The Evolution of Trees

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1:35This is In Our Time from BBC Radio 4, and this is one of more than a thousand episodes you can find in the In Our Time archive. A reading list for this edition can be found in the episode description wherever you're listening. I hope you enjoy the programme. Hello. We might take them for granted in parks, in our streets and in forests, yet trees play a crucial role in regulating our climate and providing the air we breathe. We build with them, rely on them for food and shelter, use them for paper, clothing, medicine and energy. But trees aren't simply a backdrop to human life. They are the product of a deep evolutionary history that transformed our planet.

2:22When they first appeared, they reshaped our ecosystems, altered the atmosphere, and created entirely new environments on land. Well, with me to discuss the evolution of trees are Jenny McIlwain, the 1711 Chair of Botany at Trinity College Dublin and Director of Trinity Botanic Gardens, Christopher Berry, Senior Lecturer in Earth and Environmental Sciences at Cardiff University, and Bill Baker, Senior Researcher at the Royal Botanic Gardens in Kew. Welcome to In Our Time. And Jenny, let me start with you. I think, like most of the listeners, I can say that I recognise a tree when I see one. But what is a tree?

3:06Yeah, that's actually such a hard question because we have to define a tree without using the word tree. So there is actually, there's a lot of arguments and discussion among plant scientists about what a tree is. But what we can say is that it has to have a trunk, so a main stem, and it has to have a canopy. And the canopy, we think, usually has leaves today, but not necessarily. So it is a canopy of branches. That trunk has to not die back. So we have structures that look like trees, but they're actually flowers. What do we mean by die back, exactly? So it means that it's a perennial structure so that it will last for many, many years and not just die after one or two years.

3:53And then to be a tree, the trunk and the crown have to be self-supporting. So what that means is that this beautiful organism can't rely on leaning on a wall or before walls leaning on another tree or leaning on other vegetation. and then other people use a definition of a particular height and a particular width so there's loads of arguments about that but I think a good definition is a tree has to be above three meters and then if you took your hand and at chest height measured the the width of the trunk it has to be wider than your hand to classify as a tree and then the big controversy comes with wood Yes, tell us, what is wood exactly?

4:37And can you have trees without wood? Yes, we absolutely can have trees without wood. Some botanists are really strict and they say you can't be a tree unless you have wood. But I think that's far too strict. And if you think of a tree, it can be woodless, it can have wood. Trees have worked out many ways of becoming a tree. So what is wood? Wood is a tissue made up of lots of cells. and those cells are really complex and they are hollowed out and they really look like straws in their simplest form. So they have a hollow centre and then the cell wall of the straw is thickened with a polymer that is called lignin.

5:17And lignin enables that tree to self-support and not fall down and not rely on other structures to lean on. So if the tree has lignin, we call it wood. Actually, the Greek for wood is called xylem, and that's what those cells are called. Wood cells are called xylem. And before trees came along, what was happening with plants on Earth? Yeah, so we kind of always think trees are ubiquitous today, so we kind of think they've always been here. But if you look deep into evolutionary time using fossil plants, between about 460 million years ago and 390 million years ago, the world was treeless. and in actual fact plants are just coming onto the earth's surface out of water and photosynthesising so they're green.

6:06So if you could imagine you would be walking in a world where you'd see lots of bare rock, very little soil and the plants are leafless, they are rootless and there are no trees. So the highest plants are kind of brushing your ankle, touching your knee but there's no massive tree structures on the earth before the advent of trees. So if they're rootless, that means they're sort of like mosses or something like that. Yeah, that's right. So evolutionary groups that have evolved in the earliest phase of the earth are mosses, liverworts, there's also ferns. So these plants all reproduce by spores.

6:46In the early record, they're rootless, but they evolve roots. And in the early record, they have no leaves, but they evolve leaves. OK, so Chris Berry, when did the first trees then arrive out of, presumably out of this vegetation and what did they look like? So the oldest trees I've seen are about 393 million years old from the beginning of the Middle Devonian period. And I suspect that a few million years before that there's other types of simple tree leading up to these first trees that we know about. The first tree we have is a cladoxylopsid tree, and it's called calamophyton, and it's an extinct type of organism.

