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A Background on Wood Decay Fungi
From Appendix B of the doctoral thesis by Scott M. Kent, PE, PhD
Kent, S. 2004. The effect of biological deterioration on the performance of nailed oriented strand board sheathing to Douglas-fir framing member connections. Doctoral dissertation, Oregon State University, Corvallis, Oregon.
Fungi responsible for wood decay are part of a kingdom of organisms that have membrane bound organelles and obtain their energy and nutrition requirements by metabolizing organic compounds. They are incapable of photosynthesis, although some have developed symbiotic relationships with photosynthetic microbes. Fungi cells are structured into networks of long strands called hyphae, which are further organized into mycelium (Eaton and Hale 1993).
For successful growth and reproduction, fungi require a food source, free water, oxygen, and moderate temperatures (Forest Products Laboratory 1999). Removal of any one requirement either kills the fungus or forces it into a dormant state. For example, wood preservatives work by poisoning the wood with chemicals, which typically interfere with the respiration processes of fungi. Covered bridges limit exposure of the wood structural members to water. Storage of logs in mill-ponds and structural piles driven below the water table are preservation techniques which limit the availability of oxygen. Temperature can be used in the laboratory to control fungi growth, however it is difficult to use temperature as a reliable mechanism to control decay processes in structures (Zabel and Morrell 1992).
The wood cell wall becomes a food source for fungi in the decay process. The structural components of the cell wall are macro-polymers of cellulose (homopolysaccharides), hemicellulose (heteropolysaccarides), and lignin (phenlypropane polymers). Wood decay fungi have developed enzymes capable of breaking down one or more of these wood components into simple sugars, which diffuse into the hyphae and are used to generate energy through aerobic respiration.
Decay processes require free water, which serves as an essential component in hydrolysis of wood by fungi, is necessary as a swelling agent, and provides a transport mechanism for enzymes. Water can exist in wood as free water in the lumens, vapor in the lumens, or as bound water in the cell walls. Below the fiber saturation point, approximately 30 percent for most softwood species, free water is not readily available, thereby limiting the growth of fungi. As the moisture content increases above the fiber saturation point, so does the propensity for decay to occur. For softwoods, the ideal moisture content for most types of decay fungi is between 40 and 70 percent. Above this range, the availability of oxygen becomes limited causing a decrease in the rate of decay (Eaton and Hale 1993). Snell (1929) identified the upper moisture content limit for optimum growth in Douglas-fir as approximately 70 percent. At moisture contents greater than approximately 110 percent, decay processes are inhibited.
Fungi responsible for wood decay are aerobic and require oxygen to metabolize carbohydrates. Snell's experiments (1929) indicated that 20 percent of the bulk volume of wood needs to be air in order for decay to occur. Jensen (1967) studied the sensitivity of Polyporus suiphureus (a brown rot fungus) and other fungi to atmospheres containing various concentrations of carbon dioxide and oxygen. Increasing the oxygen concentration from zero to approximately 15 percent of the atmosphere promotes the growth of fungi, but at a decreasing rate. Above an oxygen concentration of 15 percent, nutrient supply and other biological processes control growth rate. Tests by Scheffer and Livingston (1937) on the effect of oxygen partial pressure and temperature showed that the minimum oxygen concentration for fungi growth was between 0.2 and 1.3 percent. Later research by Scheffer (1985) demonstrated that fungi do not grow at oxygen concentrations below 0.4 percent. Fungi survivability without oxygen varies between I week and 24 months (Scheffer 1985).
The optimum temperature range for fungi growth is between 20 and 30° C, although some fungi flourish in temperatures up to 40° C. Fungi become dormant at temperatures below 5° C until more favorable environmental conditions arise. High temperatures (greater than 40° C) canbe an effective decay suppressant in conditions that would otherwise be favorable for wood decay. Decay fungi can be killed by exposure to temperatures over 60° C (Scheffer and Livingston 1937).
