The Many Functions of Deadwood

PROTECT, MANAGE, RESTORE – these three words are a WWF mantra for appropriate care for the environment. In the coming weeks we will look at this mantra in relation to several aspects of management of Zimbabwe’s vegetation, particularly in the form of forests.

We need to remind ourselves constantly of the importance of plants in our lives.

Without plants we humans (and all other animals) would soon cease to exist. We depend on plants for our existence – for food, shade, building timber, yes, but far more fundamental than that, we depend on plants for the very air that we breathe. Plants breathe in our carbon dioxide and breathe out the life-sustaining oxygen we need. Huge natural forests like the Amazon jungle are “the lungs of the world”.

But apart from that extreme importance of vegetation, and in particular trees, in our lives let us briefly list a few other reasons vegetation is important, such as:

• Sustaining all the life forms (including humans) that live on and off plants and that in turn provide food for all the prey species (and humans);
• Holding soil together;
• Breaking down into soil nutrients
• Fixing soil nutrients (like nitrogen)
• Carbon sink, working against global warming

We need multiple approaches to taking care of the vegetation of our planet. We need to:

• Slow down deforestation in our country;
• Promote reforestation to begin to replace what has been lost, through deforestation or even just natural tree death from old age;
• Act against invasive plant species that are squeezing out indigenous species;
• Recognise the crucial part played by deadwood in a healthy forest;
• Manage bushfires.

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“Deadwood is the richest habitat in a healthy forest ” says Keith Kirby, English Nature’s woodland expert (In article by Malcolm Smith, “Just leave the dead to rot”, The Guardian newspaper, 25^th March, 2004).

“Deadwood” is used in English idiom to suggest that something or someone is useless, pointless, a waste of space or time. From an environmental point of view this attitude to literal “deadwood” (dead trees, standing or fallen) has hugely negative implications for environmental health.

Zimbabwean National Parks staff, for example, have been encouraged for many decades to gather deadwood from the bush and chop it up for the lavish evening fires of visiting tourists and for water heating in Rhodesian boilers. On the whole people believe that if the wood comes from a tree that has died naturally then it is fine to burn it freely. But in reality that piece of firewood provides homes and food for a huge number of species and burning it has serious implications for biodiversity.

Rather than reinvent the wheel and write my own article I am going to use the text of a powerful and comprehensive report produced by WWF in 2004 entitled Deadwood – Living Forests. The brochure is focusing on deadwood in European countries, not in Zimbabwe, and I will adapt it by:

• Leaving out some references and charts of information specifically referring to Europe and where possible add some Zimbabwean equivalent – though of course I do not have access to the quality of data that WWF has.
• Leaving out WWF’s many beautiful photographs of deadwood doing what it is supposed to be doing in European forests and substitute photographs from our part of the world.
• Shuffling the order of some sections where I thought it made it easier to follow the argument being put forward.
• Starting with a summary of the argument to help you to navigate your way through a large number of words.
• Concluding with a small reflection on how we could apply this understanding of deadwood’s environmental significance in Zimbabwe.

Deadwood – Living forests

Summary

Veteran trees, standing dead or dying trees, fallen logs and branches form one of the most important – yet often unrecognised – habitats for European biodiversity.
For generations, people have looked on deadwood as something to be removed from forests, either to use as fuel, or simply as a necessary part of “correct” forest management. Dead trees are supposed to harbour disease and even veteran trees are often regarded as a sign that a forest is being poorly managed. 
Breaking up these myths will be essential to preserve healthy forest ecosystems and the environmental services they provide. In international and European political processes, deadwood is increasingly being accepted as a key indicator of naturalness in forest ecosystems.
Up to a third of European forest species depend on veteran trees and deadwood for their survival. Deadwood is providing habitat, shelter and food source for birds, bats and other mammals and is particularly important for the less visible majority of forest dwelling species: insects, especially beetles, fungi and lichens. Deadwood and its biodiversity also play a key role for sustaining forest productivity and environmental services such as stabilising forests and storing carbon.

