Rotting Hearts Are Beautiful

L: I did my first degree in biology and mathematical statistics. At the time the head of the department at Exeter was Professor John Webster – a very eminent mycologist – and he taught some of the courses I took on soil ecology, and so I started to be introduced to fungi there. I was immediately intrigued by nematode trapping fungi and mycorrhizal fungi which he talked a lot about.

Then I went to Queen Mary and worked on the ecology of wood decomposition on the forest floor. Of course the main things which decay wood are fungi. I knew that I really needed to know about the organisms as well as this overall picture of decay, so I went to Bath and we applied for a grant to work on decay in the canopy. All this wood that’s landed on the forest floor is half decayed already; clearly the place to start is in the canopy, not on the floor. So I got this grant and I learned lots about fungal communities. Then I went to Cardiff where I’ve been ever since. I’ve always had a few other little things, but most of my work has been on fungi. There’s always something exciting to discover, so it’s a very exciting life and I feel blessed that I’ve had such opportunities.

L: Part of this is that we know so little. They’ve been considered in many ways a poor relation to other kingdoms, but they are major drivers of our terrestrial ecosystems. To have such limited knowledge is absolutely ludicrous really. We need to know very much more, because they’re crucial to the nutrient cycle and feeding the plants so without them our planet would not work. And yet few people realise that.

L: Telling people about fungi is one of the great joys of life because they are so fascinating, and so I teach at all levels – but I’m also exceptionally keen to pass on information to a wider audience too be it schoolchildren, adults, professionals who encounter fungi in their day to lives such as arboriculturists. I think it’s really imperative that we pass on the knowledge we have.

During lockdown I was asked by Dorling Kindersley to produce a book called ‘The Humongous Fungus’ and I really leapt at that chance, because you know, that’s the right age! Start children off when they’re young – and get their parents who are helping them read at the same time!

L: They’re awesome books, and I’m delighted to have been asked to do that for that age group and I hope that it has a big impact. In some ways I think it’s one the most important books – if not the most important book – I’ve been involved with.

In Britain we tend to be mycophobic, and that’s not a good thing. We shouldn’t be frightened of fungi. We should be wary – obviously – you shouldn’t go out and eat just anything. We also need to have a healthy respect for them because they do rot our food and our homes and kill our crops, and we need to understand this from a self-interest point of view. But we have to pass on the information that without them we would not be here. Indeed, we need to be fungi-centric and to do things to ensure that fungi are maintained in our ecosystems and can do the jobs that they’ve evolved to do.

L: Yes, I was having a similar conversation with an artist friend of mine last week actually, you’re absolutely right we think of death as the end, as a terrible thing, but actually without death there would be no life, and I think fungi really bring this to the fore in their roles as decomposers and the fact that they live on dead material. In death there is life – it’s just a circular process that has gone on for millions of years and will go on for millions more years. It’s a cycle, in death there is life. Or, without death there would not be life.

L: Climate change is having a huge impact on fungi, and that was revealed initially by our early publication in 2007 with Alan Gange, where we analysed his father’s dataset, collected over fifty years from an area in southern Britain. We showed some important things, namely that the length of the autumn fungal fruiting season was changing, on average it was starting earlier and on average it was finishing later meaning that nowadays the autumn fruiting season in southern Britain is about seventy-six days, double what it was even thirty or forty years ago.

That’s telling us that the ecology of the fungi is changing. We shouldn’t think that everything is behaving the same; mycorrhizal fungi are behaving differently than decomposer fungi, mycorrhizal fungi attached to deciduous trees are behaving differently than those attached to coniferous trees, but mycorrhizal fungi are fruiting much later now, presumably because now their trees are photosynthesising for much longer later on in the year and sending photosynthate out to the roots so those fungi are not getting the cue telling them to start or stop fruiting.

In the decomposers we’re now finding an increase in the number of species that fruit in the spring as well as that fruit in the autumn. Before fungi can make their fruit bodies they’ve got to have obtained enough nutrients to make those fruiting bodies, so the hint is that decomposition is increasing now in the winter when it used not to be.

Obviously with a warming world enzyme activity increases, and greater enzyme activity – a speeding up of the physiology of a number of organisms – means that decay is increasing. And that’s ok so long as the plant material which is being decayed is photosynthesising, making new material, at a similar rate. But if they’re making new material at a slower rate then that is a problem, because it means that fungi are decomposing things faster than new dead material is being produced and so more CO2 is effectively going into the atmosphere than is being recaptured and that exacerbates the greenhouse effect etc. etc. So it’s a question of whether things are remaining balanced or not.

