Information

How does ultraviolet light influence the decay process of wood?


Given a piece of wood, how does putting it in the sun or not influence the rate of decay due to rot (assuming that it's in an otherwise humid environment)?


Ultraviolet light is microbicidal. See this post. As I answered in this previous post, micro-organisms are essential for rotting. Since UV kills these microbes it will reduce the rate of rotting. The heat from sun will also dry the wood and further decelerate rotting.


Breaking down wood decomposition by fungi

Credit: Amy Zanne

Through a combination of lab and field experiments, researchers have developed a better understanding of the factors accounting for different wood decomposition rates among fungi. The new findings reveal how an understanding of fungal trait variation can improve the predictive ability of early and mid-stage wood decay, a critical driver of the global carbon cycle.

Fungi play a key role in the global carbon cycle as the main decomposers of litter and wood. While current earth system models represent only little of the functional variation in microbial groups, fungi differ greatly in their decomposing ability. The researchers set out to find which traits best explain fungal decomposition ability to help improve the current models.

Among the researchers' specific findings:

  • The hyphal extension rate-or fungal growth rate-is the strongest single predictor of fungal-mediated wood decomposition.
  • Decomposing ability varies along a spectrum from slow-growing, stress-tolerant fungi that are poor decomposers, to fast-growing, highly competitive fungi that have fast decomposition rates.
  • Slow growing, stress-tolerant fungi with poor intrinsic wood decaying abilities are more likely to exist in drier forests with high precipitation seasonality. In contrast, fast-growing, highly competitive fungi tend to be found in more favorable environments and decompose wood more quickly, regardless of the local microclimate.
Credit: Amy Zanne

"Fungi are largely hidden players. We know they are critical for cycling carbon but it has been difficult to determine the effects of different decomposers in causing fast or slow decomposition. As we identify who fungal decomposers are in rotting logs and what allows a particular species to affect these rates, we can better predict carbon cycling around the globe under current and future climates. Our study takes a first step towards this, pairing complementary field and laboratory experiments to find that how fast different fungi grow and what enzymes they produce matter," said Amy Zanne, associate professor of biology at the George Washington University.

Credit: Amy Zanne

"Fungi differ massively in how quickly they decompose wood, releasing carbon back into the ecosystem. Our study identifies different fungal traits that explain this variation, which has great potential to improve predictions of the carbon cycle in forests," said Nicky Lustenhouwer, lead author and postdoctoral scholar at the University of California, Santa Cruz.

"We show that the same processes that determine where a fungus lives—that is, its ability to displace other fungi vs. survive in stressful environments—closely aligns with its decomposition ability. This connection allows us to translate an ecological mechanism into broad-scale patterns in microbial decomposition rates, helping to address a key uncertainty in earth system models," said Daniel Maynard, postdoctoral researcher at Crowther Lab, ETH Zurich.

The paper, "A trait-based understanding of wood decomposition by fungi" will be published in the Proceedings of the National Academy of Sciences during the week of Monday, May 11.


Natural Wood Defects

During its lifetime, a tree is subjected to many natural forces that cause defects in the wood. Woodworkers are quite familiar with these defects – knots, splits, ugly dark streaks or stains, worm holes, even decay. Some of the more common wood defects all woodworkers face include:

Bark pockets – Formed when a small piece of the bark protrudes into the lumber. This area is generally considered unsound.

Bird pecks – Caused by birds, especially woodpeckers, which peck on trees mainly to cause panic to the insects living in or under the bark and in the wood of the tree. This causes the insects to come out enabling the birds to eat them. Bird pecking can cause small injuries to the tree, resulting in grain changes that later show up as various forms of figure in the wood (figure is the “look” or appearance of a piece of wood).

Burls – Burls are a deformed growth formed when a tree receives a shock or injury in its young age. Due to its injury, the tree’s growth is completely upset and irregular projections appear on the body of the timber.

Continued tree growth follows the contour of the original burl deformity, producing all manner of twists, swirls and knots in the wood fiber. Usually, this results in spectacular patterns in the wood that can be used to great effect in woodworking. Burl wood is normally darker than the rest of the tree and, in some cases, may be a significantly different color altogether.

Coarse grain – If the tree grows rapidly, the annual rings are widened. It is known as coarse grain timber and possesses less strength.