Despite its enormous importance, deadwood is now at a critically low level in many European countries, mainly due to inappropriate management practices in commercial forests and even in protected areas. Average forests in Europe have less than 5 per cent of the deadwood expected in natural conditions. The removal of decaying timber from the forest is one of the main threats to the survival of nearly a third of forest dwelling species and is directly connected to the long red list of endangered species.

Increasing the amounts of deadwood in managed forests and allowing natural dynamics in forest protected areas would be major contributions in sustaining Europe’s biodiversity. Governments which have recognised the need to preserve the range of forest values and are committed to these processes can help reverse the current decline in forest biodiversity. This can be done by including deadwood in national biodiversity and forest strategies, monitoring deadwood, removing perverse subsidies that pay for its undifferentiated removal, introducing supportive legislation and raising awareness. WWF calls on European governments, forest owners and the forest industry to help conserve biodiversity by increasing deadwood in boreal and temperate forests to 20-30 cubic meters per hectare by 2030.

Wildlife species associated with deadwood (adapted from Dudley & Valauri, 2004)

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N.B. Throughout the following text you will find references to “saproxylic species” from the Latin sapros (meaning rotten) and xylon (meaning wood) – which is any species that depends, during some part of its life cycle, upon wounded or decaying woody material from living, weakened or dead trees.

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Myths about deadwood

When Europeans see a natural forest they often think that something is wrong, that the forest is sick. Unfounded myths about old trees and deadwood have developed over centuries.

Myth 1: A “clean” forest is a healthy forest

“Clean” forests are not healthy forests. The few natural forests remaining in Europe are far more stable, healthy and resistant to disease, pests and climate change. As a result, natural forests are more diverse and complex than their managed counterparts.

Myth 2: Over-aged forests are a problem

Veteran trees are often regarded as a sign that a forest is being poorly managed. As a result, we have many forests where only young trees remain and people have lost a sense of what a natural forest with uneven age structure looks like. For example in a typical central European forest we rarely find trees older than 100 years whereas many tree species could easily reach an age of 300 years and more. We might imagine a human population where everyone past their early thirties is quietly removed…

Myth 3: Dead trees harbour diseases

The most threatening pest for forest managers is the bark beetle and deadwood is often blamed for allowing the bark beetle to infest forests. In fact the evidence suggests that reasonable levels of dead trees are no danger for the forest. On the contrary, several studies seem to show that they shelter a significant group of parasitoids and predators, which more or less control the populations of pests. Although bark beetle numbers increase near significant numbers of fallen logs, research found little evidence for increased tree death as a result, mainly because the species attracted are already highly specialised to dead timber.

 Myth 4: Only young is beautiful!

A central problem in managing for deadwood is a matter of human culture. Veteran and dead trees are not attractive in a culture obsessed with youth: foresters themselves have been obsessed for decades by the question of regeneration of forest for example. Species such as fungi and beetles are not renowned for their beauty and charisma and are not well promoted by nature conservationists. Making space for dead timber is not simply a question of telling people about a few management techniques, but also involves changing the perception of what high quality management might look like and about forest ecology.

Myth 5: Deadwood brings fire

A frequent argument for the removal of deadwood is as an insurance against fire. Yet well-managed deadwood components can be integrated into existing fire management policies (and for instance can still be removed from fire breaks). Most fires start in dry weather when living trees burn readily and most fires in Europe are started by people, so a few snags and down logs will not significantly increase the risk of fire.

Myth 6: Deadwood is a health and safety risk to visitors

The greatest risk in forests is from commercial timber operations, particularly the felling operations. Deadwood is already successfully retained in many reserves and protected areas, or even city parks (like for instance in the parks around Vienna), and can be managed for instance by retaining dead trees some distance from public paths. Germany recently changed its laws to remove responsibility for accidents from forest owners, so that visitors wander off paths and through old forests at their own risk; similar changes elsewhere would make it easier private forest owners whose forest are open to public access.

Deadwood’s importance for biodiversity and people

Deadwood stabilises forests, sustains forest productivity, stores carbon and provides food and habitat for thousands of specialised species.

