A decade on from her first trip around Hampstead Heath with David Humphries, Carolyne Locher returns to find out how the park’s fungi population has diversified since her last visit and the challenges it poses to arborists. 

IT’S a chilly, white-bright Monday in late November, beneath still-green oaks and green-orange beeches on Sandy Heath at the northern edge of 320 ha Hampstead Heath. A damp, fallen Horse Chestnut left to dismantle naturally supports a multitude of life forms. Among these are horizontal bullets of perennial ‘King Alfred’s Cakes’ (a so-so firelighter), an orange ‘Pin mould’ (saprotroph) and a white ‘Slime mould’ in plasmodial phase feeding on non-woody detritus collecting in the bark ridges. This fallen tree really is a life raft afloat in an ocean of squelchy leaf litter.

“It shows the importance of managing deadwood on site for the saproxylic [fungi] species associated with it,” says David Humphries, City of London’s tree management officer, who has generously offered to guide this autumnal walk. In arboricultural terms, saproxylic is a keystone species in forest ecosystems that colonises and decomposes dead wood. “Fungi and bacteria help to break down the wood. Insects, beetles and wood wasps then utilise that habitat.” 

David’s career began with a horticultural apprenticeship with City of London (CoL) in 1985. His subsequent move into arboriculture, and specific interest in fungi, stems from witnessing how fungi and moulds interacted with fallen trees following the 1987 and 1990 storms. Seemingly working together, this ‘cooperation’ is one of the things he liked most. 

READ MORE: Storm Arwen: Langley Arb's Nick Dearden on clearing Aberdeenshire's villages

“Then, we only really knew about ‘Ganoderma fungal brackets’ and ‘Honey fungus’. I saw fruiting bodies that I could not describe to tree surgeons. Ecology books and Leif Ryvarden’s European Polypores Vol. 1 and 2 were my starting point for understanding the diversity and the range of fungi that we have and a local field mycologist, Andy Overall, became an unofficial fungi mentor.”

Now an (amateur) authority in his own right, David, known as ‘tree_myco_man’ on Instagram, has also co-authored the Arboricultural Association’s Fungi on Trees: A Photographic Reference (2021). 

“We are seeing extreme wet and dry periods, seemingly moving to a two-season scenario. Fungi are adapting, some moving to different tree hosts and new species are coming in from warmer climates. There are approximately two million species of fungi globally, with 30,000 identified in the UK. Species new to science are identified every year.”

After walking for barely 20 minutes, much of this curious new kingdom is already revealed. A ‘Ganoderma lucidum’ polypore, used in oriental medicine as a supplement to boost the immune system. Fairy rings of capped, gilled ‘Clouded funnel’ are not really poisonous to humans, also not really edible. It is the same for the not-so-tasty ‘Common yellow russula’, one of the less-prized brittlegills. The underside of a red-brown edge-curled fallen leaf reveals milky-white strands of mycelium radiating from a nibbled ‘Amethyst deceiver’ (whose gills are home to a miniscule spider) and its cousin, the ‘Orange laccaria’. David is less sure about one species being munched by a slug. He has never seen ‘Jelly bean’ fungi here before, having only heard about it last week.

Forestry Journal:

It is just over ten years since we last stood here at Pitt’s Arch Beech looking at the engineering solution that CoL had implemented – two metal rams – to prop up an archway, the last remnant of William Pitt the Younger’s house. Then it was at risk of being toppled by a 250-year-old beech, with a crust-like ‘Perenniporia’ bracket polypore growing above head height. The archway is still there, as is the beech and bracket.

This detour is a bonus, a stop on our way to Highgate Wood, to, hopefully, see one of four fungi species currently protected by UK law; a small, rare (Red Data Book) ‘Tiered Tooth Fungus’, living on the underside of another fallen Horse Chestnut. 

“This one is three or four days past its prime. It develops spores on long spines (teeth), which drop or get eaten by animals. To humans, it tastes like lobster.” 

A rarer ‘Lion’s mane’ (Hericium erinaceus) has been discovered growing somewhere on Hampstead Heath. It is the first recorded in Middlesex.

Neither flora nor fauna, fungi is its own kingdom and falls (for a layperson) within three categories: parasitic and pathogen fungi (eg. ‘Honey fungus’) take advantage of trees weakened by drought stress, compaction or pollution; saprotrophs/saprophytes feed on dead or decaying plant material; mutualistic/mycorrhiza fungi form symbiotic relationships with trees (trees giving sugars for photosynthesis, receiving water and nutrients from the mycorrhiza in return). 

There are two types of mycorrhiza: endophytes live within the cells of tree roots and have no fruiting body; ectophytes form external fruiting bodies, such as the vibrant red-with-white-spots ‘Fly agaric’, associated with pine and broadleaf oak, birch and beech. “Without mycorrhiza, trees would struggle to survive, particularly within the urban environment.”

Forestry Journal:

We enter Highgate Wood via Onslow Gate. Once part of the Ancient Forest of Middlesex, excavations within this 28 ha ancient wood of hornbeam coppice with oak standards, holly and wild service have revealed that kilns produced pottery here around the time of the Roman conquest (AD43). In the 18th century, the Crown took oaks for naval use.

