Dear Readers, this week’s talk was by David Humphries, Tree Management Officer for the City of London. He has been based in Hampstead Heath for 35 years, and recently won a special award for caring for London’s trees. I was really looking forward to this talk, and I wasn’t the only, as for the first time since the LNHS talks started, this one was sold out! Fortunately, you can still watch the whole thing here, and I’d recommend that you do so, as the photos were fantastic, and I can only capture the merest flavour of the range of the talk.

Humphries is something of a fungiphile: he gave us a quick look at his computer, where he has 22,000 photos of fungi, neatly arranged into 584 folders, one for each species. Most of them were taken on Hampstead Heath, which has over 25,000 trees, and where upwards of 600 fungal species have been recorded. Humphries thinks this is probably because, unlike in 1830 when John Constable painted a view of the Heath that shows it completely bereft of trees, there are now a substantial number of habitats and tree species.

First, we had a quick run through the variety of fungi that can be found in association with trees. There are the perennial bracket fungi such as hoof fungus (Fomes fomentarius) which persist for years. They form layers, as you can see from the photo below, but these are not necessarily annual – each layer is created when the fungus produces spores, and in one example that Humphries showed us later in the talk, it’s clear that they can be produced on multiple occasions in a single year if the conditions are right.

Incidentally, Otzi the iceman who was retrieved from a glacier in Austria and turned out to be about 5000 years old had some pieces of hoof fungus in his bag – it is used to produce amadou, which can be used as tinder. But as usual I digress.

Then there are the annual bracket fungi, such as shaggy bracket (Inonotus hispidus) which produce fruiting bodies and spores and then die every year. They may remain in the same location for many years, and on the photos that Humphries shared you could see the scars of the previous generations on the bark.

However, with so many types of fungi, many looking superficially the same, how to ID them to species level? For some, you have to use microscopy of the spores, but Humphries had some general tips:

• Take a slice through the fungus to look at the spore layer and the flesh
• Have a look at the spore colour – anything from white to saffron to darkest inky black
• Look in detail at the spore layer to see how the tubes from which the spores are released are coloured and shaped – Humphries recommended two useful resources:
  • The Great Big Index of Fungi Species by Spore Colour (from First Nature)
  • Leif Goodwin’s Keys on Spore Colour and Other ID Keys
• If you are looking at a more typical ‘mushroom’, look at the gills and check to see whether they are attached to the stem or not (the word for where gills do form part of the stem is ‘decurrent’, a new word for me!)

Then, we moved on to the three ways in which fungi can be associated with trees.

• Parasitic – it was Humphries view that parasitic fungi start to become problematic when a tree is weakened, either. A typical example would be honey fungus (Armillaria mellea)

• Saprophytic – fungi that feed on fallen leaves, dead branches etc. They recycle nutrients that would otherwise not be released back into the soil. The earthstar that I found in St Pancras and Islington Cemetery would be an example.

• And finally, there are the Mycorrhizal fungi. It’s only recently that we’ve learned what a vital part these fungi play in the health of plants – they form a mutualistic relationship with the roots of trees in this case, vastly extending the range of the roots in return for some of the benefits of photosynthesis. Some very familiar fungi, such as the edible boletus mushrooms and the traditional ‘toadstool’, Amanita muscari, are examples of mycorrhizal fungi. The fruiting bodies can often be seen exactly following the lines of the roots of the trees that are hosting them.

Humphries has, as you might expect, found some very interesting fungi in Hampstead, and one of the most attractive is the Many-Zoned Rosette (Podoscypha multizonata), of which the UK has about 80% of the European population. This is a rare species, which is being assessed by the IUCN for the Global Fungal Red List, and one reason for its rarity is that it is normally found on veteran oaks in oak pasture, a vanishingly rare habitat in the UK (though as I’m currently reading in Isabella Tree’s ‘Wilding’, it was probably once much more common. However, Humphries has noticed that the fungus has increased its range of hosts to include beech, hornbeam, lime, red and turkey oak and even horse chestnut, so maybe this bodes well for its future.

As you might expect from someone who is involved in maintaining the health of trees, Humphries has a lot of interesting things to say about the different ways that fungi can infiltrate a tree. There are broadly three colonisation strategies.

