When evaluating a tree for its potential to fail, it is important to understand the way a tree can fail, why it will fail, and the way a tree will react to defects that may increase failure probability. In this article we will explore the visual signs of these defects and combine these observations with the information in the preceding articles, to come to a basic understanding of tree risk assessment.
The term defect refers to a part of the tree that is out of the ordinary growth pattern of that species. This often indicates past events that have altered the structure of the tree. Not all tree defects indicate that a tree should be removed, and this series does not give enough information to come to a conclusion on whether the tree is a risk or not. This series is meant to give you a starting point to determine if you need to call a qualified tree risk assessor for an in depth, thorough analysis.
Cracks will often create weakness that can be exploited by future weather events. The tree will try to make adaptions to compensate for the crack, by forming new wood over the wound. If the crack has not opened up wide, it will be able to create a new layer within a few short years. This wood is often raised higher than all the wood around it, and the bark is smoother due to the rapid growth. This will appear as a vertical or horizontal rib.
Most cracks will occur along the long axis of the trunk or branch, and are caused by the opposing compression and extension of the outer edges of the trunk or branch. This type of crack will appear on the outer edges of the tree trunk or branch, and has a split strait through it.
These cracks will occur along the short axis of the branch, and is caused by the tension strain (usually wind higher in the tree) exceeding the strength of the wood. These types of cracks will occur on the upper side of the lean, or on the side of the tree where the wind force direction is most prominent.
These types of cracks will occur when the compression force pushes down on the wood fibers, leaving them no place to go but outwards. This will happen on the underside of a lean or where the wind force direction is most prominent.
A torsion crack is one that will have a spiraling path along the trunk or branch. This is typically caused by the twisting motion of the tree during a wind storm.
This form of crack is the adaption of callus wood that forms over an old wound. This kind of crack can indicate a decaying cavity just underneath the new wood – typically identified by a sunken and rounded edge where the two parts connect.
Decay in trees can be both easy and difficult to find and assess. This is on account of the visual obscurity of the wound -which can be exposed wood within an open wound, or an old wound that has left an unusual bark pattern. Internal decay can also be caused by a fungus that will “eat” the heartwood. This can often times be identified as a bulging around the area of decay.
There is no set degree of lean that can be surely defined as a risk, but it has been proposed that a critical threshold for a sustainable angle would be around forty degrees (40°) from center. These factors are contingent on several conditions, with weather being the most likely contributor. Generally, a lean that is sudden or a recent development can be an indication of potential failure or risk. Other factors to be considered are; the condition of the tree trunk, and the roots or the soil the tree is anchored in and the species of the tree.
Root and soil issues:
In general, root problems are the hardest to detect – as most of the indicators or symptoms are buried beneath the tree and soil. Some indicators that would be present are; recent “leans” that start at the base of the trunk, cracking or heaving of the soil, and a decline in health/live growth from the top down (though this particular symptom could be caused by many other issues).
Indicators and their potential causes:
Abnormal Bark patterns:
Each tree species has a unique bark pattern that will change over time, but is relatively consistent within a localized area. These patterns will often times have raised and sunken areas. In areas where there is a lot of recent growth, there will be more of the sunken areas than the raised. The sunken areas will often have a lighter coloration, and it is in this area (when accompanied with bulging) that we can observe a risk indication. This abnormality can sometimes be an indication that the tree is compensating for some internal defect or stress.
Old damage that has had time to occlude the wound will often have a smooth, rounded, or a “puckered” look.
Ribs are raised ridges and often indicate a crack that has been occluded within new wood. If the edge of the ridge is cleft, it indicates the crack has not been grafted closed. If the nose of the ridge has a sharp edge, the crack is close to the surface. The more blunted the ridge, the more enclosed the crack has become.
Bulges are areas of the trunk that typically have “tumescent” growth within a specific localized area, and are usually at or around the base of the trunk. This kind of bulge can indicate one of two situations that involve internal decay; the outer wall of the wood remaining has begun to buckle under its own weight, or the tree has produced new growth to compensate for the shrinking shell wall.
This can also, on some occasions, indicate that the tree has grown around an object.
Another type of bulging happens below the joining of two trunks known as co-dominant stems, and indicates there is a present condition called included bark. This is a very common issue here in the Northwest, and can pose a serious threat as a result of a very poor connection between the co-dominant stems. Additionally, this poses a problem as the growth between them continues to increase. At this stage, the co-dominant stems have a high likelihood of splitting apart. There are preservation techniques available(such as cabling and bracing), but this condition has to be taken into serious consideration – as we are often times dealing with a great deal of weight, and the mechanics behind this potential risk can be tricky to assess or determine.
A fissure is a flat depression on the trunk of a tree. It can indicate an area of the tree that has stopped or slowed its growth in a localized area. Not all fissures indicate decay, as some species have them in their normal growth pattern. Further investigation is needed to determine the cause of such an abnormality.
In closing this series I hope that I have outlined the basic factors that are considered when evaluating the potential risk of your trees.