We have looked at the effect of decay on things like fungal communities as well. The wood decay fungi I work on – their communities are formed largely and changed largely by the interactions between fungi. The outcomes of these interactions change depending on the amount of water there is available, on the temperature etc, so the communities in wood and other decomposing tissues will change because of climate change.

How that will affect decomposition we don’t really know, because of course different fungi decay at different rates so if you change the communities you can’t predict very easily what those subsequent behaviours will be. We’re also seeing perhaps that fungi are changing the plants with which they’re associated, presumably because of climate change, and we’re seeing this with mycorrhizal fungi too.

L: Yes, so you’re absolutely right. We have done a lot of work, and indeed are doing a lot on conservation. Both of individual species of fungi, but also of habitat in general. So at the moment, also probably the last five, six, seven years, we’ve been looking at the decaying centres of standing trees. Now, people might instinctively say oh my goodness, that fungus is rotting the heart of the tree; look at that hollow oak; isn’t that terrible what the fungi have done to it? No, it is not terrible. This is an important part of the life of a tree.

The colonisation of these inner tissues by fungi and the hollowing they cause are important because they recycle nutrients from the centre of trees, which would otherwise have been locked up there. They release them so that those trees can actually get hold of their own nutrients.

It’s also an important habitat for fungi, including threatened fungi, such as the rare oak polypore, Piptoporus quercinus, which is very rare worldwide. It’s also important for vertebrates. If we think worldwide, there are at least a thousand species of vertebrates, particularly birds and mammals, but also amphibians and reptiles, that are dependent on these hollow centres of trees. In the UK alone it’s something like 1600 species.

You only get heart-rot really in veteran trees, that is, hundreds of years old. But there’s a conservation problem, as far as these ancient and veteran trees are concerned. People are starting to plant loads of trees, which – hurrah. So we’ve got lots of new trees, and we’ve got these veterans and ancients at the other end of the spectrum, but what we don’t have, because of the way we’ve managed fires, chopped them down, used them for firewood, used them in wars, all these sorts of things – we don’t have the intermediate age groups, not very many. That causes a problem because when these very old trees eventually disappear, there isn’t a close generation coming up to replace them. So all those organisms which are dependent on that habitat – what are they going to do?

One of the things we’ve recently been doing is trying to basically veteranize younger trees, to get that habitat developing sooner than it otherwise would. This sort of thing has been going on in Scandinavia, and also some sites in Britain for maybe 20 years. People have tried to veteranize trees by maybe doing a big bit of damage, perhaps at the base of the wood, or hitting it with a sledgehammer, all sorts of horrendous things – but it’s to sort of create natural features.

There’s some evidence of some success; some of these fea – tures are beginning to cause hollowing inside, and to make cavities for nesting birds. But we’ve adopted a slightly different approach which we’re now trialling, and that’s to make heart rot, not in this rather haphazard sort of way, but to direct it; to get exactly the sort of fungi which we want to be causing that heart rot directly into the tree.

Lots of organisms are dependent on certain specific patterns or sequences of fungal decay, so we have a big experiment on site at Windsor, where we’ve cut holes into younger trees, and inside those holes we’ve put blocks of wood colonised by an appropriate fungus, to try and see if that fungus can establish in the heart and can start the decay. That’s only been going on for four years and so we’re just beginning now to find out whether we’ve had any success. We drill some wood out, and we place it onto agar, and see what fungi grow, and we can also extract DNA directly. We have got some colonisation, we’re delighted to say. But of course, this is something that won’t reveal itself really for another twenty, thirty, or maybe fifty years, when I’ve been and gone, and I’m being recycled myself. But we have made a start in that direction.

I ought to actually give a cautious word at this point, because I would not want people to go around willy-nilly thinking, oh yes, we can help, we can inoculate trees, to help with this conserva – tion. You have to be extremely careful about what you inoculate, where you inoculate it. So, for example, it’s really essential that the fungus you inoculate is something that’s threatened. It has to be a threatened species; it must be a British species. The isolate, the individual that you inoculate into the tree, must have come from a fairly local source, and you must be sure that it will increase the population, all these sorts of things.

I’m emphasising it must come from a local source, because of course on the internet, you can buy fungal inoculate, often for wood decay fungi, that you want to eat – for example, Hericium erinaceus, the bearded tooth, which is a threatened species. You can cultivate it very easily. It grows easily in the lab. But it would be a dreadful mistake to inoculate an isolate that you just bought from the internet. If you don’t know where it’s from, the chances are it might originate from outside of Britain. It certainly wouldn’t be local to the site where you’re inoculating, and you could do more harm than good by inoculating trees with fungi with unknown provenance. So people really must not dive headlong in, that would be a terrible mistake.