Fungal damage – Fungi generally damages timber or wood by discoloration and/or decay. The resulting wood is generally weaker or of a different color than is typical for that species. The more common effects of fungal damage include:
Blue stain – Common in pine, maple, and many other woods, blue stain (also called “sapstain”) is caused by a fungus that feeds on the sap. It does not live in live trees due to lack of oxygen. The bluish color (sometimes gray or dark gray) is the fungus itself, not the color of the sapwood. The color does not degrade the cellular structure and does not count against wood in the grading process.
Brown rot – A form of wood decay found only in softwoods that destroys the wood’s cellulose, eventually causing cracks across the grain. Advanced brown rot tends to leave the wood more brown than normal. It is a precursor to dry rot.
Dry rot – After the wood infected with brown rot dries out, the cell walls of the remaining wood turns into dry powder when crushed. This is called dry rot.
Heart rot – This is formed when a branch has come out of the tree. The heart wood is exposed to an attack of atmospheric agents. Ultimately, the tree becomes weak and it gives a hollow sound when struck with a hammer.
Wet rot – Some kinds of fungi cause chemical decomposition of a wood’s timber and in doing so converts timber into a grayish brown powder known as wet rot. Alternative wet and dry conditions favor the development of wet rot. If unseasoned or improperly seasoned timber is exposed to rain and wind, it easily becomes vulnerable to wet rot attack.
White rot – This is just the opposite of brown rot. In this type of fungi attack, the wood’s lignin and the wood itself assumes the appearance of a white mass consisting of cellulose compounds. Some of the white rots during their early stages of development form what is commercially termed “spalted wood.” This wood has a unique color and figure, and some woodworkers highly prize it.

Insect defects – There are a number of insects that eat wood. Many other insects use wood as a nesting place for their larvae which results in holes and tunnels in the wood. The damage they cause ranges from minor to catastrophic. Some of the more common insects include:
● Wood boring beetles – Wood boring beetles, such as buprestid, powder post, ambrosia, furniture, and longhorn, tunnel through wood to deposit their larvae. Some larvae eat the starchy part of the wood grain. Many species attack live but usually stressed trees, while others prefer recently dead hosts.
Pin-hole borers – They damage fresh-cut logs and unseasoned lumber, but also attack weakened, stressed, dying trees, and healthy trees with bark injuries.
Termites – Termites not only tunnel through wood in various directions, but eat away the wood from the cross-section core. They usually do not disturb the outer shell or cover. In fact, the timber piece attacked by termites may look sound until it completely fails.

Knots – A knot is the base of a branch or limb that was broken or cut off from the tree. The portion of the remaining branch receives nourishment from the stem for some time and it ultimately results in the formation of dark hard rings known as knots. As the continuity of wood fibers are broken by knots, they form a source of weakness. There are several types of knots:

Sound (or tight knots) are solid and cannot be knocked loose because they are fixed by growth or position in the wood structure. They are partially or completely intergrown with the growth rings.
Unsound knots (or loose knots) are knots which fall out of the lumber when pushed or have already fallen out. They are caused by a dead branch that was not fully integrated into the tree before it was cut down.
Encased knots are those which are not intergrown with the surrounding wood.
Knothole is a hole left where the knot has been knocked out.
Spike knots are limbs which have been cut across or cut lengthwise, showing the endwise or lengthwise section of the limb or knot. These knots generally have splits and severe grain deviations near them.

Raised grain – Anything that gives the wood a corrugated feel. Typically, this is caused by the harder summerwood rising above the softer springwood in the growth ring. The growth rings do not separate.

Shake – A lengthwise crack or separation of the wood between the growth rings, often extending along the board’s face and sometimes below its surface. Shakes may either partly or completely separate the wood fibers. The separations make the wood undesirable when appearance is important. Although this is a naturally occurring defect possibly caused by frost or wind stress, shakes can also occur on impact at the time of felling and because of shrinkage in the log before conversion.

There are two types of shakes:
Star Shake: A group of splits radiating from the pith or center of the tree in the form of a star. It is wider on the outside ends and narrower on the inside ends. Star shakes are usually formed due to extreme heat or severe frost during the tree’s growth. Also referred to as heart shake.
Ring Shake: Also known as “cup shake” or “wind shake,” this rupture runs parallel to the growth rings. A ring shake is not easily detected in green logs and lumber, but only becomes apparent after drying. It’s caused by any one of numerous factors, including bacteria, tree wounds, tree age, and environmental conditions such as excessive frost action on the sap when the tree is young.