7:27Just the definition of those two words. Cladoxylopsid simply means that the wood is branched. So inside the tree, there's lots of sort of network of wood rather than a solid cylinder of wood that you might expect in an oak tree or something today. Got it. And the name of the tree is calamophyton. So that's a strange word, which is, I think, a combination of a Greek word for reed and phyton, which means plant. It was considered very simple when they first found small parts of it. So these are the first trees that we know of? Yes, they are. Now that we've been able to assemble the different bits that we've known about into one organism, which is a tree.

8:08So it's a tree called Callum phyton. It's probably about two metres tall. consists of a trunk it has simple but many roots at the bottom and from the sides and particularly the top of the growing apex of the tree would be lots of branches those branches look a bit like i'd say maybe monkey arms with very long fingers and attached to those fingers would be tiny little twiggy things which weren't leaves but were probably photosynthesizing and this thing would have just grown upwards to probably about maybe two and a half metres. So Jenny wouldn't call it a tree, which is fine by me, actually. But...

8:50Would you call it a tree then? In its time, it was a tree, but we might not call it a tree now. It was definitely something that we'd look up to if we were a small arthropod crawling around on the ground back in the Middle Devonian. So Jenny, not a tree? I would call it a tree. I have a broad definition of a tree. I just gave the strict one. Okay, so we know about this because of the fossil record. Sticking with that, Chris, what do those fossil records tell us about the first forest? The first forests I look at as occurring over the next period of about 10 million years. So we find probably four really good examples of what these very primitive forests were like.

9:37If we look at the first one that was known about the famous Gilboa fossil forest in upstate New York. We find these cladozalopsid trees like calumophyton, except by 10 million years later, they're actually about a metre in diameter at the base, maybe 10, 12 metres tall. So really substantial trees. Yep, we find in the sandstone quarry in upstate New York where this forest occurs, meandering between those tree bases, we find a woody trunk, but which is lying along the ground. It's a new type of plant called a aneurophyte. Very simple leaf-like twigs, again, on little branches. If we go back maybe two million years before that, go down the hills out of the Catskill Mountains down into the Hudson Valley, there's another place called Cairo, and there's another sandstone quarry there.

10:29On the base of that quarry, there are huge great rooting systems, and that is the base of another woody tree. but this type of tree has learnt to grow upright. And you've been to the four major sites of the earliest fossilised trees that we know. Where are the other two? The other one, which is of a similar age to the two I've just described, is in Spitsbergen, about nearly 80 degrees north. Though when that forest was alive, it was on the equator. And there we see lycopod trees growing, which is a third type of tree. And that tree has a little bulbous base and then a trunk, which is just a column, which would have been covered in little leaves.

11:13And when it got to a certain height, about three metres, it would then branch, start dividing into two and two and two and two and form probably an upright crown at the top of it. These trees are still extant today in the form of little herbaceous plants. And then the last and the oldest fossil forest was found by a couple of Cambridge sedimentologists, Neil Davis and Will McMahon, down on the coast of Somerset. And they sent me some pictures of some trees and they look a bit like a tyre mark from a bicycle going across the sandstone. And I recognised immediately the pattern of branches that are found in Callumophyton, which is the oldest tree.

11:55It's from about 390 million years. Thanks very much, Chris. Bill Baker, from physical trees, early physical trees to trees as a metaphor, you've called the tree of life biology's periodic table. Can you explain what you mean by that? Well, metaphorical, the idea of the metaphorical tree of life has been around for thousands of years in religious, mythological, cultural context. Think of the tree of life in the Garden of Eden or this great tree, Yggdrasil, that connected the nine realms of Norse mythology. But what I'm talking about today is the evolutionary tree of life, so the genealogy of everything that lives and has lived.

