The way a tree dies has important effects on the species composition in decaying wood. It makes a great difference whether the tree dies suddenly, for instance because of a storm or a wildfire, or whether it dies gradually from competition, drought or old age. Different types of mortality produce dead trees with contrasting qualities, and therefore different species initiate the decomposition process. Later the decaying wood goes through major physical and chemical changes and the species composition changes completely several times until the wood is totally decomposed. The species themselves interact in many ways, and complete food webs build up and wane during the decomposition process. In combination, the different mortality factors and the decay succession have a great impact on the biodiversity in dead wood.

In addition to physically and chemically transforming the wood, the activity of wood decomposers also creates particular microhabitats such as sap exudations, insect galleries, space under loose bark, fungal fruiting bodies, rot holes and trunk cavities. Such microhabitats are important for a large number of species, and we describe them in detail in Chapter 7.

Various textbooks have described the diversity of trees (Oldfield et al., 1998;Grandtner, 2005; Tudge, 2005) and another series of books have treated theinternal anatomy, physiological functioning and defence mechanisms of trees(Blanchette and Biggs, 1992; Butin, 1995; Wagner et al., 2002; Schweingruber etal., 2006). In this chapter we describe trees from a different angle –how different tree properties have strong implications for the speciescomposition of saproxylic species after the tree has died.

Like other topics in ecology, the host-tree associations of saproxylics must beunderstood in an evolutionary context. In Chapter 10 we examine the evolution ofwoody plants with an emphasis on structural innovations. Here we simply mentionthat the origin of coniferous trees dates from about 310 million years back intime, while different broadleaved trees first evolved 100–120 millionyears ago (mya). Thus, coniferous trees and broadleaved trees represent distinctplant groups which differ in many ways.

Conifers versus broadleaved trees

There is a striking lack of scientific review publications that providequantitative information about host-tree associations among woodinhabitingorganisms. In a recent book on the ecology of wooddecaying basidiomycetes (Boddyet al., 2008 ), the topic of host-tree associations was only superi ciallytreated. Only one chapter touched on this subject and quantified the proportionof fungi in Denmark that were specific, strongly selective, or weakly selectivefor different broadleaved tree species (Boddy and Heilmann-Clausen, 2008 ). Thecorresponding chapter on fungal communities in boreal, conifer- dominatedforests did not mention host-tree associations at all. Similarly, a quite recentFrench book on forest insects (Dajoz, 2000 ), with a broad treatment ofsaproxylic insects, did not deal with host-tree association patterns explicitly.Dajoz was, of course, aware of such associations, since his chapter on communitydevelopment during the decomposition process was subdivided into sectionstreating different tree species individually. Such treatments of communitycomposition in wood from separate tree species are quite common (see Chapter 6). But it is only when information is brought together from many sources that wecan get a broader overview of host-tree ranges and the specific preferences ofwood-inhabiting species.

In this final chapter we address some issues that we consider essential for thefuture of dead wood and its biodiversity. Until now we have mainly focused onthe biological aspects of dead wood and have only to a limited extent consideredwider topics such as ecosystem functions, the future of forest biodiversity, andthe need to disseminate information about the fascinating life in decayingwood.

Value of saproxylic diversity

We are convinced that the intrinsic value of the saproxylic species is asufficient motivation for their protection. We can be fascinated by thepeculiarity, strangeness and beauty of saproxylic species, and learntremendously from the intricate interactions between them. But species living indead wood are also more directly valuable, by providing products, ecologicalservices and option values.

Ecological functions, services and resilience

The species that colonize and utilize dead trees provide a central ecosystemservice, namely the decay of organic matter and the connected recycling ofenergy and nutrients. The decomposer community, mainly fungi but assisted by amultitude of invertebrates, performs this service at no cost, allowing the otherliving components of forest ecosystems to thrive. There is a growing awarenessthat the global loss of species threatens the provision of services such asdecay and nutrient turnover. One might ask if the overwhelming variety ofspecies living in wood is really necessary from this perspective. Maybe it wouldbe enough that a few key species are present and that the vast majority ofspecies are actually redundant. In any specific case there is probably no clearand simple answer, but a growing body of evidence suggests that a criticalfeature in ecosystems is the level of resilience (see, e.g., Rockstr ö m et al.,2009 ). It is highly unlikely that the exact number of ‘necessary’species could be dei ned despite the fact that several species may perform thesame function, rather than all of them having critical and unique functions.

Preface

The last few decades have witnessed a rapidly increasing interest in the importance of dead and decaying trees for biodiversity. During their decomposition, dead trees offer habitats for thousands of species. This diversity has been studied by researchers interested in particular organism groups, such as cavity-nesting birds, wood-decaying fungi or saproxylic invertebrates. A holistic overview of the species communities inhabiting trees after their death has been lacking, and our aim is to provide such an overview here.