Split – A split is a rupture or separation in the wood grain which reduces a board’s appearance, strength, or utility. One of the more typical ruptures of this type is called ring shake. In a ring shake (also known as cup shake or wind shake), the rupture runs parallel to the growth rings. It’s not easily detected in green logs and lumber, but only becomes apparent after drying. It’s caused by any one of numerous factors, including bacteria, tree wounds, tree age, and environmental conditions.

Stains – Stains are a discoloration that penetrate the wood fiber. They’re caused by a variety of conditions and can be any color other than the natural color of the wood. A number of non-wood destroying fungi can cause stains or discoloration. Some stains may indicate decay or bacteria are present.

Spalting – Any form of wood discoloration caused by fungi. It’s typically found in dead trees, so if the wood isn’t stabilized at the right time it will eventually become rotten wood.

There are three types of spalting that are typically incorporated into woodworking as design elements: pigmentation (“sapstain”), white rot, and zone lines.

Twisted fibers – These are known as wandering hearts and caused by twisting of young trees by fast blowing wind. The timbers with twisted fibers is unsuitable for sawing.


Chlorophyll makes plants green, but it also fluoresces a blood red color. Grind some spinach or swiss chard in a small amount of alcohol (e.g., vodka or Everclear) and pour it through a coffee filter to get chlorophyll extract (you keep the part that stays on the filter, not the liquid). You can see the red glow using a black light or even a strong fluorescent bulb, such as an overhead projector lamp, which gives off ultraviolet light.


How does ultraviolet light kill cells?

The longer the exposure to UV light, the more thymine dimers are formed in the DNA and the greater the risk of an incorrect repair or a "missed" dimer. If cellular processes are disrupted because of an incorrect repair or remaining damage, the cell cannot carry out its normal functions. At this point, there are two possibilities, depending on the extent and location of the damage. If the damage is not too extensive, cancerous or precancerous cells are created from healthy cells. If it is widespread, the cell will die.

Bonnie K. Baxter, chair of the biology program at Westminster College of Salt Lake City, adds the following:

Basically, UV kills cells because of the accumulation of DNA damage. A gene product, called p53, is one of the responsible parties for slowing the cell cycle and checking for damage. If the damage is fixable, p53 sends in the repair machinery. If the damage is too extensive, it directs the cell to apoptosis, or programmed cell death.


Decomposition and decay

The former is mainly associated with things that are rotten, have a bad smell and are generally symptomatic of death. The latter is similarly viewed as undesirable. Examples include urban decay, or, on a more personal level, tooth decay. However, decomposition and decay are vital processes in nature. They play an essential role in the breakdown of organic matter, recycling it and making it available again for new organisms to utilise.

Decomposition and decay are the yin to the yang of growth. Together they form two halves of the whole that is the closed-loop cycle of natural ecosystems. Everything dies, and without decomposition and decay the world would overflow with plant and animal remains. It would also experience a decline in new growth, due to a shortage of nutrients that would be locked up and unavailable in the dead forms.

What is decomposition?

Decomposition is the first stage in the recycling of nutrients that have been used by an organism (plant or animal) to build its body.

It is the process whereby the dead tissues break down and are converted into simpler organic forms. These are the food source for many of the species at the base of ecosystems. The species that carry out the process of decomposition are known as detritivores. Detritivore means literally ‘feeders on dead or decaying organic matter’. Many of these decomposer species function in tandem or parallel with one another. Each is responsible for a specific part of the decomposition process. Collectively they are known as the detritivore community.

Nature’s unsung heroes of recycling

A wide range of organisms takes part in the decomposition process. Most of them are inconspicuous and unglamorous. From a conventional human perspective, they are even undesirable. The detritivore community includes insects such as beetles and their larvae as well as flies and maggots (fly larvae). It also includes woodlice, fungi, slime moulds, bacteria, slugs and snails, millipedes, springtails and earthworms. Most of them work out of sight, and their handiwork isn’t immediately obvious, but they are the forest’s unsung heroes of recycling. Almost all of them are tiny, and their function happens gradually in most cases, over months or years. But together they convert dead plants and animals into forms that are useable either by themselves or other organisms.