12:45And I liken it to the periodic table because the tree of life helps us understand the properties of living things, just like the periodic table helps us understand the relationships and properties of the chemical elements. so if you are looking to find your way through the complexity of biodiversity the tree of life is like a roadmap it kind of cuts your workload down because you know you don't have to if you're interested for example in a group of plants that might produce chemicals that could have medicinal value and you want to explore those plants for their medicinal properties you don't have to look at every plant you can use the tree of life and and what we call its predictive power to direct your next sampling foray.

13:36So it's really central. It's the foundation of biology in many respects and indeed the kinds of things that Jenny and Chris have been talking about are all underpinned by concepts of the tree of life. And you've been involved in the building of a complex tree of life at Kew. Can you explain to us how you go about doing that and what does it tell us specifically about the evolution of trees? In a perfect world, we'd be able to reconstruct the tree of life from what we see written in the rocks. The fossil record would tell us how everything is connected. But as Chris has just described very clearly, actually the fossil record is very incomplete and it's difficult to piece together.

14:19But there is another kind of fossil record. there's the molecular fossil record which is effectively the changes in dna that accumulate over time all of us ourselves have dna that is what encodes our properties and functions and over time dna mutates now if you can pull together dna code dna sequence as we call it from different species but for the same parts of the genome and line those up you're able to run it through a complex algorithm and infer a tree or recover a tree. We often talk about recovering as if it's a form of excavation almost. We know that there is a tree out there but that we have to somehow get to it through our molecular genetic shenanigans.

15:09I mean building the tree of life is a huge global endeavour. There are many many researchers involved in this and at Kew it's been our bread and butter for decades. For the last decade I've led a programme called the Plant and fungal trees program at Kew. And we have recently, it sounds rather glib, we have recently completed the flowering plant tree of life, by which I mean we have sequenced one representative of every genus of flowering plant. Now, the flowering plants make up 90 % of the plants that grow on Earth today. Chris has not talked about flowering plants yet. He's talked about things that come long before flowering plants, but there are 350 ,000 of them.

15:47we've sequenced about 90 % of the genera that represents about 98 % of the total diversity and we've done that using Q's amazing collections and really novel genomic methods and I think the thing we're most proud about is that everything that we have done is in the public domain and free for other people to use and it's been a really wonderful springboard for other people's research and are trees included in that because some of them are flowering and some of them aren't yes so there are many tens of thousands of the 350 ,000 species of flowering plants are trees. And what our angiosperm tree of life shows us is that...

16:28Angiosperm? Sorry, flowering plant. Angiosperm is a synonym for flowering plants, plants that produce flowers and produce true fruit. And so our flowering plant tree of life shows us very clearly that trees are not a group, that they have originated repeatedly across this enormous group. We see a wonderful pattern in the way the flowering plants diversified very early on in their origin, about 150 million years ago, there was this enormous surge in diversification. All the major lineages, what we call the orders of flowering plants appearing, but we don't think that they were trees at that time.

17:08The first groups that popped off were flimsy shrubs and climbers, or the water lilies, for example, they can't even make wood in their aquatics. There's a lack of clarity about exactly what the sequence was of diversification, but we know that trees were probably not there in the earliest stages, but by the end of the Cretaceous, they were very, very much present. OK, so as we've established, they've arrived around 380, 390 million years ago. Jenny McElwain, how did they start to change the environment? Yeah, so I think we mostly think of trees. We use the term ecosystem engineer. So they just build this structural complexity.

17:55So if you imagine you've got 20 herbs, you know, they're not trees, they're small in a space and now swap that with 20 trees. So the trees will increase the volume of complexity. So as soon as you have trees, you begin to have layers of habitats. So let's say you go hiking on a hot day. It's really hot. It's really drying. But you walk into a cool forest, you can feel the moisture increase, you can feel the shade and the cool. so what trees do in a space is they create a microclimus they they they change the light so you now have understory plants you have middle story plants and you have canopy plants they change the humidity they change the complexity of how everything interacts and they and this is just above ground below ground they're doing the same so they're extracting elements from rocks, building soil, building relationships with fungi.

