Brown Rot in Wood

I received an email with two photos of a tree that fell near a hiking path in Southeast Pennsylvania. The photos showed wood that was shattered into small cubes, and the writer was wondering what would cause the wood to do that. Could it have been struck by lightning or did something else cause it to do that?

I have seen this phenomenon in the forest many times, and I knew the wood was deteriorated and was crumbling apart, but I must admit that I never thought about what, specifically, was causing it to break apart in such a way to create the cube-shaped blocks of wood. After some research I found the answer . . . brown rot.

Decay in wood is caused by fungi. A fungus will begin to grow in a suitable host when spore is germinated.  Once this occurs, long slender threads of hyphae will begin to grow and penetrate the wood, moving from cell to cell through various openings in the cell walls and sometimes creating bore holes through the cell wall. When there are many hyphae growing together, they are called mycelium.  The mycelium grows and expands within the wood and progressively degrades the cell walls. Eventually, the fungus will consume enough wood material that the wood will begin to lose strength and weight.    

Decay fungi depend on four factors in order to survive and thrive: food, water, oxygen, and favorable temperature. When one of these factors is missing, the fungus cannot survive. The wood itself provides the food for a decay fungus the form of cellulose, hemicellulose, and lignin, depending on what type of fungus it is. 

The hyphae produce metabolites and enzymes that break down the wood constituents into simple molecules that can then be metabolized by the fungus. For the metabolites to enter the cell wall, and for the by-products to enter the hyphae (thus feeding the fungus), there must be water present in the wood. Most decay fungi do not do well if the moisture content of the wood is below 30% although there are a few that can survive with lower moisture contents. One of the best methods for preventing fungal decay in wood is to dry it and keep it dry.

Decay fungi grow very well in temperatures between 68 and 90°F. Temperatures that get below 32°F and above 100°F inhibit fungi from growing. It is important to note that when the temperature gets below 32°F, the fungus is not killed, but it will become dormant, and growth will stop until conditions are favorable for growth again. High temperatures (over 115°F) will kill fungi, and using elevated temperatures is a tool that is often used to sterilize wood if decay fungi are present.

Decay fungi can thrive in areas with oxygen levels that are much lower than normal atmospheric amounts. Due to this fact, there is no practical way to prevent decay in finished wood products by limiting the amount of oxygen. It is possible to limit the oxygen supply in logs. Some sawmills will store logs in the summertime in ponds or under sprinklers to prevent fungal problems until the logs can be sawn into lumber. The reason this works is that the moisture content in the logs is high enough that it limits the amount of oxygen that is available to the fungus. 

There are many different species of decay fungi that can be found in wood, and they can be categorized into white rots and brown rots. White rot fungi will attack not only the cellulosic components of the wood but will also attack the lignin in the wood. In simple terms, lignin can be thought of as the "glue" that holds the cells together and gives the cell walls rigidity. Because white rots will attack the lignin in the wood, it often gives the wood a bleached or whitish appearance.

Brown rot, like the one that infested the tree in the pictures sent to me, attack the cellulosic components of the wood while modifying the lignin. When the brown rot has progressed enough, the wood will have a brownish or reddish color. This color is due to the modified lignin that is left behind in high concentrations in the wood. When brown rot attacks wood, fractures develop perpendicular to the grain as well as with the grain. When the wood gets dry, it becomes brittle and will readily break into cube-shaped pieces. Because of this, sometimes brown rot is called cubical brown rot.  When the brown rot progresses far enough, the wood will appear to be dry and powdery.

Wood-decaying fungi are an important part of the nutrient cycle in our forests. Just think about how our forests would look if there were no decaying fungi to decompose dead trees. While they have a key role in our forests, they can sometimes cause unwanted problems in healthy trees as well. With fungal spores blowing around everywhere, it is virtually impossible to completely prevent a tree from becoming infected with 100% assurance. The best thing to do is to be careful not to damage trees, which will create an opening for the rot to invade, particularly during logging operations, and do what you can through sound forest management to keep the tree healthy and vigorous.