Decomposition in plants

The primary decomposers of most dead plant material are fungi. Dead leaves fall from trees and herbaceous plants collapse to the ground after they have produced seeds. These form a layer of litter on the soil surface. The litter layer can be quite substantial in volume. The litter fall in a Scots pine is around 1-1.5 tonnes per hectare per year, while that in temperate deciduous forests is over 3 tonnes per hectare per year. The litter is quickly invaded by the hyphae of fungi. Hyphae are the white thread-like filaments that are the main body of a fungus. (The mushrooms that appear on the forest floor, are merely the fruiting bodies of the fungus.) The hyphae draw nourishment from the litter. This enables the fungi to grow and spread, while breaking down the structure of the dead plant material. Bacteria also play a part in this process, as do various invertebrates, including slugs, snails and springtails. As the decay becomes more advanced, earthworms begin their work.

This decomposition process is usually odourless. It is aerobic, meaning that it takes place in the presence of air (oxygen in particular). On the forest floor it is spread out in both space and time. When people make compost heaps in their garden, they are utilising the same process. It is concentrated and accelerated by piling the dead material together in a heap, and the heat that is generated speeds up the process of decay.

Fungi that feed on dead plant material are called saprotrophic fungi. Common examples include the horsehair parachute fungus, which can be seen growing out of dead grass stems, leaves or pine needles. Another is the sulphur tuft fungus, which fruits on logs that are at an advanced state of decomposition.

In a forest, the rate of decomposition depends on what the dead plant material is. Leaves of deciduous trees and the stems and foliage of non-woody plants generally break down quickly. They are usually gone within a year of falling to the forest floor. Some plant material, such as the fibrous dead fronds of bracken, takes longer. But even these will still be decomposed within three years. The needles of conifers, such as Scots pine, are much tougher. It can take up to seven years for them to be completely broken down and recycled. The rate of decay is also determined by how wet the material is, and in general the wetter it is the faster it breaks down. In dry periods or dry climates, the organic matter becomes dessicated. Many detritivores, such as fungi and slugs, are inactive so the decomposition process becomes prolonged.

Decomposition of woody material – the rot sets in

In contrast to the softer tissues of herbaceous plants, the fibres of trees and other woody plants are much tougher and take a longer time to break down. Fungi are still, for the most part, the first agents of decay, and there are many species that grow in dead wood. The common names of species such as the wet rot fungus and the jelly rot fungus indicate their role in helping wood to decompose. The growth of the fungal hyphae within the wood helps other detritivores, such as bacteria and beetle larvae, to gain access. The fungi feed on the cellulose and lignin, converting those into their softer tissues. These in turn begin to decompose when the fungal fruiting bodies die. Many species of slime mould also grow inside dead logs and play a role in decomposition. Like fungi, they are generally only visible when they are ready to reproduce and their fruiting bodies appear.

Some decomposers are highly-specialised. For example, the earpick fungus grows out of decaying Scots pine cones that are partially or wholly buried in the soil. Another fungus known as Cyclaneusma minus grows on the fallen needles of Scots pine.

As the wood becomes more penetrated and open, through, for example, the galleries produced by beetle larvae, it becomes wetter. Being wet facilitates the next phase of decomposition. Invertebrates such as woodlice and millipedes feed on the decaying wood. Predators and parasites, such as robber flies and ichneumon wasps, will also arrive, to feed on beetles and other invertebrates. For trees such as birch the wood becomes very wet and rotten, and falls apart quite easily after a few years. Earthworms and springtails are often seen at this stage, when the decomposing wood will soon become assimilated into the soil. They can reach high densities – there can be 1 tonne or earthworms in a single hectare of broadleaved European forest! The wood of Scots pine, however, has a high resin content. This makes it much more resistant to decay, and it can take several decades for a pine log to decompose fully.

It’s a fungus eat fungus world

Most fungi are soft-bodied and having a high water content. This means they often disintegrate and disappear within a few days or weeks of fruiting. The tougher, more woody fungi, such as the tinder fungus, can persist for several years. Even so, they often have specialist decomposers at work on them. The tinder fungus, for example, is the host for the larvae of the black tinder fungus beetle and the forked fungus beetle. These feed on the fungal fruiting body, helping to break down its woody structure

Another bracket fungus that grows on dead birch trees, is the birch polypore. This fungus is itself colonised by the ochre cushion fungus, which feeds on and breaks down the polypore’s brackets. The bolete mould fungus is another species that grows on fungi, in this case members of the bolete group. (Boletes have pores on the underside of their caps and include edible species such as the cep.) The silky piggyback fungus and the powdery piggyback fungus fruit on the caps of brittlegill fungi. They speed up the process of breakdown and decay in them. Slime moulds, although not fungi, are somewhat fungus-like when they are in the fruiting stage of their life cycle. The fruiting bodies of a species called Trichia decipiens are susceptible to fungal mould growing on them. This in turn accelerates their decomposition.

Decomposition in the animal kingdom

Fungi play a key role in breaking down plants, but this isn’t the case then it comes to dead animal matter. The vast majority of the decomposers in this case are other animals and bacteria. Animal decomposers include scavengers and carrion feeders. These consume parts of an animal carcass, using it as an energy source. They also convert it into the tissues of their own bodies and the dung they excrete. These animals range from foxes and badgers to birds such as the hooded crow. They also include invertebrates such as carrion flies, blow-flies and various beetles. Their dung in turn is eaten by other organisms, particularly dung beetles and burying beetles. Some fungi, including the dung roundhead grow out of dung, helping to break it down.

Not all animal carcasses are immediately consumed by large scavengers. In these cases there are five main stages in the decomposition process. The first of these is when the corpse is still fresh. At this stage carrion flies and blow-flies arrive and lay their eggs around the openings, such as the nose, mouth and ears. In the second stage, the action of bacteria inside the corpse causes putrefaction. These bacteria produce gasses which make the carcass to swell. This is anaerobic decomposition, or decay in the absence of air. It is characterised by its bad smell, in contrast to the odourless nature of aerobic decomposition.

The next stage commences when the skin of the corpse is ruptured. The gases escape and the carcass deflates again. In this decay stage, the larvae or maggots of flies proliferate and consume much of the soft tissue. Predators such as wasps, ants and beetles also arrive, to feed on the fly larvae. In the following stage, only cartilage, skin and bones remain. At this point different groups of flies and beetles, along with their parasites, take over the decomposition process. Finally, only bones and hair remain, and they can persist for several years or more. Eventually even these are consumed – for example, mice and voles will gnaw on old bones, to obtain the calcium they contain. Clothes moths help break down hair or feathers. The progression through these stages depends to some extent on the time of year when death occurs. But typically it takes several months from beginning to end.

One example of a fungus that helps break down animal matter is the scarlet caterpillar club fungus. This species grows out of the living pupa or larva of a moth or butterfly. It converts the body of its host into a fruiting body, which is club-shaped and orange, with a pimply surface.

Decomposition feeds new growth

Decomposition and decay may appear to be unpleasant processes from our human perspective. However they are vital for the functioning of ecosystems. Just like compost in a garden, they provide essential nutrients for the growth of new organisms. They are a key aspect of the cyclical processes that maintain all life on Earth. A renewed appreciation of their importance will help humans to protect and sustain ecosystems. This appreciation may even provide inspiration for alternatives to the unsustainable unlimited growth model that drives human culture today.


Woodsense: Wood Discoloration

Non-typical coloring in wood can be good or bad –
depending on your point of view and the project at hand. We all associate certain colors with various wood species: light brown with oak, burgundy red with aged cherry and mahogany, or tan with maple. After several years of growth, the wood cells become inactive and increase extractive content like resin, gum and tannin to become heartwood. Heartwood is usually darker in color than sapwood because of a higher level of extractives that give heartwood its color attributes and odor. Major color variations are discolorations, changed by nature or man, on purpose or by neglect. These stains in wood can be classified into three groups: fungal, mineral and chemical.

This group of stains is caused by minute parasitic organisms that need water, warm temperatures and oxygen to grow. Fungi feed on sugar in the sapwood of logs and lumber with a moisture content of more than 22 percent, or when the relative humidity is more than 92 percent. The affected wood is then said to be “sap-stained.” The discoloration varies depending on the infecting organism, wood species and moisture situation, but a blue stain is the most common type. Blue stain shows as bluish to bluish-black, or gray to brown. The affected areas can be spotty or streaky. In severe cases, the entire sapwood is evenly discolored.

Even though sap stain seldom changes the strength properties of wood, heavy sap stain reduces wood’s toughness and its ability to withstand shock. Stained areas also have a higher water absorption capacity, and therefore are more susceptible to other organisms such as decay fungi.

Mold is another type of stain caused by fungi. Basically, the difference between mold and sap stain is the penetration into the wood. Mold is usually a surface growth, but it can stain the wood deep below the surface. Areas affected by mold show stain even if planed to nominal thickness – especially hardwoods such as oak. On softwood, the discoloration is shallow and can be removed by planing the lumber. Sap stain can affect the entire sapwood of a tree, which in some species is more than half of its volume, or entire boards cut from a log.