The first is fungal-induced dysfunction, as favoured by our old friend honey fungus. Basically, rhizomorphs, which are a ‘rope’ of hyphae (the filaments of the fungi) travel through the soil and colonize a tree which already weakened. Once they’ve found such a tree, they fan out under the bark and infiltrate the vascular system, preventing the tree from transporting water and nutrients. In honey fungus the rhizomorphs are often called ‘bootlaces’ and you can see why.

Secondly, some fungi infiltrate the sapwood when it’s suddenly exposed, whether by storm damage, lightning, injudicious pruning, or, in the case of the poor tree on my road, sudden collision with a skip. Examples include the beefsteak fungus, which at least has the benefit of being edible.

And finally, there are the fungi that are living in the tree already, but which can only proliferate when the tree is weakened (endophytic fungi). These remind me a bit of the bacteria that live happily on our skin for ages, until our immune systems take a knock and then they lurch into action (Staphylococcus springs to mind). An insect attack, storm damage, root rock in high winds can all be starting points for such fungi (one example would be the birch polyphore (Fomitopsis betulinus). Humphries noted how, when a tree is cut down, these fungi can appear remarkably quickly once the sapwood is exposed to the air.

Trees can live quite happily with fungal infestations, sometimes for decades. However, many fungi will eventually cause problems. Some cause white rot, which is where the wood turns white and spongy because the fungus has ‘eaten’ the lignin which provides stability – this is what honey fungus does. Some cause brown rot, which is where the cellulose is ‘eaten’ instead, and the tree becomes brittle – an example of this would be chicken of the woods (Laetiporus sulphureus). Some trees will eventually be hosts to both. And it isn’t just trees in forests, either.

Humphries mentioned two fungal diseases that are affecting that icon of the capital, the London Plane (Platanus x hispanica). One is Massaria Disease, caused by the fungus Splanchnonema platani. Humphries is of the opinion that this used to largely appear during droughts, but as most street trees have roots that are compacted, and as climate change affects rainfall in unpredictable ways, it has been seen in the UK. It normally causes branch fall in trees over 40 years old.

The second is elbowpatch crust (Fomitiporia punctata). According to the Forest Research UK site, this seems to affect a particular clone of the London Plane which has a propensity to develop weak forks. When infected by the fungus, it can drop whole branches, which is something of a health hazard considering how many there are.

Humphries spent some time explaining how part of his work is assessing trees, and deciding whether or not to save them, and how. There are various techniques that can be used to assess the amount of damage – a microdrill can be used to take a core through the tree without harming it, to see how far any rot has progressed. The whole tree can also be fitted with what sonic tomography receivers, which used sound waves to detect the integrity of the trunk – the photo of the tree in Humphries’s photo makes it look rather as if it’s getting an ECG. And there is much that often can be done, in terms of reducing the wind load that the plant has to bear in storms to prevent it being knocked over, and to support the tree. However, when the worst comes to the worst, the standing wood is endlessly useful for everything from beetles to woodpeckers, and fungi themselves are food for many invertebrates and other creatures (I’ve even watched a fox take a speculative bite out of a puffball.

However, the lockdowns and the increased footfall in Hampstead have caused additional challenges for fungi, and for the people who care about them. The big enemy seems to be compaction of the soil – no one seems to know how much this will damage the underground hyphae of the mycorrhizal fungi, without which many of the trees on the Heath will no longer thrive. Soil health is an issue for all of us, wherever we are, and it’s something to which we pay far too little attention in my view. I worry about the trampling in my local wood, but am also uncertain what we can do about it.

I really recommend this talk. It was stuffed full of information, and some of the photos that Humphries presented were wonderful. I learned so much, and I think I’ll probably watch it again to pick up some of the things that I missed or didn’t understand the first time round. So if you have an hour to spare and are wondering what to do during lockdown, here’s something to keep you entertained (along with all the other LNHS talks). The amazing world of fungi awaits!

Photo Credits

Photo One By George Chernilevsky – Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=10577678

Photo Two by Stu’s Images, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

Photo Three by Stu’s Images, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

Photo Four by Amanita_muscaria_3_vliegenzwammen_op_rij.jpg: Onderwijsgekderivative work: Ak ccm, CC BY-SA 3.0 NL <https://creativecommons.org/licenses/by-sa/3.0/nl/deed.en>, via Wikimedia Commons

Photo Five by Lukas from London, England, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons

Photo Six by Ericsteinert, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons

Photo Seven by Dan Molter, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

Photo Eight by Bernie Paquette, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

Photo Nine by Gargoyle888., CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons