6 Wood Discoloration

174 8 Habitat of Wood Fungi
After wounding, tree-own discolorations (deposition of heartwood substances)
develop by living cells, followed by microbial stain and finally by
wood rot (Shigo and Hillis 1973; Hillis 1977; Shortle and Cowling 1978; Bauch
1984; Rayner and Boddy 1988; Fig. 8.6).
Depending on the fungus and tree species, brown, white, or even soft-rot
decay can develop in the tree. Sapwood and/or heartwood can be colonized.
Fungi may be saprobionts of parasites. Development and spread of decay are
influenced by the tree species, which can be susceptible, like birch or poplar,
or exhibits natural durability in its heartwood due to inhibiting accessory
compounds.
It has to be distinguished between passive mechanisms, which are already
present before damage, and active defense mechanisms, which trees trained in
the course of their phylogenetic development to limit wounds, infections, and
senile damages (Blanchette 1992; Duchesne et al. 1992; Rayner 1993).
After the xylem is wounded, two defense functions have to be differentiated:
First, the tree must avoid an interruption of the transpiration stream by air
embolism, and second, limit the spread of invaded microorganisms (Liese and
Dujesiefken 1996).
When a softwood tracheid is injured, its lumen is filled with air at ambient
pressure. Thus, a pressure drop exists across the pit membranes of the watercontaining
neighboring tracheids. Their tori are therefore pulled against their
pit borders, and the air-blocked tracheid is thus sealed off from the waterconducting
tracheids (Zimmermann 1983). Conifers protect themselves from
Fig.8.6. Model of successive changes in
the stem wood after prior injury to the
bark. w wound, c callus margin, f fruit
body, b barrier zone, r rot, m microbial
wood discoloration, t tree-own wood
discoloration (after Shigo 1979)
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