A third kind of organism is decay fungi, more commonly referred to simply as rot. Even though decay fungi aren’t stains, they should be mentioned here because they live and thrive under the same conditions as mold and sap stain . Spalted wood is a good example of decay fungi. The blue-black zone lines are boundary markers of fungi colonies coming into contact with each other.

Unlike wood with other fungal stains, spalted wood is rare, high-priced and in high demand. The reason is that the timing has to be just right to harvest spalted wood. If a spalting log is cut into lumber too soon, the color characteristics are less pronounced. When the log is cut too late, the wood is too decayed and the lumber is useless.

Organisms like fungi need food, water, oxygen and warm temperatures to live and grow. Temperatures of 75-85 degrees are optimal for fungal growth. But the growth rate slows to half the optimal rate at temperatures of 32-50. The organisms are killed at temperatures over 130. Another characteristic of fungi is that they can go dormant for long periods of time. Only high temperatures and chemical treatment kills the organisms. Any fungal stain is permanent and cannot be removed or lightened by chemicals like bleach or oxalic acid.

Mineral stains occur in living trees and should not be confused with fungal or chemical stains, which occur in cut logs or lumber. Mineral stains are mostly the result of an injury or biological attack on the tree and no treatment is known. Typical discoloration in oak shows as dark-brown streaks, in maple as green or brown, and in poplar as purple to dark-red. Mineral stains do not affect the strength properties of the lumber.

While some see mineral streaks in lumber as a defect, others see them as aesthetically pleasing. However, one should keep in mind that the brilliant colors will fade to a light-brown or medium-brown over time.

Chemical stains result from the oxidation of naturally occurring chemicals in the wood cells of the hardwood. This is especially true with species with the highest proportion of sapwood. The discolorations vary and can be yellow, pinkish-brown, blue, or shades of gray.

Chemical stain develops just below the surface and can penetrate deep into the wood if conditions are favorable. Requirements for the development of chemical staining are similar to that of fungal staining – temperatures above 50 degrees and a moisture content above 20 percent. Since chemical stain is an oxidation process, oxygen must be present in the wood cells. That means the wood needs to be partially dry. And then, of course, chemicals have to be present.

The problem with most chemical stains is that they don’t show on the surface of the lumber. Only when the lumber is planed does the discoloration become visible. And even then, it’s sometimes hard to detect unless a piece of wood without stain is used as a comparison. This is common on white maple. The stain may be so even throughout the wood that it isn’t noticed, but when a finish is applied to the wood, the stain becomes magnified, giving greater contrast between stained and unstained wood.

One common type of chemical stain is iron stain. These are unsightly blue-black or dark-gray discolorations mostly in oak, redwood, cedar and cypress. The heartwood of these species has a high content of tannin and tannin-like extractives. When iron or steel comes into contact with the wood, a chemical reaction takes place, resulting in discoloration. This contact can be as simple as rubbing out a finish on a project with steel wool, or driving a nail into a living tree.

There is one characteristic that all chemical stains have in common: The chemical reaction can be reversed. Many times the discoloration can be softened with household bleach. A more powerful bleach is oxalic acid, available in pharmacies and paint stores. Oxalic acid is a good spot remover for small areas of iron stain. However, the treatment isn’t permanent unless all the iron is removed from the wood. If not, the acid will break down over time and the staining will continue. Use extreme caution when using oxalic acid, as it causes irritation of the skin, eyes and mucous membranes. Ingesting just a few grams can be fatal.

In contrast, fungal stains cannot be bleached out. An application of bleach may lighten the stained area temporarily, but it will return to its original color after a short time.

All wood darkens with age. No treatment with colored stains or dyes can duplicate the beauty of naturally aged wood. Some species, like cherry, darken very fast and care should be used when working with them on large-scale projects. Finished parts of the project should be kept in dark rooms or covered to prevent light from prematurely aging the wood.

Weathering is another change in wood color through photo-oxidation caused by ultraviolet radiation of sunlight. Add this to the effect of the elements, and weathered wood turns grayish regardless of the species’ original color. Exterior projects should always be protected with a finish containing UV blocking. Luckily, the color of aged or weathered wood is only skin deep and can be sanded or planed out.

Born and educated in Germany, Udo Schmidt came to the United States in 1979. After 12 years in the lumber-export industry, he started his own cabinet shop. Schmidt has written numerous magazine articles and is the author of “Building Kitchen Cabinets” (Taunton Press, 2003).


