1 Introduction

Even in early cultures, special individual trees were placed under protectionbecause they were deemed to be the abode of deities and spirits. Fromantiquity right up to modern times, legal regulations to protect trees havebeen established for reasons of taboos, aesthetic perceptions and competingforms of utilization, influenced by constantly changing concepts of value. Anew consensus has emerged in the last two decades which considers it sensi­ble to protect trees in urban situations simply for environmental reasons.Society's desire to preserve trees may have profound adverse consequences

for people and property in the event of damage in densely populated urbanand rural areas with high traffic density. With increasing frequency, ques­tions are being asked: what dangers may arise from trees? And how can thehazard potential be estimated or evaluated? Such questions concern not onlytree owners and tree experts, but also the general public.Wood-decay fungi are of particular importance in this connection, as they

can so severely impair the stability and fracture-resistance of trees that theyno longer meet the requirements of traffic safety. Accordingly, in order toavert serious consequences, visual tree assessments are made at regular inter­vals by the VTA method (Mattheck and Breloer 1994; HotZel1996; Mattheckand HotzeI1997), which comprises a visual assessment of foliage, branching,defect symptoms, fungus infection etc. Only if there are clear grounds forsuspicion are further investigations made. The VTA method systematizes theuse of various methods of diagnosis.Fungus identification represents just one contribution towards assessing

the stability and safety (fracture resistance) of infected trees. However, it isonly valuable when further questions can be answered as well, e.g.:1. How great is the extent of wood decay?2. What consistency (mechanical properties) does the decayed wood exhibit?3. How rapid is the expected progression of the decay?

The first two questions involve an assessment of the actual situation, but thethird one involves a prognostic estimate of the future expected progression ofthe decay.These questions are important for several reasons, as the mere occurrence

of a fungus fruit body on a tree does not indicate the extent of the decay(Lonsdale and Schwarze 1995; Schwarze 1995; Schwarze et al. 1997). Degrada­tion processes, host differences and environmental conditions are toodiverse. Numerous empirical investigations show that the occurrence ofwood decay by the fungus Meripilus giganteus in beech will often involve a

F. W. M. R. Schwarze et al., Fungal Strategies of Wood Decay in Trees© Springer-Verlag Berlin Heidelberg 2000

2 Introduction

loss of stability. In contrast, wood decay by the fungus Fistulina hepatica inoak leads only very rarely to the failure of infected stems, and then only inthe later stages of decay. Indeed, there are indications that even wood decaycaused by the same species of fungus will have different effects in differenthosts. In this connection, the fungus Inonotus hispidus represents the clearestexample. Studies on its wood decay pattern, i.e. the way this fungus breaksdown the wood in ash and in London plane trees, have shown that the fungusis much more dangerous on ash than on London plane (Schwarze 1995;Schwarze et al. 1995a; Schwarze and Fink 1997). Moreover, not all fungusinfections, e.g. the mold fungi (Hyphomycetes), will lead to extensive struc­tural changes in the tree. Finally, decays often affect only a small amount ofwood in the tree, so that stability and safety (fracture-resistance) are notimpaired. Information on such differences can help in accurately assessingthe hazard posed by damaged trees. They also help to avoid unnecessaryinterventions resulting from exaggerated safety worries, such as crownreduction measures on safe trees or even fellings.Various diagnostic instruments are now available for discovering and eva­

luating the extent of decay (Eckstein and Sass 1994; Schwarze and Fink 1994;Mattheck et al. 1995; Lonsdale 1999). Valuable criteria have been defined innumerous field studies, which are suitable for estimating the fracture-safetyof a tree more objectively than before, from the ratio between the extent ofdecay and the adjacent load-bearing residual wall thickness (Mattheck andBreloer 1994; Mattheck and Kubler 1995). However, as wood-decay fungi pos­sess different abilities to influence the mechanical properties of a tree, it is offundamental importance that the diagnostic instrument used should alsodetermine the most informative of the changing strength properties(Schwarze and Fink 1994; Schwarze 1995; Schwarze et al. 1995b). Otherwise,far-reaching conclusions cannot be drawn on the extent of the decay. Forexample, Ustulina deusta can drastically reduce the strength properties of atree, while the modulus of elasticity (stiffness) remains unaffected (Schwarze1995). Moreover, even a slight reduction in the modulus of elasticity by 14%can mean that the impact bending strength is reduced by more than 60%(Trendelenburg 1940; Wilcox 1978).When assessing a fungus-infected tree, mere recording of the actual con­

dition will in many cases be unsatisfactory or insufficient for the client or theperson responsible. Decay with a small extent, or massive decay in trees withobviously severe loss of vigor on a site exposed to traffic are obvious excep­tions. In the former case no further detailed investigation is needed, while inthe latter case necessary measures must be taken to remove the danger orrestore the stability and fracture-safety of the tree. In many cases where theextent of decay is approaching the threshold of the VTA failure criterion (i.e.load-bearing residual wall thickness/tree radius=0.3) the question arises:how fast is the decay likely to progress in the future? This will depend on thetree's own responses and conditions such as the nature of the wood, its con­tent of anti-fungal substances, moisture relationships, formation of reactionzones etc., and also on what mechanisms the fungus possesses to overcomethese limitations.

Introduction 3

Although many studies have been made on active reaction mechanisms ofa host to wood-decay fungi in sapwood (Shigo 1986), there are only very fewstudies on pathogenicity and thus on the way in which many fungal speciespenetrate the tree's own reaction zones (Pearce 1997). Recent studies showthat fungi classified as white rot, e.g. Inonotus hispidus, can switch to a softrot mode in parts of a reaction zone. Thus the fungus avoids the anti-fungalconditions there and will penetrate reaction zones successfully (Schwarzeand Fink 1997). Basically, when making a prognosis for a fungus-infected treeone must ask oneself:1. Is any reaction- or barrier-zone actually present?2. Can a reaction- or barrier-zone actually prevent the spread of the particu­lar fungus species in the long term?

These aspects are discussed in detail in Chapter 4: Host-Fungus Interactions:Development and Prognosis ofWood Decay in Sapwood.First, however, in Chapter 2 we shall discuss fundamental information on

cell-wall structure and wood decay phenomena. Then, in Chapter 3, we shallpresent detailed information on individual fungi on different hosts. Themacroscopic and microscopic changes caused by fungi in the substrate willbe explained by means of a fungus description with illustrations.