Types of Wood Decay Fungi
There are three general types of wood degrading fungi, classified based on their effect on the wood: brown rot, white rot, and soft rot (FPL 1999). Brown rot fungi consume the cellulose and hemicellulose leaving the decayed wood looking dark brown, dry, and heavily checked across the grain. Brown rot is considered the most destructive and economically important decay type for commercial softwoods. It is estimated that up to 80 percent of the cases of wood decay in wood members used as building materials are caused by brown rot fungi (Green and Highley 1997). Interestingly, this group accounts for only six percent of the wood deterioration in forested areas (Gilbertson and Ryvarden 1980). Many white rot fungi can use all wood components and cause the wood to look bleached. Soft rot fungi flourish in high moisture content and nearly anaerobic conditions unfavorable to the other types of decay fungi. Soft rot damage is characterized by a shallow region of spongy decay on the wood surface(Carll and Highley 1999).
Many basidiomycetes responsible for decay of wood in buildings are capable of producing distinctive fruiting structures during late stages of decay. The mycelium is typically light in color, often white or yellow. Wood decay caused by brown rot fungi is sometimes erroneously called dry rot in its late stages due to the dry crumbly appearance of the damaged wood. There is, however, one type of brown rot, a true dry rot called Serpula Iaciymans, which can transport water over long distances across inert materials such as stone, concrete, and masonry (Eaton and Hale 1993).
Mechanical Property Changes
A variety of mechanical properties of wood are adversely affected by the biological activities of fungi. Fungi obtain energy from the breakdown of cellulose, hemicellulose, and lignin in the wood structure, converting them to carbon dioxide, water, and other byproducts of aerobic respiration. Significant reductions in some mechanical properties occur in the early stages of decay at weight losses less than 10 percent. For a given weight loss, wood samples subjected to brown rot fungi tend to experience greater losses in the mechanical properties than samples subjected to white and soft rot fungi (Wilcox 1978). The effects of brown rot fungi on the mechanical properties of wood become detectable by non-destructive methods and microscopic evaluation between 5 and 10 percent weight loss (Wilcox 1978).Strength loss in the early stages of decay by brown rot fungi may be attributed to depolymerization of the cellulose polymers, a linear chain of repeating glucose monomers each joined by an oxide bridge, through hydrolases. Hydrolases is a classification of enzymatic action that breaks down compounds at an oxide bridge. Brown rot fungi broadcast exocellular enzymes that diffuse in the free water. Some of the enzymes, termed exohydrolases, reduce the long polymer chains to monomers or dimers by successively attacking the ends of the polymers. Other enzymes, termed endohydrolases, are capable of breaking polymers at any point, thereby increasing the number of sites available for the exohydrolases. The suite of enzymes used by fungi to digest cellulose is called cellulase (Zabel andMorrell 1992). Degradation of the cellulose often occurs at a higher rate than brown rot fungi can metabolize the material. This results in a significant decrease in mechanical properties, particularly toughness, at low weight losses. In contrast to brown rot fungi, degradation of the wood structure bywhite rot fungi tends to be localized, causing thinning of the cell walls and more gradual losses in mechanical properties. White rot fungi degrade wood in the proximity of the hyphae ends rather than the broadcasting technique employed by the brown rot fungi (Akande 1990).
Chemical Property Changes
Winandy and Morrell (1993) and others (Curling et al. 2002) proposed hemicellulose depletion as a mechanism for early loss of mechanical properties due to decay by brown rot fungi. Early strength losses were associated with arabinose and galactose degredation, whereas glucose, the monomer of cellulose, was only slightly degraded by the brown rot fungi and not significantly degraded by the white rot fungi. In the study by Curling et al.(2002), bending strength losses between 0 and 40 percent occurred at an average weight loss less than five percent and were accompanied by hemicellulose degradation, specifically the galactan and arabinan carbohydrates. Between 40 and 80 percent bending strength loss, mannan and xylan hemicellulose carbohydrates were degraded. Bending strength losses in excess of 80 percent were noted by glucan degradation, 90 percentof which was from the cellulose. Winandy and Morrelt (1993) indicate that although cellulose depolymerization may account for a portion of the early strength loss by brown rot fungi, hemicellulose degradation may be responsible for the majority of early strength loss.