The scope of the book is global, but we admit that it has a strong north European bias. There are two reasons for this. Firstly, much of the research and many of the scientific publications about species living in dead wood originate from northern Europe, although during the last decade an increasing number of papers dealing with saproxylic organisms have also been published in North America, Australia, Japan and elsewhere. Secondly, our own studies have taken place in Fennoscandia, and our empirical knowledge is mainly derived from the boreal and temperate parts of Europe. We admit that we only have superficial first-hand experience of tropical forests and the temperate and evergreen forests of other continents.

Dead wood and saproxylic species do not occur only in forests. A rich saproxylic community also inhabits dead wood in habitats created by people, both in agricultural and urban landscapes, such as pasture woodlands and parks. Human-maintained habitats can provide important sites, or even the last footholds, for surprisingly many rare saproxylic species. Until recently, the biodiversity value and management of trees in agricultural and urban settings have not received the attention they deserve, despite the fact that these environments often contain greater concentrations of ancient and valuable trees than managed forests. In fact, most readers of this book are likely to find not only the closest populations of saproxylic species, but also the closest populations of threatened saproxylic species, only a few kilometres away, in the nearest park where old, hollow trees occur.

In this chapter, we deal with the occurrence, conservation and management of saproxylic species in different kinds of cultural environments, excluding forests managed primarily for wood production, which were treated in Chapter 13.

Saproxylic species are one of the most threatened organism groups. As with all forest species, they are suffering from the dwindling of forests. But the habitats of saproxylic species may also be decreasing in regions where, although the forest area is currently increasing, such as in Europe, practically all forests and other wooded areas have been taken into intensive economic use, resulting in a greatly reduced abundance of large over-mature trees and large-diameter dead wood (see Chapters 13 and 16). As a consequence, many saproxylic species dependent on these habitat structures have drastically declined and have become threatened.

In this chapter we examine threatened saproxylic species, their threat factors and the assessment of their threat status. The historical development which has led to so many saproxylic species becoming threatened is best known in Europe, and hence we first give a short account of the endangerment history of saproxylic species based on European examples. Next we list the current threat factors which reduce the number and extent of habitats for wood-inhabiting species. We also discuss the knowledge base, methods and criteria used in assessing the threat status of species. Knowledge on threatened species needs to be improved, and in the final section we examine survey methods. We have described, in other chapters, general measures that can be taken to maintain the overall diversity of saproxylic species in managed forests (Chapter 13) and in cultural habitats (Chapter 16). Most of these methods benefit threatened saproxylic species too.

Some of the species associated with snags, logs and hollow living trees do not depend on dead wood as a source of nourishment. Instead, they use cavities and other dead-wood microhabitats for various purposes such as nesting, roosting, denning and hibernation. These species may be obligate saproxylics if the availability of dead wood is essential for their survival during some part of their life cycle. Many facultative saproxylic species use dead wood more opportunistically, without being dependent on it. In this chapter, we describe these uses of dead woody material, concentrating on saproxylic species that do not belong to the saproxylic food web described in Chapter 3.

Vertebrates

Nesting and roosting in cavities

Many forest-dwelling vertebrates utilize holes and cavities in trees. For instance, in Australia, over 300 vertebrate species are known to use cavities (which are generally referred to as hollows in the Australian literature). The list includes 83 mammals (31% of the total terrestrial mammal species in Australia), 114 birds (15%), 79 reptiles (10%) and 27 amphibians (13%) (Gibbons and Lindenmayer, 2002).

A particular challenge for saproxylic species is that they live in ephemeral habitat patches (‘sinking ships’): in dead trees that decompose and gradually vanish. The substrate units they inhabit will inevitably disappear, and they then need to colonize new suitable substrate units. This means that the reproductive success of an individual (and the fate of the local population) depends not only on its ability to reproduce in a suitable host tree now, but also on the availability of suitable host trees in the future. Different types of dead-wood substrates are highly variable with respect to their abundance and persistence time, and therefore contrasting life-history strategies and dispersal abilities have evolved among saproxylic species.

Using the arguments from the well-known evolutionary ecologist, Southwood, the conclusion is that the ‘habitat is the life history template’ (Southwood, 1977; Figure 14.1). This statement emphasizes the central problem that all species face: how to track suitable habitat given its distribution over time and space. In the most fundamental sense there are two basic questions: whether the individual should reproduce here or somewhere else and whether it should reproduce now or later. This connects strongly with change in habitat quality over time and space and how predictable this change is. Given the ‘sinking ship’ situation for saproxylic species, selection for efficient dispersal is obviously strong in most cases. In this chapter we briefly outline some of the basic issues on population dynamics, with a specific focus on the life histories of saproxylic species.

When you sit beside a campfire you can easily feel the energy that is tied up inwoody material. As the wood burns, it is transformed to carbon dioxide, watervapour and minerals – the elements that the tree tied up throughphotosynthesis when it was alive and growing. The combustion of wood in thecampfire takes only a few hours. In temperate and boreal forest ecosystems theequivalent degradation of a tree typically takes 50–100 years and iscarried out by numerous wood decomposers working at a much lowertemperature.

This chapter deals with the activity of these decomposers – how theydegrade and recycle dead wood in forest ecosystems all over the globe. Fungi arethe principal decomposers in terrestrial ecosystems, and especially among thebasidiomycetes we find many effective wood-decaying species. Also a large numberof invertebrates, such as beetles and termites, take part in the process of wooddecomposition. Before we explore this fundamental ecosystem process, we shalldescribe some key aspects of wood structure.

Structural wood components

Wood is made up of three structural components: cellulose, hemicellulose andlignin. The chemical composition, synthesis and degradation of theseeconomically important wood constituents have been important research topics formore than 50 years – and they still are. As a result, we have a goodunderstanding of their biochemical properties. It is beyond the scope of thisbook to go into great detail about these specialized topics, which are regularlyreviewed in books and scientific journals (see Buswell, 1991 ; Markham andBazin, 1991 ; Jeffries, 1994 ; Schwarze et al., 2000b ; Vicuña, 2000 ; Martínez et al., 2005 ; Baldrian, 2008 ).

The surrounding environment strongly influences the conditions inside the wood and is fundamental to determining whether a saproxylic species is able to utilize a certain piece of dead wood. Many species show a clear preference for wood in sun-exposed and dry habitats, while others prefer shady and moist conditions. The tree’s position, whether it is standing or lying, also determines the degree of sun exposure, temperature and moisture in the wood. In addition, the species composition varies according to the surrounding medium. In terrestrial habitats, the vast majority of species utilize the above-ground wood, although some species are specialized to use dead roots buried in the soil. Other species only utilize submerged wood from trees that have fallen into rivers or lakes, and yet others occur on wood in marine waters. In addition, man-made wooden constructions create opportunities for saproxylic species. When these species occur inside houses, they can attack and severely damage the wooden construction materials (see Box 9.1).

In addition to the direct effects, the surrounding environment also has an indirect effect on dead wood through the conditions experienced by the living tree. The local conditions determine the annual growth increment and wood density, and events such as physical injury and insect attacks affect the chemical characteristics of the wood. These wood properties may strongly influence the saproxylic species that later utilize the dead tree. Some of these aspects have partly been addressed in Chapter 6, but deserve some additional attention in this chapter.

Many saproxylic species are only able to use dead wood of a particular size or diameter. Some species prefer large trunks, while others favour small trees or thin branches. Some can use dead wood of many sizes while others are specialized within a narrow diameter range. In an individual dead tree, trunk sections with different diameters tend to be used by different species.

In this chapter, we describe the factors that contribute to the niche separation of species according to tree size. In general, the basal diameter of a tree is closely correlated with other dimensions such as height, surface area and volume, and each of these correlated factors can be important for individual species. For simplicity, the terms ‘diameter’ or ‘size’ are used in the text to refer to all the diameter-related effects, and the other factors (height, surface area, volume) are mentioned only when their effects are specifically considered. The preferences of individual species are reflected in species richness and species composition patterns that can be observed in dead-wood units belonging to different diameter classes. These patterns are reviewed in this chapter.

Almost every ecology textbook includes a chapter or section on food chains or food webs. Such texts are normally accompanied by an example that starts with a photosynthetic primary producer (the lowest trophic level), followed by a herbivore feeding on the producer, next perhaps some medium-sized predator, and finally a conspicuous top predator (third or fourth trophic level). Subsequently, the example might be expanded with additional organisms at each trophic level to illustrate the concept of a food web. The specific example could be from the marine environment starting with photosynthetic plankton algae and ending with a seal or whale species. Alternatively, the example might depict an African grassland food web, ending up with the powerful cheetahs and lions as top predators.

It is typical that nearly all food web examples have herbivores at the second trophic level. We tend to view decomposer communities as simpler systems composed of two organism groups: decomposers, i.e. bacteria and fungi, and detritivores, i.e. animal consumers of dead matter (Begon et al., 2006). This is far from the truth for the communities associated with decaying wood, which represent complex food webs with several trophic levels above the primary producers (the trees). One can also find all kinds of species interactions, such as predator–prey, competition, parasitism and symbiosis, that are well known from other communities.

For millions of years, natural forest dynamics have created the variety of deadwood hosting the diversity of saproxylic life. This chapter describes thestructure and natural dynamics of forests that develop without management orwith negligible human interference. We will deal with stand-replacing dynamicsdriven by fire, storm events, or insect attacks; continuous-cover dynamics,including gap dynamics, in coniferous and broadleaved forests; and ripariandynamics caused by flooding and natural erosion. ‘Parklanddynamics’ in open wooded land maintained by large grazing herbivores willbe presented in Chapter 16.

In this chapter, we also emphasize the abundance and variation in time and spaceof the qualities of dead wood. The patterns of habitat occurrence represent theenvironment to which saproxylic species are evolutionarily adapted. Theconsequences for their life strategies are discussed in Chapter 14.

The suitability of a dead tree as a habitat for a particular saproxylic speciesis partly related to the causes of its death. Different mortality factors openup different decomposition pathways, resulting in divergence in speciescomposition during the decay succession (see Chapter 6 ). In natural forests,the full range of factors causing tree mortality is present, which means that agreat variety of characteristics of dead trees becomes available. Thus, not onlyis the volume of dead wood higher in natural forests than in managed forests butalso, and perhaps more importantly, the diversity of dead wood is muchhigher.

Both living and dead trees provide a number of distinct microhabitats for saproxylic species. By ‘microhabitats’ we mean discrete parts of a tree that host different species assemblages. Some of the microhabitats are only present in living trees, typically in mature or old individuals (Figure 7.1), and additional ones become available after the trees die. Wounds, rot holes and cavities; attached dead branches and roots; phloem (inner bark), sapwood and heartwood; fruiting bodies and mycelia of decomposer fungi etc. each host very different species assemblages. In this chapter, we describe in more detail the various dead-wood microhabitats, and the many ways in which saproxylic species can be dependent on their particular microenvironments. We present the microhabitats in the order in which they appear in a tree, starting from wounds and sap exudations that can occur even in young trees, followed by cavities and other microhabitats that develop as a result of decay in mature living trees, and ending with those microhabitats, such as subcortical space, that become available only after a tree has died. Some microhabitats, especially sap exudations and cavities, are far more common in broadleaved than in coniferous trees.

Wounds and sap exudations in living trees

Many factors can cause injuries to trees. Mechanical damage includes branch and stem breakages by wind, wounds made by falling trees hitting adjacent trees, lightning strikes, fire scars, frost cracks and snow breakage. Pathogenic fungi and bacteria can create necrotic patches, termed cankers, in the phloem layer. Most of the fungi that induce cankers kill the phloem and do not bring about sap flows. ‘Bleeding cankers’ are generally caused either by bacteria (Schmidt et al., 2008) or by Phytophthora species (Oomycetes) (Brown and Brasier, 2007) and produce only small amounts of exudates. Some vertebrate species, such as woodpeckers, and many invertebrate species can injure the bark, which may result in sap exudation and provide an opportunity for microbial colonization. For instance, cicadas (Hemiptera: Cicadidae) are known to cause sap exudation by piercing the bark with their stylet-like mouthparts to feed on sap (Yamazaki, 2007), and carpenterworm (Lepidoptera: Cossidae) larvae can initiate and maintain sap exudation by gnawing the bark (Yoshimoto and Nishida, 2007).

It is evident that planet Earth hosts several million species. The actual number of species is unknown but a much cited study has calculated a global figure of 12.5 million species (Hammond, 1992), while a recent detailed revision came close to 11 million species (Chapman, 2009). In addition to these calculations, there are also estimates that have arrived at global figures as high as 30–100 million species (Erwin, 1982). Nobody has tried to estimate the number of saproxylic species on a global scale. This is quite understandable, since many groups are poorly investigated and large areas are minimally explored for wood-inhabiting species. But there is one region where most saproxylic species are well documented – in the Nordic countries of Europe. We therefore highlight some of this knowledge and present an overview of the diversity in various groups. Despite major knowledge gaps on a global scale, we also make an attempt to calculate the relevant numbers indicating the global diversity of saproxylic species.

Saproxylic diversity in northern Europe

There is a long and strong tradition of documenting species diversity in the Nordic countries of Sweden, Finland, Denmark and Norway. This tradition is rooted in the work of Carl von Linné, who made Sweden the European centre for alpha-taxonomy (i.e. the description of new species) in the 1700s. The Linnean school also had a great local impact in Sweden and the neighbouring countries. During the 1700s and 1800s, the majority of terrestrial species in the Nordic region were described and identification keys were made for large groups of insects and fungi.