The rotters of Inverewe

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.

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.

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.