Owned by the Corporation of London since 1885, in 1886 it was dedicated as ‘an open space forever’ and designated a ‘Site of Metropolitan Importance’ in 1990.

These woods host an array of wildlife: large numbers of birds (including the golden oriole and sparrow hawks); five species of bat (CoL has a bat handler); 180 species of moth; 80 species of spider. The fungi found here include the‘Clouded agaric’ and the ‘Blusher’.

Welcoming up to a million visitors in a normal year, facilities include a café/bar, a well-used play park and cricket pitch. The pandemic saw increasing numbers of visitors and dogs, latterly professional dog walkers, resulting in new paths being formed, causing potential issues for the soil, fungi and ground-nesting birds. “The increase in ‘Spindle shank’ could be a soil issue related to compaction and climate change. We have sent soil and fungi samples to Bangor University. We are one of seven woodland sites participating in a Tree Soils Health study.”

Managing ancient woodland in an urban setting, rather than street tree stock, is a unique role. 

“Lapsed coppice and not felling for timber brings its own issues. These trees are ageing and suffering a lack of light as the understorey grows. We cyclically manage the conservation areas (small fenced-off zones protecting tree roots and encouraging soil and woodland regeneration) and open up (halo-prune) the canopies around older trees (as seen around the beech at Sandy Heath).”

Forestry Journal: One of the ‘Orange laccarias’, the mycorhizzal cousin of the ‘Amethyst deceiver’, one of 75 species in this genus.One of the ‘Orange laccarias’, the mycorhizzal cousin of the ‘Amethyst deceiver’, one of 75 species in this genus.  

Additional works will further protect the roots of veteran trees and manage the decline of a tree growing adjacent to a footpath, which was hit by lightning two years ago.

At the café, members of CoL’s constabulary and site supervisor Declan O’Brien stop for a chat. During the first lockdown, David managed the five-strong arb team remotely from Suffolk. 

“We split into single units for inspections, coming together in multiple vehicles for safety works. All trees within falling distance of the roadside, café and playground are inspected every year and the rest of the site every five years. We were able to inspect more trees than ever before. The backlog is still being cleared.”

David suggests walking clockwise around the perimeter. From Onslow Gate, walking in parallel with southbound traffic on Muswell Hill Road, the path is hard underfoot, possibly compacted. Hornbeam leaves glow yellow against their green-black trunks and the single stems of maiden oaks. Leaves float silently to rest beside fallen deadwood and the odd forest school wigwam. Woodland access gates shut at dusk, deterring any would-be pyromaniacs or after-hours revellers setting alight the wigwam poles.

Veering off-piste early on, we are mindful of where we tread, avoiding fungi and dog pooh. A ‘Honey fungus’ energised by its consumption of a decaying hornbeam stump puts much energy into fruiting. David expects to see many more in the coming months.

Spying a small ‘Sulphur tuft’ with bright blue-green gills half-munched by a slug, he says, “Fungi are not really poisonous to the touch, but you wouldn’t want to lick your fingers after picking this one. This species used only to fruit in the autumn months. I now have records of it fruiting every month of the year. What we see now was not here a month ago and will be replaced by other species in a month’s time.”

Amidst leaf-fall alongside a dead hedge, just past an oak with a fibre buckle putting on extra wood due to the decay inside, rings of ‘Clouded funnel’ are abundant. “Without fungi,” David says, “we would be walking in deadwood 10-foot high. Saprotrophs break it down and feed it back into the soil for the roots to reabsorb.”

A standing sycamore has put on buttresses (five–10 years old) either side of a soil phytopthora burn that has grown a fruiting body crust to sporolate. The crust is being fed on by fine miasma. 

“We think it (soil phytopthora) is spreading through the sapwood. A digital microprobe revealed decay only an inch deep.”

Forestry Journal: David and Highgate Wood site supervisor Declan O’Brien.David and Highgate Wood site supervisor Declan O’Brien.

To identify a species, David often flips over the fruiting bodies (mushrooms and brackets) to see whether they have pores or gills (from which the spores drop)and by examining the way that they are attached.

A fallen cherry hosts a cluster of overlapping stalkless ‘Smoky bracket’ whose caps grow side-on. 

“Orientation happens through gravitropism (up and down, aligned with gravity). If a trunk falls slowly, they adjust their angles creating geotropic clusters.”

Whenever we approach a fallen tree, David hopes he might see an ‘Eyelash fungus’, an orange disk fringed with black hairs, or the tiny ‘Green elf cup’, a delicate turquoise disc that stains its woody host and is much appreciated by woodturners. Today is not that day.

Inhaling sharply in short, shallow bursts, he exclaims, “Rotting flesh! ‘Stinkhorn’! I just can’t see it! They start as a gelatinous egg and erupt into a phallus-shaped fruiting head with a single gleba (gelatinous form of spore) that smells of rotting flesh to attract flies.

The Romans used to eat it as a delicacy! I don’t know why.”