18:54So per unit space, trees add complexity and they engineer space and climates and resources for other plants to come in and other animals and fungi, other groups to come in. So very fundamental really in terms of how life evolves. The ultimate cookout starts with the ultimate ingredients. At Whole Foods Market, no antibiotics ever, burgers and kebabs are prepped and ready to throw on the grill. Fire up a juicy ribeye, grab creamy potato salad and savory flatbreads from the prepared foods department and round it all out with 365 brand condiments, chips and dips at everyday low prices. Whole Foods Market, make your summer sizzle.

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21:02I feel like the American dream is alive but not well. From the BBC, it's the United States at 250. Listen on bbc.com or wherever you get your podcasts. Chris, are trees from 390 million years ago recognisable to us? Or are today's trees very, very different from what first emerged? And if so, where do those differences lie? So if we look at the Archaeopterus tree, the one with the big roots that we saw at Cairo in the forest, this would be a recognizable form to us because it's got big sturdy roots it's got a woody trunk with some sort of bark on the outside of it it may have branches but it definitely has leafy frond-like branches attached to it so it would have nice green leaves and so it'll probably reach a height of maybe at that time 20 meters or so so does it look like something like a pine tree or yes to it yes to a certain extent that would be the basic thing that you might recognize a sort of leafy pine tree if you could imagine such a thing the lycopods um such as we saw in spitzbergen were quite different i think to anything that we really see today because they would grow up as a leafy trunk from the ground and they would grow up to um at this time three meters maybe and then start branching and have this branching crown of leafy smaller branches at the top which would then terminate in a cone that would be where the reproductive part was and i don't really know of anything that looks particularly like that today there's some cactuses and things which maybe do a little something a little bit similar but the the cladizylopsids are really fairly unique because the way that they support themselves is with this very much divided woody structure around a hollow interior and it's a bit like the Eiffel Tower inside the plant holding it up and then with these monkey arm branches on the side of them which are then dropping off as the apex grows upwards it's a form that we might recognize from a palm I'm sure Bill might talk about this or a tree fern but without actually having any leaves.

23:17So a quite unusual type of structure, but quite distinctive. So when do things like oaks and ashes and birches that we know today, when do they start evolving or do we not know? This will not be till probably about 200, 250 million years later. So these early Devonian forests are really quite distinct. And one thing you should perhaps recognise is there's no seeds there, there's no flowers there's almost no insects maybe just millipedes no birds very low diversity not this tropical ecosystem that we think of today as being a tropical forest this is really something very distinctive and without the level of interactions that jenny's talked about perhaps apart from the fungi in the soil which might be quite interesting well and you mentioned tropical forests rain Rainforests.

24:09Bill, you've done quite a lot of work on palms. What can you tell us about palms and rainforests? Rainforests obviously are iconic places for trees. They're also really important for biodiversity. We think maybe half of all plant species occur in rainforests, which take up just 7 % of the world's land surface area. So they're really important. And that idea begs the question, well, how on earth did that come about? So the palms provide a fruitful line of inquiry on this subject because palms are found all over the tropics. They're widespread, 2 ,500 species, but they are mechanically kind of bound to the tropics.

24:53They can't cope with freezing on the whole. There are a few exceptions. Don't write in. on the whole palms can't cope with freezing because freezing causes catastrophic embolism of their plumbing basically you get a bubble and it can't function anymore so this makes them what we call niche conserved they are bound to a certain environment and as you can tell already i love building the tree of life and i my early days were spent frenetically building the palm tree of life and we have used that tree to try to see what that might tell us about the rainforest by combining it with fossils to turn this branching diagram actually into a proper chronology if you like so that the individual splits in this tree fit to a time we've related onto that tree the environments in which the modern species occur and reconstructed what the original the ancestral condition might be and that tree told us that an analysis published in 2011 told us that palms appear to have evolved in the rainforest 100 million years ago somewhere in the northern hemisphere this caused a bit of a drama because in fact from a paleo perspective the fossil evidence for rainforest doesn't exist at that time the earliest fossil assemblages that look like They characterise a rainforest, don't come till after the meteor impact and the extinction of dinosaurs around 60, 65 million years ago.

