This book is about life in dead trees. All over the world one can find a fascinating diversity of life forms in decaying wood – first and foremost a wide variety of fungi and insects. These organisms carry out the hidden but highly important work of wood decomposition.

A fundamental question frequently revisited in this book is: ‘Why is the species diversity of wood-inhabiting organisms so tremendously high?’ In most chapters we approach this question indirectly by highlighting the key properties of dead wood, along with the environmental factors and processes that bring about the diversity we can observe. We also discuss species richness explicitly in Chapter 11. There are at least two good reasons for addressing the biodiversity in dead wood. One is that the diversity of wood-inhabiting organisms is a multifaceted and interesting phenomenon that deserves attention for its own sake. Another reason is that this diversity is being seriously threatened due both to the loss and fragmentation of forests and because of the greatly reduced amount of dead wood in managed forests and other woodlands. Thus, we need to understand the role of dead wood for biodiversity in order to manage and maintain it while efficiently utilizing forest resources.

This chapter presents various forestry practices, from modest selective timberharvesting to plantation forestry. Particular attention is paid to differencesin the amount, quality and dynamics of dead wood between managed and naturalforests. The chapter also discusses management options and processes that mightimprove conditions for saproxylic species.

Many kinds of forestry practice exist, but it is beyond the scope of this chapterto give a full treatment of all aspects. Common to all types of forestry is thefact that trees are cut and removed from the forest. This clearly represents asituation of resource competition for the species dependent on dead wood. Inmany regions, clear-cutting and removing all the trees is the most commonharvesting method and has obvious negative effects on many saproxylic species(Figure 13.1). But other management regimes also show negative effects on thedead wood biota. Balancing the extraction of wood with the demands of saproxylicspecies is a difficult task – a topic that is discussed in the secondhalf of this chapter.

Amount, quality and dynamics of dead wood in managed forests

The most obvious ef ect of forestry is the extraction of trees for commercialpurposes. This will result in a loss of dead wood resources for saproxylicspecies, in terms of quantity, quality and dynamics.

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.

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.

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.

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.

Target setting has become a familiar concept through the international policy debate on global climate change. In contrast with greenhouse gas emission targets, the type of targets we emphasize in this book must be developed from ecological knowledge rather than from a socio-economic analysis and a desired outcome. The focus on value-free, quantitative approaches to target setting we imposed from the start could not be applied to all chapters, however. Contributors to this book represent a wide array of professional backgrounds and, accordingly, they approached conservation target setting from a variety of perspectives. Hence, we were not surprised when some contributors argued that targets should integrate socio-economic considerations. Does this reflect insubordination on their part or, rather, the complex socio-economic ramifications of conservation issues? It would be naive to expect a large group of intellectuals to abide by a rule and, therefore, we suspect that both hypotheses may apply here! Including ourselves, most of this book's contributors work with forest managers and policy-makers on a weekly basis unless they are practitioners themselves. Thus, they are well aware of the practical limits to the development of conservation policy and its implementation. None the less, to paraphrase George Bush, we as co- editors decided to stay the course and separate ecological and socio-economic considerations in the target-setting approaches presented in the various chapters. Our stance on this issue generated interesting discussions, which provided insight for this synthesis.

Ecological degradation is both ubiquitous and relentless. Human activities have left a footprint even in the most remote locations. Some species benefit from certain forms of degradation whereas many others are expected to decline to extinction under current or increasing land-use intensity (Vitousek et al. 1997; Norris and Pain 2002). While the optimal allocation of conservation efforts and funding at the global scale is being debated (Myers et al. 2000; Balmford et al. 2002; O'Connor et al. 2003; Lamoreux et al. 2006), target setting at the landscape scale should be viewed as equally important because, for many taxa, this is the scale over which most human activities take place and management regulations are applied. A landscape can be defined as a mosaic of habitat types whose extent reflects the perspective of target species or taxa (Wiens et al. 2002). However, it should be noted that this organism-centered perspective of the landscape must interact with human perception and action. Forest managers perceive the landscape as that of the “forest” or “forest management unit”, which may cover hundreds of square kilometers.

The landscapes we tend to envision when considering human activities such as timber harvesting or agriculture may match those perceived by many birds and mammals, but not those over which the dynamics of most species (e.g. plants and insects) take place. With the exception of some mega-projects, most human activities tend to alter relatively small patches (e.g. a forest stand or a field).

INTRODUCTION

Biodiversity is a term now commonly used in the political arena. However, it has a fairly strict definition that is widely recognized in ecology. In essence, biodiversity refers to genes, species, and ecosystems as levels of organization, and it includes ecosystem structure and function (Noss 1990). These different aspects of biodiversity must also be the starting point for setting conservation goals for forest landscapes. However, when applied to forest management, biodiversity objectives must be broken down into measurable targets based on clear and, preferably, functional links to the overall goals.

Around the world, relatively pristine forest ecosystems have been preserved through the foresight of a few individuals, have been restored at great cost, or they simply persisted by default owing to slow economic development. In regions that are still undeveloped (e.g. portions of the boreal forest or the Amazon basin), targets may be set as proactive measures to limit impacts of foreseeable economic development (see also Chapter 4, this volume). In regions where conservation planning has maintained an intermediate level of ecological integrity, targets must still be set to protect sensitive species or critical ecological processes (see Chapters 8, 9, and 10, this volume). Finally, conservation targets may also represent useful tools to monitor the success of ecological restoration (see Chapter 11, this volume) in regions where major habitat loss and conversion have taken place.

Any specific target is relevant to a temporal and spatial domain. In addition, it may relate to different levels of conservation ambition.

INTRODUCTION

Habitats for species vary substantially in their stability, whether temporally or spatially. Some habitats, such as rocks and lakes, may be present for centuries and only subject to changes in climate, whereas others, such as dung patches or carcasses, may be available only for a single season or even shorter. This variation in predictability constitutes an important selective force for associated species. It has been suggested that the habitat is the template for ecological strategies (Southwood 1977). In this sense, individuals of a species are confronted with the challenge of determining whether reproduction is best achieved “here” or “somewhere else” as well as “now” or “sometime in the future”. By addressing these tradeoffs, the successful reproductive strategy will represent the life history of the species. Species adapted to long-lasting, predictable habitats are expected to generally be more sedentary; for them, the spatial distribution of the habitat is particularly important. This is a basic ecological starting point for appreciating the need to include time and space to a greater extent in forest conservation management. If only total habitat amount is considered, but the temporal and spatial distribution is ignored, important factors influencing the long-term viability of focal species will be missed and thus, at least for some species, the risk of decline and extinction will be severely underestimated.