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13 Dec 2022

The rotters of Inverewe

Written by John Hedger, local fungi expert
Small brown mushrooms that look a little like burst blisters lie on a pile of twigs.
Blistered cup fungi | Image: Stuart Colly
Much of the autumnally discarded leaves, branches and flowers in Inverewe Garden (the plant ‘litter’) is recycled by a mostly unseen army of rotters.

Fungi, bacteria and lots of soil animals, like millipedes, woodlice and earthworms, consume rotting plant material. It is a pretty efficient system: over the winter, much-needed nutrients are returned to the soil for use by the plants in the spring. This amounts to tonnes of material recycled per hectare in a woodland. A few plants, like rhododendrons, drop leaves and twigs that are more difficult for the recyclers to process, and this can accumulate on the floor as a thick thatch of litter. Even whole trees blown down by storms, as earlier this year at Inverewe, would eventually be broken down by this army of recyclers if the gardeners did not intervene first. Different tree species vary in how fast their wood rots: fallen birches and sycamores can largely disappear within a decade or so; the heartwood of a large oak may take 100 years or more.

A man stands beneath the root plate of a fallen birch tree in a woodland garden.
A fallen tree in Inverewe Garden

The fungi are at the forefront of the recycling team and use their mycelium (a complex of fine threads made up of tubular cells called hyphae) to penetrate the plant litter and wood. They do this by mechanical means – individually, they are tiny but together they can exert considerable force; mushrooms can lift paving stones as they ‘mushroom’. The mycelia have an added trick: as they advance, they release enzymes that digest the walls of the plant cells, like a nail excavating its own hole. Fungi have a better battery of these enzymes than any other organism on the planet – some of them can dissolve the tough cellulose of the plant cell walls. Other enzymes can even tackle the lignin in wood, which provides its durability. If the fungi remove both cellulose and lignin, the wood bleaches (unsurprisingly called a white rot). If the lignin is left alone, the wood turns into a soft brown powder or a brown rot – ‘dry rot’ in houses is a good example.

All of these rotting processes need oxygen. This explains why plant remains accumulate in our local peatlands, where the saturation with water (due to the high rainfall) denies a supply of air to the fungi and prevents recycling. Walkers in the hills around Inverewe will find stumps of the forest from warmer, drier times. These remains had been engulfed by peat and stayed un-rotted for thousands of years until exposed by ditch digging or track construction.

A peat bog landscape, with a large pool of water and mossy hummocks and reeds sticking out. Mountains can be seen in the distance.
Peat bog on Inverewe Estate

The recycling fungi are most obvious when they produce their fruit bodies, the mushrooms and toadstools. This is when we notice them around Inverewe, usually in autumn although many can still be found in December and January. The wood decomposers are easiest to spot – those with leathery or woody shelf-like fruit bodies (bracket fungi) can persist for a long time on logs and stumps. Examples include dyer’s mazegill (Phaeolus schweinitzii) which is common on the base of conifers along the woodland trail and conifer mazegill (Gloeophyllum sepiarium) on fallen pines. Both are brown rots.

In the gardens, the most common bracket fungus is the aptly named turkey tail (Trametes versicolor), which occurs on deciduous tree wood (even on rhododendron) and bleaches it a pure white. Perhaps our most beautiful wood-inhabiting fungus is the aptly named angel’s wings (Pleurocybella porrigens), which is a soft-bodied gill fungus and occurs as tiers of white mushrooms sitting directly on the wood surface. It does not have the stem normally associated with mushrooms. This beautiful fungus is very common on fallen conifer logs in Wester Ross but is a bit of a rarity further south.

The fungi that rot the litter on and in the soil produce a bewildering variety of shapes, ranging from the recognisable mushroom-like gill fungi through to club (or coral) fungi, cup fungi, puffballs and stinkhorns. Most are fleshy and much less common in the winter months, although a mild December and January will still see them fruiting. The shaggy inkcap (Coprinus comatus) is prominent in the grassy area of the garden and likes to digest grass cuttings. The gills dissolve into inky drops, coloured by black spores as they age. The yellow-brown Hypholoma marginatum appears as hundreds of small toadstools when heavy mulch has been applied in the gardens, and has become more common as mulching has increased. Alongside it grow the brown cups of the blistered cup (Peziza vesiculosa), which also likes to rot mulch and is common in UK gardens.

Of the puffballs, the pestle puffball (Calvatia excipuliformis) is especially prominent along the sides of paths. It has a leathery fruit body consisting of a tough column topped by a rounded sac, which puffs out spores when hit by drops of water from the trees above. After some months of weathering, only the column remains. Club fungi also occur everywhere in the gardens and in the Inverewe woodland. The wrinkled club (Clavulina rugosa) has a beautiful, soft, white, much-branched ‘coral-like’ fruit body, usually rooted in mosses.

All of these fungal rotters are doomed to die out when they have used up all the wood or leaves, and so they rely on immense production of spores from the fruit bodies to reach new places to grow. They also explore their surroundings with usually white, root-like cords to find more wood or litter to rot. And so, it is important to leave wood piles in our garden and not rake up too much leaf litter – too much tidying is bad for the fungi and for the invertebrates of the recycling community. Fortunately, at Inverewe there are plenty of places for the recyclers to do their bit for the ecosystem.

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