As wood degradation by fungi involves enzymes, which are active in aqueous environment, and because hyphae consist of up to 90% of water, wood fungi need water. Water is also used for the uptake of nutrients, the transport within the mycelium and as solvent for metabolism. Without water, the metabolism rests. The resting phase occurs by means of spores, in wood fungi particularly by chlamydospores.
survived. Water is taken up from the substrate wood, the soil, and from masonry etc. Altogether the moisture content of wood is the most important factor for wood degradation by fungi and thus also for wood protection. Moisture in wood exists in two different forms: Bound or hygroscopic water occurs within the cell wall by means of hydrogen bounds at the hydroxyl groups mainly in the cellulose and hemicelluloses and to smaller extent in the lignin.
Free or capillary water in liquid form is located in the cell lumen as well as in other holes and cavities of the wood tissue (e.g., Siau 1984; Smith and Shortle 1991). There are several methods of measuring wood moisture content (Vermaas 1996): oven-drying method, microwave drying Danko (1994), distillation, Karl Fischer-titration, moisture meters based on electrical and dielectrical proper-ties, continuous moisture meters, capacity admittance moisture meters, and hygrometric methods. Determination of the moisture content without destruc-tion is done electrically by means of resistance measurement (Skaar 1988; Du et al. 1991a, 1991b, Bohner et al. 1993. With increasing moisture content of wood from the oven-dry phase to the fiber saturation range (about 30% u) the electrical resistance decreases approximately by the factor 1:106.
Moisture can be rapidly determined in practice using an indelible pencil that is thepencil line runs if the fiber saturation point is exceeded. Theproportional wood moisture (% u) is determined gravimetrically by the wood mass before and after drying a wood sample at 103 ± 2 °C: u (%) = [(MW — MD) : MD] x 100 (MW = mass of wet wood, MD = mass of dry wood) Wood samples may be also conditioned to specific relative humidity conditions prior to and after decay, e.g., at 20 ± 2 °C and 65 ± 5% relative air humidity. With the latter method, the theoretical dry weight (MDt) of a sample results from: MDt = (100 x MC): (100 + u) (MC = mass after conditioning, u = % wood moisture after air conditioning).
However, weight loss meth-ods using moisture-conditioned wood samples instead of oven-dry blocks are influenced by changes in hygroscopicity: For brown-rot, mass loss is slightly overestimated, for white rot, no difference occurs, while for soft rot, mass loss is slightly underestimated using the moisture-condition method (Anagnost and Smith 1997).
To quantify the moisture content of fungal nutrient substrates, including wood, only the proportional water content of the substrate was considered in previous investigations. At the disposal to microorganisms, however, not the whole water content of the substrate is available, but only that part of the total water, which is not bound by solved substances (salts, sugars, etc.). The relative vapor pressure of a substrate (water activity aw, 0-1) results from the quotient
survived. Water is taken up from the substrate wood, the soil, and from masonry etc. Altogether the moisture content of wood is the most important factor for wood degradation by fungi and thus also for wood protection. Moisture in wood exists in two different forms: Bound or hygroscopic water occurs within the cell wall by means of hydrogen bounds at the hydroxyl groups mainly in the cellulose and hemicelluloses and to smaller extent in the lignin.
Free or capillary water in liquid form is located in the cell lumen as well as in other holes and cavities of the wood tissue (e.g., Siau 1984; Smith and Shortle 1991). There are several methods of measuring wood moisture content (Vermaas 1996): oven-drying method, microwave drying Danko (1994), distillation, Karl Fischer-titration, moisture meters based on electrical and dielectrical proper-ties, continuous moisture meters, capacity admittance moisture meters, and hygrometric methods. Determination of the moisture content without destruc-tion is done electrically by means of resistance measurement (Skaar 1988; Du et al. 1991a, 1991b, Bohner et al. 1993. With increasing moisture content of wood from the oven-dry phase to the fiber saturation range (about 30% u) the electrical resistance decreases approximately by the factor 1:106.
Moisture can be rapidly determined in practice using an indelible pencil that is thepencil line runs if the fiber saturation point is exceeded. Theproportional wood moisture (% u) is determined gravimetrically by the wood mass before and after drying a wood sample at 103 ± 2 °C: u (%) = [(MW — MD) : MD] x 100 (MW = mass of wet wood, MD = mass of dry wood) Wood samples may be also conditioned to specific relative humidity conditions prior to and after decay, e.g., at 20 ± 2 °C and 65 ± 5% relative air humidity. With the latter method, the theoretical dry weight (MDt) of a sample results from: MDt = (100 x MC): (100 + u) (MC = mass after conditioning, u = % wood moisture after air conditioning).
However, weight loss meth-ods using moisture-conditioned wood samples instead of oven-dry blocks are influenced by changes in hygroscopicity: For brown-rot, mass loss is slightly overestimated, for white rot, no difference occurs, while for soft rot, mass loss is slightly underestimated using the moisture-condition method (Anagnost and Smith 1997).
To quantify the moisture content of fungal nutrient substrates, including wood, only the proportional water content of the substrate was considered in previous investigations. At the disposal to microorganisms, however, not the whole water content of the substrate is available, but only that part of the total water, which is not bound by solved substances (salts, sugars, etc.). The relative vapor pressure of a substrate (water activity aw, 0-1) results from the quotient
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