Weight Loss Metric
Laboratory techniques such as the soil-block test (AWPA 2000) are used to characterize fungal attack under controlled environmental conditions. Although this test method exposes wood samples to a severe decay exposure, exceeding that of non-ground-contact wood-based building products in-service, the test is primarily used to compare the durability of various wood species and the effectiveness of preservatives. Results from soil-block and similar in-vitro durability tests are typically expressed as weight loss, expressed as a percentage of the original oven-dry weight. Nilsson and Daniel (1992) noted that the weight loss metric might be misleading when results, especially decay rate, of different experiments are compared that used different size samples. The specific surface (total surface area of the specimen divided by its volume) of a test specimen influences the rate at which decay occurs; small samples have more surface area per unit volume than large samples. Decay potential, defined as the weight loss, expressed in mass units, divided by the original wood volume, may provide another useful metric to compare decay rates between experiments.
Akande, A. 1990. Failure in wood related to decay weight losses. Forest Products Journal. 40(7/8): 47-53.
AWPA. 2000. Standard method of testing wood preservatives by laboratory soil-block cultures. E10-91. AWPA book of standards. Granbury, TX.397-407.
Caril, C., and T. Highley. 1999. Decay of wood and wood-based products above ground in buildings. Journal of Testing and Evaluation. 27(2):150-1 58.
Curling, S., J. Winandy, C., and A. Clausen. 2000. An experimental method to simulate incipient decay of wood by basidiomycete fungi. IRG/WP/00-20200. In: Proc. 31st Annual Meeting International Research Group on Wood Preservation. May 14-19, Kona, Hawaii. International Research Group on Wood Preservation, Stockholm, Sweden. 12 pp.
Eaton, R., and M. Hale. 1993. Wood decay, pests, and protection. Chapman and Hall, London, England. 546 pp.
Forest Products Laboratory (FPL). 1999. Wood Handbook: wood as an engineering material. Agriculture Handbook 72. Washington, DC: U.S. Department of Agriculture. 466 pp.
Gilbertson, R., and L. Ryvarden. 1980. Wood-rotting fungi of North America. Mycologia. 72(1): 1-49.
Green, F., and T. Highley. 1997. Brown-rot wood decay - insights gained from a low-decay isolate of Postia placenta. Trends in Plant Pathology. 1: 1-17.
Jensen, K. 1967. Oxygen and carbon dioxide affect the growth of wood decaying fungi. Forest Science. 13(4): 384-389.
Nilsson, 1., and G. Daniel. 1992. On the use of percent weight loss as a measure for expressing results of laboratory decay experiments. IRG/WP/2394-92. In: Proc. 23rd Annual Meeting International Research Group on Wood Preservation. May 10-15, Uppsala, Sweden. 5 pp.
Scheffer, T. 1985. 02 requirements for growth and survival of wood decaying and sapwood-staining fungi. Canadian Journal of Botany.64(9): 1957-1963.
Scheffer, T., and B. Livingston. 1937. Relation of oxygen pressure and temperature to growth and carbon dioxide production in the fungus Polystictus versicolor. American Journal of Botany. 24(3): 109-119.
Snell, W. 1929. The relation of the moisture contents of wood to its decay. II. American Journal of Botany. 16(7): 543-546.
Wilcox, W. 1978. Review of literature on the effect of early stages of decay on wood strength. Wood and Fiber. 9(4): 252-257.
Winandy, J., and J. Morrell. 1993. Relationship between incipient decay, strength, and chemical composition of Douglas-fir heartwood. Wood and Fiber Science. 25(3): 278-288.
Zabel, R., and J. Morrell. 1992. Wood microbiology - decay and its prevention. Academic Press, Inc. San Diego, California. 476 pp.