The abundant life of dead wood, part 1

Over the winter of 2021-2022 Scotland has been hit by a number of named storms, with Storm Arwen in particular causing considerable damage in the coastal area of Moray near Findhorn. On the night of 26th November 2021 Arwen blew down more than 100 trees in the small area of pine woodland on the land surrounding the Findhorn Community that is managed by the Findhorn Hinterland Trust (FHT).

I’ve been a trustee of the FHT for several years now, with a special focus on the land and its biodiversity, and part of my role involves raising awareness about both the biodiversity of the site and the importance of various habitats there. In our modern-day society, dead wood is often seen either as a problem, as fallen trees can block paths and access, or merely as a source of firewood (and I do have a wood stove in my house, which burns some firewood from the Findhorn Hinterland area!). However, it’s also a vitally important component of a healthy woodland ecosystem, and provides the habitat and home for a wide range of species – in this blog I’ll be writing about a few of them.

When a tree dies, and especially when it falls to the ground, the nutrients that it has accumulated in its trunk and branches throughout its life become available as a food source for a whole host of organisms. The most significant and important of these, at least in the initial phase after the tree has died, are fungi. Many species of fungi live in symbiotic, mutually-beneficial mycorrhizal relationships with living trees, but it is a different category of fungi – saprotrophic fungi – that live on dead wood.

The microscopic spores of fungi are widely distributed everywhere, and some will soon become established in any dead wood in an ecosystem. What we see as fungi, such as the turkeytail fungi (Trametes versicolor) shown here, are the fruiting bodies that release the spores. The main body of the fungus is a network of fine white threads called hyphae that spread within the dead wood itself.

These can readily be seen by pulling apart a decomposing log, and can either be a thinly scattered network of threads or a much denser concentration of hyphae, depending on the stage of decomposition that the wood is at.

Identifying a fungus from the network of hyphae, or mycelium as it is known, is not possible in most cases, as the characteristic features needed to reach a specific determination for an individual species only become apparent when the fruiting bodies appear. Often, various different species of fungi will colonise the same dead wood, and some fungi fruit inside the rotting wood of a log, as in this example here.

In addition to hyphae, other fungal structures that can be seen inside dead logs are rhizomorphs. Sometimes known as bootlaces, because of their resemblance to the dark laces of shoes and boots, these are typically produced by honey fungi in the genus Armillaria, as well as other species of fungi. They are also called mycelial cords, are composed of specialised hyphae and are used for transporting nutrients over a considerable distance in dead wood.

The different types of fungi that feed on and break down dead wood can be grouped together, depending on which part of the wood they utilise. White rot fungi, such as the turkeytail fungus, break down the lignin in wood, and this group includes several other well known fungi like the tinder fungus (Fomes fomentarius) that is common on dead birches. By contrast, brown rot fungi such as the Dyer’s mazegill (Phaeolus schweinitzi) specialise in breaking down the cellulose in the wood.

Whilst carrying out some research on the Hinterland site for this blog I came across another example of a brown rot fungus – a species called Serpula himantioides (it doesn’t have a common name). I found it on the underside of an old pine log, and it is the wild relative of the notorious ‘dry-rot fungus’, Serpula lacrymans, which causes decay in the woodwork of poorly-ventilated buildings.

Close examination of this fungus reveals that its spore-bearing surface consists of an intricate labyrinth of interconnected, folded ridges that provides a remarkable example of the natural beauty of fungal architecture.

A large number of fungal species feed on dead wood, and quite a lot of them produce crust-like fruiting bodies that hug the surface of the logs they are helping to decompose. Some of them produce similar types of pore-bearing structures, which are very interesting to look at in close up, either with a hand lens, or with a camera and macro lens combination that gives a magnification of life-size or greater.

Other species of fungi that feed on dead wood produce fruiting bodies that are more conventional in their appearance, with a similar shape and form to the mushrooms that are grown commercially as food. The clustered pine bonnet is, as its name suggests, commonly found on the fallen dead wood of pine trees, where it fruits in small groups or clusters like this.

Other fungi have fruiting bodies that are more amorphous in their shape, especially in wet conditions when they are fully hydrated. After a dry spell of several days, species such as the yellow brain fungus (Tremella mesenterica) and the jelly ear fungus (Auricularia auricula-judae) shrivel up and become much duller in their colour.

Fungal fruiting bodies come in a wide variety of shapes and forms, and some of those that appear on dead wood resemble miniature versions of the massed organ pipes or elaborate candelabras of Gothic cathedrals in their structure and complexity.

A number of species, including the turkeytail and Dyer’s mazeglll fungi featured above, show a distinctive radial pattern of growth, with concentric rings visible on the fruiting bodies. These usually indicate that the fungi grow, and persist, over a long period of time, perhaps several years, with the rings being a fungal equivalent to the annual growth rings in trees.