Deadwood is not an optional extra, but a critical component in forest functioning, which plays five major roles in the ecology of a healthy, natural forest:

■ Maintaining forest productivity by providing organic matter, moisture, nutrients and regeneration sites for new trees – some tree species germinate preferentially on logs

■ Providing habitats for creatures that live, feed or nest in cavities in dead and dying timber, and for aquatic creatures that live in the pools created by fallen logs and branches

■ Supplying a food source for specialised feeders such as beetles and for fungi and bacteria

■ Stabilising the forest by helping to preserve slope and surface stability and preventing soil erosion in the event of storms, heavy rainfall and other climatic extremes

■ Storing carbon in the long-term, thus mitigating some of the impacts of climate change (seesection on climate change below). Even before a tree dies completely, it attracts specialised species; for example around 115 species of hoverfly (Syrphidae) are saproxylic, but almost exclusively in dying rather than dead timber. Veteran trees provide specific habitat and nesting spaces for some species, such as the black stork (Cigonia nigra). (In Zimbabwe many species of raptor nest at the top of veteran or dying/dead trees.)

When a tree is newly dead it attracts specialised organisms capable of breaking down the tough lignin layer that protects it, principally fungi (like the familiar bracket fungi) and bacteria. These colonisers open up the resources locked in the wood, by making cracks in the tough outer skin and modifying the heartwood so that it can be assimilated by other feeders.

Next to arrive are plant and animal species that eat the “evolved” organic matter, including many beetle species. Research in hardwood floodplain forest in South Moravia in the Czech Republic found 14 saproxylic ant species and 389 saproxylic beetle species. Similarly 37 per cent of beetles in La Massane old-growth forest in France were associated with deadwood, and there are about 100 saproxylic beetles species in the Mediterranean cork oak forest of Les Maures. Around Lake Vatten in Sweden forests harbouring rare saproxylic beetles had on average 10-30 times more deadwood than other forests. Also in Sweden around 2500 fungi species rely on dead timber along with over 50 moss species. (All these examples are European but you just have to translate – species that depend on dead wood include saprolyxic flies, ants, beetles and fungi.)

As soon as herbivores move in, their predators arrive as well. Woodpeckers are the best known, with their deep drumming accompanying any walk through a natural forest. Many are highly dependent on deadwood particularly in winter. For instance, the great spotted woodpecker (Dendrocopus major) relies on insects from snags or down logs for 97 per cent of its winter food. Between 80 and 130 ha of old-growth forest is required for one breeding pair of three-toed woodpecker (Picoides tridactylus), which forages mainly on recently dead spruce.

Larger animals also make use of dead timber for shelter. All but one of the eleven European woodpecker species excavate nesting holes in dead timber, and at least ten European owl species use tree holes as do species like flycatchers (Muscicapidae), nuthatches (Sitta spp.), treecreepers (Certhia spp.), tits (Parus spp.), and even ducks like the goldeneye (Bucephela clangula). Other users include many bat species and large mammals like bears. In La Massane in the French Pyrenees, a quarter of mammals and over a sixth of birds are associated with deadwood. (In Zimbabwe many species of bird make their homes in hollows in living or dead trees – e.g. woodpeckers and owls, as in Europe, but also hornbills. Such hollows also accommodate bats, mongooses, and a variety of lizards and snakes, to name but a few.)

When a tree falls in the forest it creates disturbance that helps some plants to germinate and grow. Deadwood falling into streams and rivers also provides important habitats, including assisting the creation of gravel bars and pools which reduce water flow, creating fish and insects’ habitat and providing valuable substrate for algae. These slow flowing areas retain up to 70 per cent of the litter fall thus increasing nutrients. Research in the USA found that pools created by logs and branches provide over 50 per cent of the salmonoid spawning and rearing habitats in small streams.