How Does Thermally Modified Wood Look Different?

The high gradual heat process creates permanent reactions and gives the wood a rich, deep brown appearance. The darkened color brings an unexpected tropical look.

The chocolate color can be maintained in exterior applications by finishing the wood with a UV-inhibitor sealant on all sides and ends of the wood. If not finished, the wood will naturally weather to a shade of gray because of exposure to the sun’s ultraviolet rays.


Soil organic matter dynamics under decaying wood in a subtropical wet forest: effect of tree species and decay stage

Decaying wood is an important structural and functional component of forests: it contributes to generate habitat diversity, acts as either sink or source of nutrients, and plays a preponderant role in soil formation. Thus, decaying wood might likely have measurable effects on chemical properties of the underlying soil. We hypothesized that decaying wood would have a stronger effect on soil as decomposition advances and that such effect would vary according to wood quality. Twenty logs from two species with contrasting wood properties (Dacryodes excelsa Vahl. and Swietenia macrophylla King) and at two different decay stages (6 and 15 years after falling) were selected, and soil under and 50 cm away from decaying logs was sampled for soil organic matter (SOM) fractions [NaOH-extractable and water-extractable organic matter -(WEOM)] and properties (WEOM aromaticity). NaOH-extractable C and WEOM were higher in the soil influenced by 15-year-old logs, while the degree of aromaticity of WEOM was higher in the soil influenced by the 6-year-old logs. Decaying logs did influence properties of the underlying soil with differing effects according to the species since there was more NaOH-extractable C in the soil associated to D. excelsa logs and more WEOM in the soil associated to S. macrophylla older logs. It is proposed that such effects occurred through changes in the relative quantity and quality of different SOM fractions, as influenced by species and advancement in decomposition. Through its effect on SOM and nutrient dynamics, decaying wood can contribute to the spatial heterogeneity of soil properties, and can affect process of soil formation and nutrient cycling.

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CONCLUSIONS AND FUTURE PERSPECTIVES

What is the origin of bacteria in wood how and over what timescale does colonisation occur?

What are the major biotic and abiotic determinants of bacterial communities, and by what mechanisms do these operate?

Are interactions with fungi predominantly beneficial or antagonistic? Does one partner consistently benefit at the expense of the other?

How, and to what extent, do bacteria influence ecosystem-level flows of carbon and nitrogen in the context of dead wood?

One of the major features that emerges with regard to fungal–prokaryote interactions is just how hard it can be, in any given case, to distinguish the exact identity of the association. If fungal growth increases in the presence of a bacterium (or vice versa), is it mutualism, commensalism or parasitism? Do we truly see mycophagy rather than saprotrophy, endosymbiosis rather than opportunism? When fungi alter the bacterial community, are they selecting for their specific symbionts or simply unable to outcompete the remaining bacteria—or a mixture of the two? Such associations can be deceptively hard to disentangle.

Happily, a suite of methods is coming of age that will hopefully assist in answering such questions. Metagenomics gives a snapshot not just of the taxonomic identities of the community, but also of their genomic potential (although it still has limited ability to marry the two). Metatranscriptomics and metaproteomics offer insight into which of these potential abilities are realised in a particular situation. Metabolomics explores the complete metabolic signature of a microbial community under given conditions. At the same time, new culture methods offer hope for isolating key community players, allowing physiological characterisation and manipulative experiments (Ling et al. 2015 Oberhardt et al. 2015 Kielak et al. 2016a). Each of these techniques comes with associated limitations, pitfalls and benefits, and it will require judicious use of these approaches, combined with appropriate statistical and mathematical methods, to pick apart fungal–prokaryote associations.

The overwhelming conclusion regarding the current state of knowledge is that despite the work already done on saproxylic bacteria and their interactions with fungi, we have still barely scratched the surface. Results can be disparate or even contradictory depending on the environmental conditions, identity of the organisms involved or methods employed, frustrating the chance of drawing together a robust theoretical framework. With so much ground still to cover, the microbiota of dead wood remains a lively and underexplored area of ecological research, but one that is likely to be highly rewarding and will be furthered by deployment of modern genomic and post-genomic approaches.

The authors wish to thank Prof. Eshwar Mahenthiralingam for his helpful comments on an earlier draft, and two anonymous reviewers for their contributions to improving the manuscript.


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