In addition to their role in breaking down dead wood, fungi also perform another ecologically important function there, by helping to provide some of the nutrients that are required by xylophagous (ie wood-eating) insect larvae, especially those of beetles. The wood is composed of cellulose, lignin and resins, which in turn consist of carbon, hydrogen and oxygen, but are lacking in nitrogen, phosphorus, potassium and trace elements such as manganese and zinc that most invertebrates need for their metabolisms. Fungi are able to transport these nutrients from outside the dead wood, and this, plus their presence and action in breaking down the wood, prepares the way for the beetle larvae that follow.

In Britain, over a hundred species of beetles live in dead wood during the larval stage of their lifecycle, including those in the bark beetle and longhorn beetle families. Adult female beetles insert their eggs, or oviposit, into a suitable site and once the larvae have hatched they feed on the dead wood. In some cases they can do this for up to five years, because of the low nutritional value of the wood.

The larvae chew through the wood just under the bark of a dead tree, forming tunnels, or galleries, as they do so, and leaving behind piles of frass, which is the undigested material that the larvae excrete. Sometimes the galleries are densely packed together, creating beautiful patterns from the meanderings of the larvae through their food source – the outer layers of wood in a log.

Eventually, when the beetle larvae are fully grown, they make an oval-shaped chamber at the end of a gallery, where they pupate and from which the adult beetles emerge in due course. Some pupal chambers can be seen in the photograph on the right below.

The larvae of one group of beetles, the click beetles in the family Elateridae, are known as wireworms and have a distinctive linear and segmented body shape. More than 70 species of click beetles occur in the UK, and the larvae of about half of them occur in dead wood. Most wireworms are saprophagous, meaning they feed on dead organic matter, but a few of them are predators of other invertebrates.

Beetle larvae can be quite abundant in dead logs, and that makes them a food source for other creatures. This is particularly the case with woodpeckers, which specialise in excavating small holes in the wood to extract the larvae feeding inside. Woodpeckers will spot holes in the wood surface that a beetle will have used to lay its eggs and then listen for the sound of the larvae chewing the wood to target where they peck to find their prey.

It’s not just dead wood that beetle larvae develop in, as there are specialists that feed in some of the tougher and more persistent woody bracket fungi that fruit on dead wood. For example, the larvae of 35 species of beetles have been recorded feeding inside the fruiting bodies of the tinder fungus (Fomes fomentarius), including those of the black tinder fungus beetle (Bolitophagus reticulatus) shown here. This also illustrates another example of the varied relationships between fungi and beetles in terms of their roles in breaking down and recycling dead wood.

Apart from beetles, a whole host of other invertebrate species live in, and utilise, dead wood. Perhaps the most obvious and commonly seen of these are the woodlice. They are terrestrial crustaceans (most crustaceans, such as crabs, live in marine environments) in the taxonomic Order of Isopods, and have heavily-armoured, segmented exoskeletons. They have seven pairs of short legs, all of which are alike, and have given rise to the name Isopod, which is derived from ancient Greek, with ‘iso’ meaning ‘same’ and ‘pod’ meaning ‘foot’.

35 different species of woodlouse occur in Britain, but the common shiny woodlouse (Oniscus asellus) is the one that is most frequently encountered – anyone opening up a few pieces of dead wood in a forest is almost guaranteed to see one or more of these. It’s one of the largest woodlice in the UK, reaching a length of 16 mm. It feeds on dead organic material and can often be encountered in small groups under tree bark or amongst rotting wood.

Like all invertebrates with exoskeletons, woodlice go through periodic moults, when they cast off the old exoskeletons that they have outgrown, so that their soft internal bodies can reach a larger size. Unlike most other invertebrates, woodlice moult in two stages, shedding the back half of the exoskeleton first, with the front part being cast off a few days later. The exoskeleton shown here is complete, so it is almost certainly from an individual that died there, rather than being shed during moulting.

When a woodlouse has moulted its new exterior is often translucent at first, as is the case for many invertebrates, and it can take a few days for the new exoskeleton to harden fully and darken in colour. The common striped woodlouse (Philoscia muscorum) in this photograph (another species that is widespread and ubiquitous in the UK) has almost certainly recently moulted.

The next part of this blog will feature a whole range of other organisms that can be found in or on dead wood, including slime moulds, springtails, mites, overwintering adult insects, millipedes and sundry other interesting and unusual life forms. There’s a whole miniature world of tiny creatures to encounter there, as shown by this photo of a springtail (Tomocerus vulgaris), which is dwarfed by the 16 mm woodlouse beside it!

(I’m grateful to Liz Holden for her assistance with the identification of some of the fungi depicted in this blog, and for carrying out the detailed fungal surveys on the Findhorn Hinterland area.)