Deadwood : supporting never-ending forest cycles

Deadwood is not a single habitat, but instead a complex range of different micro-habitats, which change and evolve over time. The quality of deadwood, and its usefulness for different species, depends on how long it has been decaying and also on the tree species, age at time of death, cause of death, position (standing, fallen, etc) and size, and on the surrounding climatic conditions. In Sweden fine deadwood material forms a richer habitat for fungi like morels, and cup fungi. Elsewhere, research shows that small logs and branches do not decay in the same way as large trunks, so that necessary habitat types will not occur. The process of deadwood recycling can sometimes take hundreds of years to complete and includes three main phases:

■ A short colonisation phase during which the wood is invaded by primary and vigorous saproxylic organisms, often longhorn beetles associated with fungi, which attack the wood when it is still hard.

■ A long decomposition phase during which the primary saproxylic organisms are joined or replaced by secondary saproxylic organisms, which feed on material that has already been partially converted by colonisers, or are their predators.

■ A long humification (formation of humus) phase through which the saproxylic organisms are progressively replaced by scavenging organisms like springtails or millipedes, who incorporate wood residues into the ground when it has been sufficiently transformed during the decomposition phase.

Managing for deadwood requires a thorough understanding of the numerous habitats and associated species. The following typology provides an initial and simplified guide to deadwood likely to be found in forests under natural conditions.

Any one tree will not go through all these stages, and the trajectory of decay will depend on how an individual tree died along with other external environmental factors.

Loss of deadwood means loss of life

Many threatened species are associated with deadwood in Europe, ranging from simple organisms to complex, mobile species like woodpeckers. Deadwood is now one of the most threatened habitats in the forest.

Over much of Europe, forests have been “managed” for hundreds of years. Management has already taken a heavy toll on species associated with deadwood and we know from sub-fossils in peat deposits and insects preserved in amber that many saproxylic beetle species have already become extinct in the last few thousand years, almost certainly because their habitat disappeared.

It is likely that the extinctions we know about are a small proportion of the real losses. In this impoverished environment, those that remain are precious. Unfortunately they are also often highly at risk. Species associated with deadwood now make up the largest single group of threatened species in Europe.

For example, of the 1,700 species of invertebrates in the UK dependent for at least part of their life cycle on deadwood, nearly 330 are Red Data Book-listed because they are rare, vulnerable or endangered. In Sweden, one of the most densely forested countries in Europe, 805 species dependent on deadwood are on the national Red List.

 (The following list is, of course, threatened European species but I have left it in because you can just translate to similar species in Zimbabwe.)

■ The white-backed woodpecker (Dendrocopos leucotus) has declined dramatically because of the disappearance of old-growth deciduous forests (sallow, alder and birch). It is highly threatened in Sweden and Finland, with 90 per cent of the Fenno-scandian population (1700 pairs) now confined to coastal forests of Norway.

■ The Bechstein’s bat (Myotis bechsteinii) is one of many bat species to have undergone a catastrophic decline: it is now very rare in some countries and vulnerable throughout its range. In Italy, sub-fossil records suggest it was once abundant in the region and the current decline is attributed particularly to loss of the hollow trees used as summer roosts.

 ■ The Hermit beetle (Osmoderma eremita) lives in hollows of dead trees in 33 European countries but is declining and protected by the Bern Convention. In Poland, for instance, researchers conclude that its survival is threatened by the felling of hollow or rotting trees. Most of the beetle species living in hollow trees do not fly more than a few tens of metres making dispersal difficult in a fragmented forest landscape.

■ Many fungi confined to deadwood are now under threat, although data on distribution and status in Europe are very incomplete. Threatened species include Laricifomes officinalis growing principally on veteran trees in pine-larch woods, and Pycnoporellus alboluteus, confined to thick logs in herb-rich spruce forests in Fenno-scandia. Both are protected by law in some countries and were recently recommended for listing in the Bern Convention. The importance of fungi is often underestimated; for example in the Alps, a single hectare of spruce forest can support over 300 species of fungi. These threats are unlikely to disappear very soon because current problems will be increased by fragmentation and future shortages. Even where natural forest fragments are conserved or where deadwood components are restored, dispersal problems make surviving populations vulnerable. And in some countries, even where deadwood currently exists, lack of young or middle-aged trees will cause a problem in the future unless there is intervention.

