The Science Behind Wood Rot Resistance
Have you noticed how some rotten logs appear brown while others appear whiteish? This is due to different kinds of fungi acting.
Some wood species naturally resist decay, such as old-growth cypress and cedar. Well-known domestic examples are osage orange and black locust; their durability decreases depending on stem position.
Cellulose
Cellulose is a complex carbohydrate or polysaccharide composed of thousands of glucose molecules linked together by b-1-4 glycosidic bonds, found throughout plant cells and believed to be one of the most abundant organic compounds on Earth. Although indigestible by humans, herbivorous animals like cows and horses digest it via bacteria in their digestive systems; providing structure and strength to plant cells while acting as the main source of dietary fiber in our diets.
Wood’s strength lies in the combination of its three chemical constituents – cellulose, hemicellulose and lignin – as its three core chemical constituents. Lignin is an aromatic polymer that increases hardness of its cell wall while binding microfibrils of cellulose together (Gindl et al. 2002). Cellulose and hemicellulose consist of long chains composed of glucose molecules linked together into long glucan chains; while lignin incorporates both types into its cell wall together with cellulose. Hemicellulose components include xylans, glucomannans galactoglucomannans to form integrated wood cell wall structures (Gindl et al. 2002).
Lignin is an extremely strong protective polymer, offering protection to tree cells against decay fungi and other organisms, while contributing to coloration of bark and sapwood. When wood is exposed to elements, lignin becomes degraded first by water then decay fungi; once degraded water moves into cells to start attacking their cellulose content.
Fungi attacks cellulose to break it down and disintegrate into smaller pieces that are then digested by them to release their nutrients, which in turn help the fungi thrive and reproduce.
As wood decays, its integrity deteriorates as its cell walls become compromised by decay fungi that attack its cellulose and hemicellulose components, leaving holes for moisture to enter and promote further decay fungi growth. This creates pores in the wood which allow decay fungi to grow within it and infect other nearby spaces with their pathogens.
Wood rot resistance lies in maintaining moisture content below 20%, known as fiber saturation point. At this moisture level, wood won’t swell, acting as food source for decay fungi instead.
Lignin
Lignin is a dense aromatic compound found in plant cell walls that forms an elastic matrix around cellulose and hemicellulose molecules to provide strength, stiffness, and protection from microbial attacks. Wood contains embedded lignin that makes its presence felt due to being embedded within cellulose/hemicellulose networks and difficult to break down; additionally it acts as an inefficient water conductor and delays moisture movement, keeping humidity under control and helping avoid excessive humidity build-up in wood products.
Fungi are nature’s main decomposers of lignin, with white-rot and brown-rot basidiomycetes serving as particularly effective decomposers of this material. Fungi break down lignin by nonenzymatic modifications and degradation processes such as oxidation or the removal of methyl groups from aromatic rings; they may also cleave ether bonds within molecules to separate polysaccharides from them – although they cannot completely break down its structure.
Studies on wood decay resistance have demonstrated that the composition of lignin monomers plays a key role in susceptibility to fungal degradation, with Syringyl and Guaiacyl monolignols more resistant than Eugenol or P-Coummarate monolignols to degradative reactions than Eugenol or P-Coumarates for instance. Furthermore, concentration as well as presence of ether bonds or phenolic groups play important roles as well.
Studies of transgenic poplar trees with modified lignin monomer composition showed that changes to the ratio of syringyl to guaiacyl monomers significantly increased fungal-induced cell wall degradation, suggesting that altering this trait might provide an effective method for increasing wood rot resistance by modulating fungal degradation.
Wood rot fungi require free water in order to attack cellulose and hemicellulose structures within tree cell walls, leading it likely to only occur when moisture content exceeds what is known as its fiber saturation point (FSP).
Obst et al. discovered, after conducting tests to investigate various species of trees for resistance to fungal degradation, that bark acts as a primary defense mechanism against infection; no stem decay fungus had ever invaded an intact bark area.
Water
Wood’s resistance to decay depends heavily on its water content, measured through cell wall pore water saturation levels. Therefore, an understanding of this relationship between pore water saturation levels and fungal decay is critical in order to interpret durability test results accurately and devise efficient wood protection systems.
Wood absorbs water through its cell wall via interactions between cellulose and lignin compounds and water molecules, with higher temperatures increasing water absorption while lower ones decrease it. Humidity and temperature play an equally significant role when it comes to wood’s ability to take in moisture; faster absorption is seen at higher temps while absorption decreases at lower temps.
Fredriksson and co-workers define the hygroscopic moisture range (up to 95-98% RH) as capillary condensation of macro-voids within cell walls such as pit chambers and lumens, where water from air can enter via capillary action and be stored by form of hydrogen bonds with hydroxyl groups found within their structures.
Super-hygroscopic water can significantly decrease wood’s resistance to fungal decay (Fredriksson and Thybring 2018). At RH levels of 95-98% or above, over-hygroscopic moisture range occurs where water absorbs through capillary condensation rather than active transport from its surroundings and has less of an effect on cell walls and is thus less resistant to fungal degradation (Fredriksson and Thybring 2019).
Water can affect both rot resistance of wood and its penetration by brown-rot fungi. Brischke et al. (2018) demonstrated this fact by showing that mycelia of these organisms can penetrate cell walls more readily when present; Brischke et al. found that both brown-rot hyphae can reduce pore water saturation within its hygroscopic moisture range while simultaneously decreasing electrical resistance at certain moisture conditions provided that their network stays alive.
Wood rot can be prevented or reduced through proper site drainage and waterproofing measures, along with appropriate ventilation of wooden structures to reduce fungal colonisation risks. Furthermore, foundations must not come in direct contact with groundwater or soil; gutters and downspouts can help direct water away from their foundation while creating a waterproof barrier against it.
Soil
Soil plays an essential role in supporting plant growth, providing nutrients and water for crops, protecting archeological treasures and sequestering carbon. Recent scientific research has also demonstrated its vitality for sequestering carbon emissions, reducing water runoff and improving biodiversity. With increasing consumer demand for healthy foods, more farmers are adopting sustainable farming practices such as no-till or cover cropping to help boost soil health.
Wood rot is caused by the breakdown of cellulose and lignin in wood, leading to its degradation and subsequent degradation of buildings or other structures. As this can result in costly repairs and safety hazards, homeowners must be aware of factors contributing to wood rot development so they can take steps to prevent it.
As moisture is necessary for fungal growth in wood, its development begins through rain penetration, poor construction, or lack of ventilation. Once moisture enters the wood surface and enters its core, fungi are free to start the rotting process and bring forth irreparable destruction.
Fungus needs oxygen to convert cellulose and hemicellulose into sugars for nourishment, so when its source is cut off it dies; hence why it is crucial that moisture levels in wood remain low.
Temperature also plays an essential role. Fungus grows more rapidly in warm weather than cold. Each species of fungus has an ideal range for its optimal growth; Forest Products Laboratory tests have determined that most thrive between 65-95 degF; however, growth begins slowing and eventually stopping at temperatures slightly beyond freezing point.
An additional factor in wood rot resistance is density of wood. According to research on brown rot fungi in poplar trees, density was directly correlated with decay resistance – the higher its density was, the better its resistance against brown rot; as this is because the fungus mostly spread through cell lumens, parenchyma cells, and cell wall pits.