing through their leaves. And that means that both the fungus and the tree can get access to the nutrients and resources they need to survive together.

Will - And if you are, as you say, linking up all these trees through a network, does that imply that the trees and themselves might be able to, communicate might not be the right word, but impart information?

Tom - Well, yes. So what we do see is some of these fungi will actually connect to multiple trees, in fact to large networks of trees in some of the forests. They're also all connecting with other different fungi. And through this massive growing network, which some people refer to as the wood wide web, it is possible that nutrients and signals get moved around throughout the forest in this incredible underground network. Now, some people will say that that is communication between the trees. Quite a few other scientists are not in favour of that wording because it suggests that the trees are really trying to talk to each other. But the reality of the situation, I think, is actually even more magical. It's that there are these thousands of signals that are just constantly moving throughout the forest, and the trees are responding to those signals.

Will - What kind of information is being sent between these trees?

Tom - So the most obvious thing that is being moved is nutrients. The fungus is accessing these nutrients and spreading it towards the trees, but sometimes that fungus is connected to multiple trees. And so the fungus isn't actively trying to help one tree over another, it's just redistributing its nutrients to wherever it's needed. So if you imagine that you have an injury, a lot of additional energy is relocated to that location with the injury. So that injury can heal, which means that within the fungal network, if there's an area where maybe one of the host trees is dying or one of the host trees is struggling, then that fungus might choose to allocate more nutrients in the direction of that host tree so that its entire system can be functioning better.

Will - All of this interconnectedness between the trees and the fungi kind of implies that we need to stop thinking of trees as individuals and instead think of them as part of a greater whole.

Tom - I think you could not be more right. If there's one thing we know in ecology, it's that no species exists in isolation. Everything is interdependent on everything else. And if you took away the mycorrhizal fungi, you don't have trees. If you took away trees, you don't have mycorrhizal fungi. And those are just two of the thousands and thousands of species that are fully dependent on one another.

Will - Given how wet we know fungi like it, and given how much drier and hotter it's going to be with current climate projections, are we expecting to see a shift in this symbiotic relationship?

Tom - Well, unfortunately, yes, warmer drier soils are not beneficial for these fungi. And so they are really damaging our forests and they're limiting the capacity of those forests to capture carbon and help us in the fight against climate change. But on top of that, what we realised in this global analysis of the mycorrhizal networks is that it's changing the composition of the fungi. Two certain types of fungi, we call them ectomycorrhizal fungi. They grow in these very large networks and they grow very slowly because of the way they behave. They end up storing huge amounts of carbon in the soil. Other types of fungi, which we call arbuscular mycorrhizal fungi, dominate in warm dry areas and they tend to be a little bit smaller networks and they tend to store slightly less carbon in the soil. And both of those fungi are incredibly valuable and important to the functioning of our planet. But what climate change is doing is it is changing the composition. So we're getting fewer and fewer of the large ectomycorrhizal networks that we see in the cold regions and, as they leave, they are being replaced by the arbuscular mycorrhizal fungi. And on its own, that wouldn't be a big concern, but what that means is it's speeding up carbon cycling in the soil. And that means that we are releasing more and more and more carbon into the atmosphere, causing a feedback loop that could really start to spiral out of control and accelerate the right rate of climate change to a terrifying degree. What we do know is that when we protect intact healthy ecosystems, we are able to improve the diversity of those networks in the soil, and that can lead to massive additional carbon sequestration. So it really is good evidence that protecting diversity is essential for our climate change fight.

Tree disease

How do diseases affect trees?
Leigh Greenwood, The Nature Conservancy

Trees are currently coming under increasing attacks from parasites and pathogens. Perhaps the most renowned case is that of dutch elm disease, a fungal disease spread by bark beetles. Since its arrival in the UK in the 1960s, the disease has killed tens of millions of trees, and there is no cure. So, what causes this disease , and others like it, to be so destructive? And is there anything we can do about it? Leigh Greenwood is the director of The Nature Conservancy Forest Pest and Pathogen Program…

Leigh - So when we talk about the different pests that affect trees, sometimes it's easiest to understand the ones that we can physically see. So sometimes we think a lot about the beetles because you can see a beetle, you know what it is, you know how it works. But oftentimes the beetles themselves aren't the primary way that the damage happens. They are spreading a fungus in many, many cases, and that beetle fungus relationship is actually what kills the tree. And these diseases can be fungal, they can be bacterial, they can be other kingdoms. They often clog up the circulatory system of the tree. So what happens is that they disrupt the sap movement in a very, very critical way. And so the circulation system of the whole tree fails and that's why they can cause such severe damage.

