Dead wood

Britain's primeval forest: what an experience it would be to visit the Wildwood as it once was. Picture a rich forest-scape with lush undergrowth and trees of all ages, from tiny seedlings, to huge, gnarled giants, some of which are thousands of years old. The forest floor strewn with fallen limbs and trunks, the scents of fresh vegetation and decay mingling in the air. A bear emerges from the forest, and pads deliberately across the cool, dappled glade towards a large, rotting stump, which it effortlessly begins to prise apart in a search for grubs...

Most of our remaining woodland is very different from those prehistoric forests, and a key difference is the amount of standing and fallen dead wood in them. Studies in temperate forests in North America and elsewhere suggest that in a healthy, wild forest dead wood can comprise as much as 30% of the overall woody biomass. Today, the average British woodland has far less. Before examining why, let us look at the crucial role which dead wood plays within the forest ecosystem.

The value of dead wood

Dead wood (coarse woody debris or CWD) is extremely important to the health of the forest, and this is being increasingly recognised by conservationists. Not only is it an aspect of the process of nutrient cycling, providing a steady, slow-release source of nitrogen, but it is also thought to play a significant role in carbon storage. Fallen logs can also increase soil stability within a woodland.

Microhabitats

Standing dead trees (snags) and fallen debris provide a fantastic array of 'microhabitats'. There is a breathtaking range of saproxylic (deadwood-dependent) organisms including fungi, lichens, invertebrates, mosses and birds, many of them having very specific requirements, and some specialising exclusively on one particular microhabitat. A remarkable 40% of woodland wildlife is dependent on this aspect of the forest ecosystem.

Invertebrates

Among the invertebrates, specialist flies and beetles are particularly well represented. A number of these have now become extremely rare, as a result of loss of habitat. Certain insects depend on the fungi which grow on dead trees. For example, the black tinder fungus beetle (Bolitophagus reticulatus) (which occurs in Glen Affric) and another beetle (Bolitothorus cornutus) live in the fruiting bodies of the tinder fungus (Fomes fomentarius) which is only found on dead birches.

Then there are some endangered species which specialise on Scots pine (Pinus sylvestris), such as the pine hoverfly (Blera fallax) which breeds in wet pockets of decay in large pine stumps. This species is found in only a few places, including Abernethy, and is threatened by loss of habitat. Another rare hoverfly (Callicera rufa), whose larvae rely on rot holes in old pines, has been recorded in Glen Affric.

At present Trees for Life are taking a particular interest in the saproxylic insects associated with aspen (Populus tremula), such as the aspen hoverfly (Hammerschmidtia ferruginea). There is in fact a whole specialist community of insects which depend on the dead wood of aspen in various stages of decay, and one of these species - a fly (Ectaetia christiei) - was only identified by scientists for the first time in the 1990s. In order to fully flourish, this community requires a minimum of 4.5 hectares of woodland containing decaying aspen and other broadleaved trees.

Birds, bats and other vertebrates

One third of all woodland birds nest in holes or cavities in dead trees, and large, hollowing trees provide ideal roosting sites for species such as the great spotted woodpecker (Dendrocopos major) and various owls. The crested tit (Parus cristatus) is highly dependent on dead pines for nesting, and excavates its nest hole in the softer, rotten wood of large diameter pine snags. Some birds also rely on the invertebrates in dead wood as a food source.

At least ten of our fifteen bat species use tree holes for summer and winter roosts. Raptors (birds of prey) will frequently use snags as lookouts and food handling points. In addition, logs are a valuable physical feature for other vertebrates, and are used for cover, feeding, lookouts, resting, sunning, drumming and hibernating.

Plant and fungus habitat

A wide range of mosses and lichens can be found on decaying wood, and the Black Wood of Rannoch and Culbin Sands have been designated as Sites of Special Scientific Interest, partly because of this. The characteristic hummocks which are found in ancient pinewoods frequently form on old tree stumps. These lush, vegetated mounds develop as layers of lichen and moss colonise the stump, followed by larger plants such as blaeberry (Vaccinium myrtillus), ling heather (Calluna vulgaris) and eventually even birch (Betula spp.) and pine. These attractive features add further diversity to the forest ecosystem. There is also a whole range of saprotrophic fungi (fungi which obtain nutrients from dead organic matter) which are associated with pinewoods, including the yellow staghorn or jelly antler fungus (Calocera viscosa), which sends up yellow branching coral-like fruiting bodies from pine stumps.

The 'backbone of forest streams'

Large woody debris plays a vital role in creating diverse ecological niches within forest streams and rivers. Fallen logs, for example, can provide cover for fish, as well as creating pools and gravel deposits which are used for spawning. Debris dams gather leaf litter, providing food for fish and invertebrates. By slowing the velocity of a stream, dead wood also helps to reduce soil erosion and regulate flooding.

The decay succession

It is said that there is more life in a dead tree than a living one, and studies in the Pacific Northwest of North America confirm this. In the bole of a living tree there are around 5% living cells by volume. However, in a dead one there can be as many as 40%, largely made up of fungi and nitrogen-fixing bacteria, both of which are crucial to the health of the forest.

