6 Wood Discoloration

178 8 Habitat of Wood Fungi
for branches without branch collar are also part of the European Tree Pruning
Guide. According to the branch attachment (branches with or without a collar),
the cut has to be done outside the stem so that the branch bark ridge is not
damaged. Flush cuts have to be avoided. When pruning dead branches, the
distinctive swelling at branch base must remain at the stem. Regardless of the
time of year and the tree species, radical tree pruning, e.g., a drastic removal
of crown parts, should not be done. If possible, branches greater than 5 cm
in diameter of weak compartmentalizing trees (e.g., Aesculus, Betula, Malus,
Populus, Prunus, Salix), and greater than 10 cm of strong compartmentalizing
trees (e.g., Carpinus, Fagus, Quercus, Tilia) should only be reduced partially
rather than completely.
For organizational reasons and due to nature protection laws, pruning is
usually done during the dormant season. However, wounding of maple, birch,
beech, oak, ash, lime tree and spruce showed on the basis the intensity of
the wood discolorations that injuries should be avoided in hardwoods during
the dormant stage and in spruce from late summer to winter due to different
wound reactions (Lenz and Oswald 1971; Armstrong et al. 1981; Dujesiefken
et al. 1991; Schmitt and Liese 1992b).
8.2.3
Wound Treatment
In the 50s and 60s, large stem wounds were shaped out and filled with concrete.
Since concrete and wood shrink and extend differently under weather
influence, shakes develop, water penetrates and leads to rot. Since the 70s, the
cleaned wounds were treated with wound dressings or with wood preservatives.
Disinfection of the opened wood body with ethanol or alcoholic iodine
solutionbeforewoundtreatmentdidnotledtoapreventionofdiscoloration
and decay in beech and ash (Dujesiefken and Seehann 1995). The use of wood
preservatives was disputed for tree care measures, as they are not developed for
theprotectionoftreewounds.Thetreatmentofartificialwoundswithwood
preservatives resulted in beech in more intensive discolorations behind the
wound area than at wounds, which were only treated with wound dressings.
Wound dressings belong to the plant preservatives. In Germany, they must
be tested according to efficacy and environmental compatibility to become
licensed (Balder 1995).
Alternatively, cavities can be foamed with polyurethane (Dujesiefken and
Kowol 1991). Figure 8.10 shows reduced discoloration in a beech tree after
filling the wound with polyurethane.
Currently, traffic wounds on street trees are covered by black plastic wraps,
which promote the development of a surface callus overgrowing the wound
area (Fig. 8.11).
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