 The final stages in the life cycle of a tree – from veteran to dying trees and deadwood – attracts specialised species which play a key role in maintaining the forest’s health and stability.

 Deadwood and Climate Change

As the reality of climate change is widely recognised, carbon sequestration (the storing of carbon in ecosystems) is gaining attention as one way of reducing greenhouses gases.

Major forest carbon pools include trees, under-storey vegetation, deadwood, litter, and soil. Deadwood is important as it is both a store and source of carbon but is generally the least studied of the carbon pools. This will now change because national carbon inventories are required under the Kyoto Protocol of the 1992 United Nations Framework Convention on Climate Change.

Initial discussion on carbon storage focused on fast-growing rotations of exotic plantations. However, while these can quickly accumulate carbon, storage is very temporary: average retention time of carbon in plantation trees is only a few years because most of the fibre is used in paper and other short life products that are either burned or degrade quickly in landfill. Deadwood itself releases carbon to the atmosphere – becoming a carbon source – during microbial respiration from decomposer organisms.

But in ecosystems in cool climates, microbial activity is restricted and decomposition very slow, so that deadwood tends to act as a long-term storage site. Much of the carbon in long-lived and slow decaying trees, such as Scots pine, can remain sequestered for over a thousand years. Dead trees and old-growth forests are therefore usually better carbon stores than the new forests which replace them.

In British Columbia, research found that at a rotation age of 80 years, regenerating stands stored approximately half the wood carbon of nearby old-growth forests (predominant age 500 years), indicating that conversion of old-growth forests to younger managed forests results in a significant net release of carbon. On the other hand, in impoverished forests, restoring deadwood by retention can store carbon for many decades or centuries. Calculations in France suggest that creation of new protected areas (with no logging) can store the same amount of carbon as afforestation.

Deadwood in European forests: not enough! (I have left out charts of the deadwood remaining in forests in different European countries.)

A severe lack of deadwood in managed forests and inappropriate protected area management are key reasons for biodiversity loss in European forests. A few remaining old growth forests in Europe can be used as a direct evidence of the links between deadwood and biodiversity.

Naturalness is more than just a question of what species occur, but relates to the pattern of the forest canopy, the way that the forest functions ecologically, its resilience to change, the extent to which it has been fragmented and the process by which it regenerates itself. Because naturalness is so complex, it is often measured by using a suite of indicator species or microhabitats likely to be present in a natural forest. Deadwood is one of them.

Deadwood is an indicator that captures many elements of naturalness and is becoming a general reference for natural forests in Europe. If there are enough of the right kinds of deadwood in a forest then it is likely to be fairly natural. Researchers and governments are now surveying deadwood in forests to find out how much deadwood should be present in a natural forest as a reference, and how much is present in managed forests. Volume of deadwood depends on productivity, pattern of natural disturbance, successional stages, forest history and human intervention.

Deadwood type and decay trajectory (the way in which the tree decays over time) are influenced by the way in which it dies (lightning strike, storm damage, drought, disease etc). Deadwood type and volume vary between different forest types and management systems. Some types of disturbance are particularly linked to the production of either standing deadwood (e.g. dry-out), or fallen deadwood (e.g. storm damage)5. In unmanaged European broadleaf forest, deadwood will eventually rise to anything from 5-30 per cent of the total timber, with volumes normally from 40 to 200 cubic metres per hectare with for example an average volume of 136 m3 /ha6 in old-growth beech forests.

Deadwood can rise even higher after a catastrophic event like a storm. These figures contrast dramatically with deadwood volumes in managed forests, even those that are managed in quite a natural manner. For instance, deadwood in the Jura Mountains of Switzerland, which are managed under continuous cover forestry with large areas in an IUCN Category V landscape protected area, was only 6.3 m3 /ha7 in 1993-95. Less natural forests, such as plantations of Eucalyptus or spruce, result in a further significant reduction of deadwood volumes.

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There’s a use of deadwood that the WWF article doesn’t mention because it doesn’t occur in European forests and that is – elephant rubbing post