We do find a leathery ‘Puffball’ detached from its mycelium. When squeezed or hit by rain, the smell of ‘autumnal woodland’ is released in a grey-white cloud of spores looking to land and begin the growing cycle once again. When young, the white flesh of a ‘Giant puffball’ is (apparently) delicious. Cracking open this leathery specimen reveals an unappetising mass of spongy, grey-black ash.

Forestry Journal:  Images taken by tree_myco_man. Images taken by tree_myco_man.

The spores of the spindle-stemmed, frilly-gilled ‘Spindle shank’, growing at an oak’s base, drop to the ground from the gills to then be kicked about by animals and humans.

“The ‘Zombie fungus’ (Cordycep) attaches itself to an insect host and feeds on them until it kills them. The fungus will then grow from the insect’s head and it sporolates from there. You can get a good idea of species characteristics from the spore colours which range from white, through yellow, red, cocoa brown to black.”

Now is probably not the time to mention it, but these woods are well-used: walkers, more walkers, dogs, strollers, dogs, joggers, dogs and the odd illegal cyclist leave little opportunity to be ‘alone’.

Turning to follow the woodland’s boundary with the A1, a tasty ‘Wood blewit’ with vibrant, broken lilac gills grows beyond the playpark and cricket practice nets. Inside a fenced-off conservation area with soil as hard as concrete, brambles and assorted regeneration are taking over.

Quite animated, David kneels and clears away leaves from a beech base revealing a nibbled, warty-capped pink-tinged ‘Blusher’. A mycorrhizal member of the Amanita family, it should not to be confused with a deadly ‘Panther Cap’. 

“The Vikings saw how it affected reindeer, sending them berserk, and ate it before going into battle. It’s probably how they got a reputation for being fierce (crazy).” The leaves are replaced so that children (and dogs) are not drawn to this mushroom. When toxic species are seen fruiting, they are removed.

In the young spinney alongside a London Underground storage line we spot an edible and medicinally beneficial ‘Turkey tail’ growing on a broken willow. This is followed with a mostly-devoured polypore ‘Hen of the Woods’ sprouting on an oak. Snapping off a cap releases the smell of autumn. Flipping the cap over reveals small pores.

‘Hen of the Woods’ is highly prized by foragers.

Forestry Journal: ‘Clouded funnel’.‘Clouded funnel’.

“Foraging is an issue. People posting ‘finds’ on social media encourage more foraging,” blanket-stripping a food source vital to wildlife. By-laws prohibit any wild foraging in Highgate Woods and Hampstead Heath.

Found to be bioluminescent and more recently to contain the poison muscarine, a ‘Lilac bonnet’ is no longer on any forager’s menu. 

Woodland paths grew wider during the pandemic and more desire lines (smaller paths) were created through holly and young birch regeneration. More ‘Clouded funnel’, more ‘Amethyst deceiver’ and rich, friable soil (leaf litter) tells David that these soils are slowly recovering.

Approaching Bridge Gate, standing deadwood oak has been consumed by ‘Cubicle brown rot’ caused by ‘Chicken of the Woods’. ‘Chicken of the Woods’ grows on a number of trees, but growing on a yew, it is poisonous to humans because it takes in the yew’s toxins. This decay is good for habitat.” 

Wandering between dead hedges dividing two bluebell woodland boundaries on the hill where kiln earthworks were uncovered, David says he likes this part of Highgate Wood. 

“Being somewhere so long, you get to know the site and its character throughout the seasons.”

Forestry Journal: David Humphries, City of London’s Tree Management Officer, pointing out white ‘Slime mould’.David Humphries, City of London’s Tree Management Officer, pointing out white ‘Slime mould’.

Were he not here, he would be out with the team inspecting trees along property boundaries on Hampstead Heath.

We keep our distance from the lone artist sketching an unpopulated woodland view by quickly moving on.

Skirting a cluster of Hemlock planted in the 1980s, our final approach to Onslow Gate is marked by a cluster of inedible past-its-best purpley-brown ‘Zoned rosette’ growing near an oak. 

“Ancient woods and parkland in the South East hold the majority of this population for the whole of Europe. They are quite rare and vulnerable because oak habitat is declining.”

City of London’s Trees and Conservation and Sustainability Manager, Jonathan Meares, is on his way out through Onslow Gate. He asks how the morning has been. This walk takes place late in the season and David replies: “There are not large numbers of fungi around today, but we saw a wide variety: different types of mycorrhiza, sapotrophs and a few parasitics.”

Forestry Journal:  ‘Amethyst deceiver’, cap nibbled, gills home to a miniscule spider. ‘Amethyst deceiver’, cap nibbled, gills home to a miniscule spider.

Before returning to the office in Golders Hill Park, David shares some images of his favourite fungi not seen today: the cheerful ‘Scarlet elf cup’ (grows on decaying branches or beneath leaf litter on the forest floor) and the furry cap of the larger, rarer ‘Silky rosegill’, which emerges from knotholes or areas of damage found higher up in standing trees.

Last year, David won the London Tree Officers Association ‘Individual Commitment Award’, given to professionals within the capital who have shown consistent dedication and commitment over a prolonged period. He concludes, “This morning I reread the article from ten years ago. It offered an opportunity to reflect on where we were then and where we are today. I enjoyed it.”