6 Wood Discoloration

64 3 Physiology
intermolecular cavities in the cell wall dry (liquid movement in wood: Siau
1984; Skaar 1988).
From the view of a hypha, a low water availability begins to become critical,
if free water is no more located in the cell lumen void space, but liquid water
exclusively within the cell wall and only water vapor in the lumen, or in other
words, if the cell walls are fully hydrated yet with no water contained in the
cellular spaces. This condition is defined as fiber saturation point or range
(Babiak and Kúdela 1995) and lies at about −0.1 MPa (0.9993 aw), according to
1.5µm pore radius (Table 3.5) and about 30% u wood moisture for woods of
the temperate zones. The lower limit for wood degradation by Basidiomycetes
is about −4 MPa (0.97 aw).
Below fiber saturation, not only fungi are influenced by the moisture content,
but also all technological properties of wood. With increasing moisture, e.g.,
elastic, strength, and insulation properties decrease.
Relative air humidity (RH), which is in equilibrium with a substrate, and
water activity of a substrate stand in the relationship: RH(%) = aw × 100. For
example, 99.93% RH correspond to 0.9993 aw and thus to fiber saturation, so
that the critical range for Basidiomycetes of 0.97 aw (Table 3.5) is exceeded by
condensation in buildings. The S-shaped sorption isotherms, which indicate
the dependence of the wood moisture on the relative air humidity of the environment,
are shown by Siau (1984) and Kollmann (1987). Wood is dry at the
relative vapor pressure of 0, and fiber saturation is reached at 1 (100% RH).
Spruce sapwood samples placed over a saturated solution of K2SO4, which
results in 97% RH and 26.5% u, showed 4.5% mass loss after 3 months of
incubation with S. lacrymans (Viitanen and Ritschkoff 1991a). Wood samples
in 93% RH according to 23–24% wood moisture content were overgrown by
S. lacrymans and Coniophora puteana (Savory 1964). For the initial colonization,
21% u was necessary (Huckfeldt et al. 2005; cf. Table 8.7). Coniophora
puteana colonized wood samples of 18% moisture content when a moisture
source was 20–30 cm away from the wood. Timber in buildings reached however
till 45% humidity in the winter during night by condensation (Dirol and
Vergnaud 1992).
According to Skaar (1988), the wood moisture content of living trees
amounted to 83% u in hardwoods in the sapwood and to 81% in the heartwood
(average of 34 species) and in conifers to 149% in the sapwood and to 55% in
the heartwood (average of 27 species).
The moisture content in dead wood is determined by several factors:
– fungal decay: For example, the wood moistures of dry heartwood samples
of different wood species increased during decay by Trametes versicolor in
84 days to 78–236% and by Oligoporus placenta to 108–286% (Smith and
Shortle 1991). Regarding the sorptive capacity of wood (Cowling 1961; Anagnost
and Smith 1997), Rawat et al. (1998) showed that the moisture content
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