Last week I hope I was able to adequately impart an understanding of how trees deal with decay in their woody structures. This week we will discuss the types of decays that will invade the woody structures of trees.

Decays that are caused by fungus have three basic stages that are useful in risk assessment. The first marks the initial stage of decay and can be hard to detect. This is called incipient decay, as the fungus has just started to break down the cell walls and has only lost a small amount of its strength. In the intermediate decay stage the wood begins to become discolored and the strength of the wood has been significantly compromised, but the cell walls remain intact. After many years (the amount of time depends on tree species) the wood reaches the advanced decay stage, at which point all wood strength and cell structure has been lost.

There are three basic types of decay that breaks down wood; they are divided by what cells are broken down by the invading fungus:

White rot

White rots will break down the lignin which is responsible for the brown pigmenting and compaction strength in wood, hemicellulose and cellulose which is responsible for the tensile strength in trees. White rots are divided into two types; one that breaks down lignin and cellulose at the same rate, leaving the wood severely compromised. The other will break down the lignin at a much faster rate, leaving the trees tensile strength but removing its ability to withstand its own weight. Trees under the latter influence will show bulging on a vertical trunk as the tree buckles under its own weight.

White rots are more commonly found in broadleaf trees but can also be found in conifers. Wood decayed by white rot will be fibrous, wet, spongy and white or yellow due to the oxidation and loss of lignin.

Brown rot

Brown rot is exclusively caused by the fungi basidiomycotina, and primarily affects conifers. This type of rot will break down the cellulose and hemicellulose but will leave the lignin intact. When the wood is attacked by these fungi, the wood is left brittle and dark brown due the loss of cellulose and the oxidation of the lignin. As the decayed wood dries, the wood will shrink and breaks into cubes.

Brown rots are considered more serious than white rots as there is a dramatic loss of flexibility in the tree, and make them much more vulnerable to wind.

Soft rot

Soft rots can be cause by both fungi and bacteria and will break down lignin, hemicellulose and cellulose – but only in localized areas that spread slowly, making them less serious than brown rots and white rots. Soft rots will normally only infect broadleaf trees and only infect pockets of decayed wood that is softer than the wood around it. There is one fungus Kreutschmaria (Ustulina) deusta that will cause more extensive damage and create major structural issues.

These three types of rot are further classified by the region of the tree they infect.

Root rot

The fungi will enter the tree through root grafting on an infected tree, and by fungal rhizomorphs in the case of Armillaria species. Roots rots can also be contracted by waterborne or airborne spores. Root rots can affect trees in three ways: They can cause dieback of the fine roots and root hairs, lessening their ability to uptake nutrients and gas exchange. This will stress the tree and often lead to crown dieback. The fungi will move through the tree by developing on the underside of the roots, where others will move into the heartwood of the roots and spread into the butt of the trunk.

Root rot can be difficult to assess and detect, as the roots are buried and beneath the standing tree. More often than not, it is impractical to expose the required amount of roots to gather a full understanding of the extent of infection. At the right time of year (depending on the age of the fungus, the type of fungus, and the time of year the tree is assessed), there is a chance of finding an identifiable mushroom at the base of the tree. Often times this process is identified by exposing small patches of roots, and sampling for signs of infection.

Butt rot

Butt rot can be caused by decay moving up into the trunk through the roots, or by injuries to the trunk giving entry to formally protected wood. Tree injuries at the lower-trunk are the easiest to detect and assess, as the tree leaves behind “body language” that will tell a story of how the tree has compensated for the decay. This will be the subject for next week’s blog.

Heart rot

Heart rots are located in the heartwood at the center of the tree trunk. The fungi generally need a wound to gain entry into the otherwise-sound wood of a tree. These wounds can be caused by branch failure, pruning wounds, fires, vandalism, animals, insects, or mechanical damage. Heart rot can also be caused by root rot spreading up into the trunk of the tree. In all cases, the bark and cambium has been damaged and the sap wood has become exposed to oxygen and airborne pathogens.

Sap rot

Sap rots are often found on dead wood or are a secondary fungal invasion after an area of the tree has been killed by other factors (IE; bark beetles or mechanical or weather related injuries). Sap rot often will progress rapidly but are not normally associated with structural failures.

Now that we have prefaced how trees deal with rot and what types of rot are to be found in trees, we are now ready to get to the real point of this series: How to recognize hazard trees. Please stay tuned for next week’s blog; The body language of trees.

The majority of the information given is taken from the “Tree Risk Assessment In Urban Areas And The Urban/Rural Interface” Manual written byJulian Dunster and published in 2009