Will - Do we know what is boosting these? Are they becoming more common or are we becoming better at monitoring them And that's why they seem more common? Or are cases actually on the rise?

Leigh - Cases are actually on the rise because nearly all invasive forest pests and pathogens are being moved due to human activity, including international trade tourism, immigration, and human caused changes.

Will - Is there a climate angle to this as well? Because as the climate warms, we are seeing the redistribution of many animal species outside of their regular habitat distributions and trees can't move. So it seems like they're going to be first affected if a new wave of a certain bug or beetle comes into town.

Leigh - Yeah, there's several different important climate aspects here that are really interacting when a tree is at the edge of its sort of comfort zone. So if it's at the southern border of where that tree's ecosystem keeps it healthy and thriving, then as climate change pushes that southern border into a hotter and drier state in the soil or the air, that tree will stress and stress. And both native and non-native pests will have much more dramatic effects on it because the defences that that tree could mount are probably so busy with the other stressors that that becomes a much more vulnerable tree. Another element is that because global trade is driving a lot of this problem, when you trade from areas that have a well matched environment, such as a hot and wet forest to a different hot and wet forest, you're much more likely to move a forest pest and pathogen that could survive at a new place. But when you're trading from somewhere that's hot and wet to somewhere that's cold and icy, it's basically not going to happen. When you add climate change on top of that idea, more places are getting hot, so more places match and the pests and pathogens can match up when they go to the new place.

Will - As with every disease, it feels like an ounce of prevention is worth a pound of cure. So is there anything that can be done to prevent these outbreaks before they get worse?

Leigh - Absolutely. There's two different important elements to prevention, one of which is really strong rules and adherence to the existing rules of international trade and travel. So when you see a sign that says, you know, don't bring any fresh fruit on this trip to a beautiful tropical island, take that seriously. Don't do it. It's not so they can steal your fruit. It's to prevent the movement of some sort of, you know, agricultural pest. But then beyond that, if you see an unusual pattern of damage or even tree death in your neighbourhood, you should absolutely tell somebody because sometimes these things, they're seen first by somebody who's just really familiar with their street and their trees and their neighbourhood, and they know they've never seen an insect like this or they've never seen a tree die in this very odd way. And reporting that newness, that moment of sort of, wait, I've really never seen a tree look like that before, is how with the vast majority of these forest pests and pathogens get reported to the authorities

Will - And anything could be done for the unfortunate populations that are already suffering their blights?

Leigh - That's such a great question because in many cases, the horses have left the barns. When we look at the forest pests and pathogens that we know the names of that have been here for a long time, some of them do not have strong management for existing trees. And for those we have to look to the future of selective breeding. If you find two trees that are doing very well, when all the other trees around them are suffering from these blights and fungi or beetles, you can actually take them into a tree nursery and breed them and then multiply that effort out across the whole ecosystem and create a generation or more generations of improved trees that have resistance. We have several different types of trees that we are creating improved tree programs for, some of which are showing very good success right now, some of which are still in their infancy. We have to look really strategically into the future. If we cannot fix it today, what will make it better in the future?

Tree sapling

23:04 - Does planting trees actually help the climate?

The right tree in the right place can make all the difference...