26:24So what's happening there if the evidence is so contradictory? Well, we, of course, what is science for if not for revisiting later? So the update to all of this is that we have managed to build an even bigger tree of life for palms with 80 % of the 2 ,500 species in it now. that gives us a more nuanced insight that shows us that palms are even older, perhaps 120 million years, but that it appears now that they evolved in a non-rainforest environment but quickly moved into them around 60 million years. So by doing more work, we finally get a close match that I hope will cause less controversy among my paleo friends.

27:07Thanks for that clarification, Bill. Jenny, we heard from Bill that palms exist in rainforests, or I think it was you, that they can't exist in different temperatures. So what makes some trees resilient and other trees not resilient? Yeah, that's such a great question. And I actually think nobody can answer it, yes. So I think this is really an active area of research for plant scientists in all disciplines. and we really need to know which trees are going to be resilient in the face of climate change in the future. And there's lots of ways of answering those questions. You can come from lots of different disciplines and answer in different ways but I suppose thinking of my own research area, we know from looking back at the fossil record at times of natural global warming in the past, so this would be caused by volcanism, that if you push the climate too far and too warm, we know that trees have limits.

28:10So you push the climate beyond their niche or their adapted envelope and they suddenly disappear. And we don't see them as fossils anymore. And instead we see the really ancient plants thrive like ferns and those mosses that we talked about in the early phase of the earth. So what we do know is that trees, all trees have limits. We see this if you go to the Arctic today or the Antarctic. There are no trees. It's a treeless landscape. It's because it's too cold. And then you see it in a desert. In the most extreme deserts, it's too hot. So I kind of think we can't be complacent. We know trees, we love trees, but we absolutely can't be complacent.

28:54and we've set up a project in Trinity College Botanic Garden called the Witness Tree Project and the idea is that all trees are censors and they're witnesses of climate change and because they're long-lived organisms, many live for thousands of years, most live for hundreds of years or certainly decades, we are tracking 21 trees and it was very difficult to choose which ones, 21 trees every year over time to answer the question you posed which trees are most resilient and we're trying to understand it's called a common garden experiment because we have trees from all over the world growing in the same botanic garden so we have trees from the subtropics trees from the from cold environments through trees from very warm environments and we're trying to understand fundamentally what is it about their traits that enables some to be resilient and others to not withstand a changing climate.

29:57So Chris Berry, in that case, presumably even resilient trees would find it difficult to adapt year on year. So you're looking at tree resilience over millions of years, hundreds of millions of years. Are you able to say, ah, yes, this tree faded away but became an oak tree or something like that? Or do you see hard cut-off points? One of the interesting things about how trees grow is where the growth occurs, where the primary growth occurs. And for my very old trees, this tends to be at the actual apex of the trunk. So at the apex of the trunk, there's a thing called a meristem, which is producing new cells which become part of the trunk.

30:47And in these ancient types of trees, then pretty much the whole body plan of the tree was organised from there. So it would give instructions as to where the branches would grow when they grow in geometric patterns coming back from the meristem. Problem with this is that that's extremely vulnerable because if you take out the meristem, if you have frost or something, of this big bulbous thing on the top of the trunk, then it can easily be destroyed. And it's the same for the lycopod trees as well. I think archaeopterus, the big, tall, woody tree that we see at Cairo in upstate New York, is a really interesting thing because with my PhD student, Amy Wyatt, we've been in Spitsburg and collecting what we think are trunks of these archaeopterus trees.

31:33And up to a certain size, they seem to have a very geometric insertion of the leafy branches that they had, which seems to fit in with this model. It's all coming from the apex. But work by American, particularly French colleagues, looking at the anatomy of some of these trees shows that there's a new type of branching which can develop in Archaeopteryx. And that's to say that it starts growing new branches from essentially the surface of the wood. So it can produce a new branch anywhere on the trunk and that can grow into a big branch, a long live branch with fronds on it itself or even into a new trunk.