In dead wood, there is a typical pattern of decay and breakdown, which is largely the result of colonisation by other organisms. When a tree dies, the sapwood will be invaded by wood-boring beetles. Along with fungi, bark beetles (Scolytidae) are among the earlier dead wood colonists, and use the cell contents of the cambium and sapwood. These soon attract predators and parasites including spiders, false scorpions, and ichneumen wasps, as well as allowing more fungi to enter. Fungi tend to begin the work of decaying the less nutritious heartwood, as their threadlike mycelia penetrate the tissue and allow entry to other organisms.

Some specialist beetles such as longhorns (Cerambycids) are found in the early stages of wood decay and are again followed by predators and scavenging beetles. In Scots pine these predators include the larvae of the large robber fly (Laphria flava) which feeds on longhorn beetle larvae. Hoverflies, millipedes and mites are associated with the mid-stage of decay, and in the later stages the wood may even be used by small mammals. The 'humification phase' is the final stage of decomposition, in which saproxylic insects are replaced by soil organisms. Most of these organisms feed on the bacteria and microfungi that convert the wood to humus. This is in a sense the completion of a cycle, in which the nutrients that have been stored within the tree for decades return to the soil.

The high resin content in Scots pine means that pine logs decay more slowly than those of other species such as birch, and they can provide a habitat for the species associated with dead wood for decades. It can take well over forty years for a fallen Scots pine to decompose, due to the high resin content, and snags can last as long as 100 years.

The decline in dead wood

Obviously wood from the forest has played a crucial role in human society for many thousands of years. Fuel is essential for our survival, and our ancestors harvested both fallen wood and living trees to meet their needs. In some situations this spelled the destruction of an area of woodland, especially if the area was subsequently used for agriculture. In others it meant the ratio of living to dead wood was dramatically altered.

More recently, in modern forestry practice fallen dead wood has often been 'tidied up' in an attempt to control pests and fungal diseases in managed forests, as well as for 'aesthetic' reasons. However, attitudes are now changing, and bodies such as Forest Enterprise (FE) recognise the importance of leaving dead wood within the forest.

A study carried out in 1997 by a student from Edinburgh University on the amount of dead wood in the native pinewoods in Glen Affric found that there was less CWD than expected, and less than some other comparable pinewoods elsewhere in Scotland. This may be due to the past history of exploitation in the glen.

Clearly, the fact that CWD is so valuable means that our forest ecosystems have suffered tremendously as a result of its removal. Twelve of the 45 British species of saproxylic beetles present in the Bronze Age are now extinct, and those which are left are very rare. The fact that there is so little native pinewood left increases the problem, and in many Caledonian Forest remnants, even when dead wood is present, the lack of natural regeneration means that it can be overexposed to the wind and sun, thereby drying it out and rendering it unsuitable for organisms which require cool, damp conditions. Furthermore, the current skewed age distribution in native pinewoods means that there will possibly be a decline in dead wood before there is an increase. When the current generation of old trees die (mostly during the next 30 - 50 years), there may be a gap of up to 100 years before the trees which are regenerating today will become old enough to produce large quantities of CWD.

Tackling the problem

 

A key part of our approach at Trees for Life is to restore as natural a forest as possible. At present we make a point of leaving felled non-native trees in situ - 'felling to recycle' - which is also part of Forest Enterprise's policy within Glen Affric. The aim of this is to provide habitat for the dead wood-dependent species, and to return valuable nutrients to the soil. By increasing the size of existing aspen stands, and linking them up, we aim to provide more habitat for the saproxylic insects which depend on dead aspen wood. Other organisations are tackling the problem in different ways. For example, in Beinn Eighe National Nature Reserve, management by Scottish Natural Heritage has included artificially creating rot holes, which have been successfully colonised by a number of endangered invertebrates.

As a result of these and other initiatives, there should, in the long term, be more and more dead wood and old trees in Scotland's regenerating native forests. We envision that those future forests will once again teem with the life forms which thrive on decay and old growth, and that some of them will perhaps even resemble the primeval forests of long ago.

Dan Puplett

Sources & further reading

  • Ferris-Khan, R., Lonsdale, D. & Winter, T. (1993) The conservation management of deadwood in forests. Forestry Authority Research Information Note 241
  • Life in the Deadwood (PDF). Forestry Commission
  • Kirby, K.J., Reid, C.M., Thomas, R.C.& Goldsmith, F.B. (1998) Preliminary estimates of fallen dead trees in managed and unmanaged forests in Britain. Journal of Applied Ecology 35(1), 148-155.
  • Peterken, G.F. (1996) Natural Woodlands - Ecology and conservation in northern temperate regions. Cambridge University Press: Cambridge.
  • Rackham, O. (1990) Trees and Woodland in the British Landscape (revised ed). Dent: London.
  • Speight, M.C.D. (1989) Saproxylic invertebrates and their conservation. Council of Europe: Strasbourg.
  • Stubbs, A.E. (1991) Insects in dead wood in standing and fallen trees. Fry, R.& Lonsdale, D. (eds.). Habitat conservation for insects - a neglected green issue. Amateur Entomologists Society: Middx.
  • Ancient Tree Forum. Woodland Trust

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