Does planting trees actually help the climate?
Mark Maslin, UCL

The recurring underlying driver behind the threats that trees face is of course our warming climate. But, even though I’m sure you’re ‘sycamore’ climate doom, we know for a fact that trees can act as a clean and relatively quick way of storing carbon. Indeed, tree planting has, in recent years, become a very ‘poplar’ activity. China and India have planted over 4 billion trees combined in the past few years. The Great Green Wall is an international effort to plant a belt of trees across Africa to hold back the spread of the Sahara desert. The Trillion Tree Campaign is a global effort to… well I’m sure you can work that one out. But, whilst the task of planting a tree seems simple, the what, the where, and the how are often anything but. Mark Maslin is a professor of earth system science at UCL…

Mark - There were 6 trillion trees on the planet before agriculture, and we've cut down 3 trillion trees, so we've cut down half the trees on the planet. So we know Earth can actually support a lot more trees. And it's really interesting that it's sort of like people are saying, 'oh, but there's so many people on the Earth.' What is interesting is that actually we are becoming more urbanised. So even though the population will go up to about 10 billion by the middle of the century, we are living in more urban areas. So those places that were deforested, that were populated, and now becoming unpopulated, giving us lots of opportunities to replant forests everywhere.

Will - Are there cases of trees not acting as carbon sinks? Because there's quite troubling studies coming out from Southeast Asia and from the Amazon saying that the Amazon may no longer be a sink for that much longer. So are there instances in which a tree can take out more than it takes in?

Mark - So you have to think of trees not as a single static source of carbon storage. Remember, when you plant a sapling, it's really small, and as it grows, it's going to take on more and more carbon. And then when it becomes a mature tree, then that carbon absorption will slow down. And so the key thing here is which part of the life cycle are you at? How big is the forest? And therefore how mature is the forest? But the interesting thing is just because some of the mature tropical rainforests are no longer absorbing carbon, that doesn't mean we don't protect them, because if you lose them due to deforestation or increased drought length due to sort of like severe climate change and elongated El Niño, then you suddenly lose all that carbon. They may not be storing extra carbon, but they're storing a huge amount of carbon now, which you don't want to lose.

Will - I'd like to play you a clip from Tom Crowther, who we heard earlier in the program. He was part of the research that went into the 1 Trillion Trees Project, and he had this to say about reforestation

Tom - Planting trees should be done by local people to bring back the local biodiversity they depend on. It should not be this idea of mass plantations, plant rows and rows, carpets of monocultures, of species, of an individual tree species because that can be devastating to biodiversity and even worse for the people who depend on it. So it's really critical that we get the right type of restoration in the right way.

Will - What do you make of that?

Mark - Well, I think Tom has really hammered home the most important key message, which is the right trees in the right place. In the 1990s, Western China was becoming a desert, and the politicians were obviously very worried about this, and what they did was ask their scientists, 'well, look what's gone wrong.' And the scientists said they've cut all the trees down. So they went through a massive classic Chinese massive reforestation project. They reforest about a hundred million hectares of land. What happened is the trees stabilised, the soil stopped flash flooding, but because they planted so many, it stabilised the local rainfall and the consequences of that was agricultural production increased massively. So there is no necessary conflict between actually trees and agriculture if you get the balance right, because of course the trees provide the environmental requirements like stable rainfall soils and things like that, which are essential for agriculture. So it's getting that balance right and making sure that the local people are fully involved. In the case of western China, it was extremely poor. So the Chinese used it in two ways. One, they wanted to restore the environment, but they also then turned around to these incredibly poor farmers and go, you will take this money to plant these trees. So it was a social manipulation as well as an environmental manipulation. So I think Tom is on the right track, which is to support local people to actually plant the right trees to support biodiversity, but also to support their own sort of agriculture and local culture.

Will - With all that being said then, what do you think is the most effective way of utilising trees in order to combat the climate crisis?

Mark - I'm going to qualify that the best way of fighting climate change is to stop burning fossil fuels. Now, even if we planted a trillion trees, that would only take 3 to 4 years of global emissions out of the system, and it would take 50 to 60 years to do that. So if we go back, what's really important is to stop burning fossil fuels. Second thing is, yes, of course we should plant trees because they are our carbon sinks, so that will help us on getting to net zero. But they're also incredibly important culturally, but also for biodiversity. Remember, we've cut half the trees down on the planet, so we've got a long way to go to actually restore all of that incredible nature, all that incredible biodiversity. And of course, we all love forests.


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