L: Yes, and you’re so right. I mean, even things like trees. We really should be sourcing our tree seed locally and growing it locally, not sending it out hundreds of miles away to grow into plants, and then bring them back to where you want to plant them, because of course, you can still be bringing back the wrong thing – or a tree adapted for the wrong environment.

L: I think it probably was brought into the country, but likely would have got here anyway. And we’re only 20 odd miles away from continental Europe, aren’t we? But I think the evidence suggests that it was brought into the country originally.

L: On the positive side, it looks like there might be genotypes of trees which are immune or can fight their way back. So one hopes that ash is not going to disappear completely from our landscape. We’re not going to see the same wonderful ash landscape again, but hopefully, all is not lost. I can report that the ash tree that’s in my garden is still okay. We are surrounded by considerable ash dieback, but I keep a very close eye on it. And there are things we can do as individuals who have got a few ash trees of our own.

So, for example, when the disease gets into the leaves and into the stalks they become coated in fungal material all around the outside and they look black. Now obviously, you can’t do this over a forest, and normally I would say you absolutely do not remove leaf litter. But if I find any under my tree that are colonised by Hymenoscyphus, the ash dieback pathogen I shall be removing them before they fruit and release ascospores, in the hope that my tree can survive. But of course, it’s surrounded by all these other dying ash trees and ascospores will be arriving.

L: I think so, we have to try.

L: So most of my current work is on the heart rot that I’ve told you about, and also on ash dieback. But one of my huge interests from the past which hasn’t gone away, but which I have done rather less recently is on cord forming fungi. They are wood decay fungi, but with most wood decay fungi you’ve got some logs or twigs on the forest floor, and all the fungi stay in there doing their decaying until they make fruit bodies and then they spread spores.

But there are some fungi which can actually grow out into the soil. Rather than hoping that their spores will just land on a suitable new bit of dead stuff, they can grow out and search, and find new dead stuff. And then when they find a new woody resource, they send messages back through their mycelium. Now this mycelium you can actually see – individual hyphae come together and form cords that look almost like roots except that typically they’re white.

We get these networks all over the forest floor, so I’ve done a lot of work looking at how these fungi behave when they find new resources. One of the rather fun things which we’ve done in recent years is to look at memory in fungi. A Japanese researcher, Yu Fukasawa, came over and spent two years in my lab a few years ago, and he was interested in looking at memory.

So we set up some experiments. We had trays of soil straight from the woodland, and on it we put a block of wood that was colonised by one of these cord forming fungi, let it grow out, then gave it another block of wood, a fresh resource, not colonised by anything, it of course started to colonise that wood resource. Then we cut the mycelium off the original one, took the wood block away, and put it on a fresh tray of soil.

We were asking the question, does that fungus ‘know’ in which direction that second resource that it had colonised was. In other words, will more mycelium grow out from the same side that originally ‘found’ the new block fastest, and sure enough it did. That shows that fungi have some sort of memory.

When you say that people immediately think ‘oh, fungi – it’s a brain’. It’s not a brain in the sense that we have a brain, but it’s certainly a mechanism for sending messages, and in some sort of ways for ‘remembering’. We shouldn’t imagine that it’s exactly like our memory, because it’s certainly not, but there are analogous points and we should think about what that means. So that’s been a very interesting thing lately.

L: Yes. And on that, one of the things we can do as individuals which can really help, is to not take the wood away that falls to the ground, we need to let it decompose there. If you walk around our woodlands, most of them, you’ll hardly see any wood on the forest floor. If they’re managed for timber, then it will have been taken away or otherwise people will have come in and used it for firewood or what have you. We’re taking away all these resources which could be used by other organisms; fungi and invertebrates – and vertebrates as well.

If you look at our forest floors, they are very depauperate in wood. If you’re lucky enough to go to places which are tiny relics of something close to the primeval forest – and there’s not many of those in the country now – you’ll see it’s a totally different situation. There are masses of downed trees, so to walk a hundred yards could take you half an hour. So a really good conservation thing is not to remove fallen wood. In a broader sense that’s something which we can all do: not put ourselves above nature and think oh yes, let’s tidy this up, that’s untidy, etc.

L: Most people are, like me, perhaps inherently lazy. Let’s use laziness to our advantage here.