Assessing Tree Risk

All trees have the potential to fail but fortunately few trees do. Trees can have defects. Most trees do.

The key is being able to evaluate the defect and the likelihood of failure and take remedial action if necessary. Removal is the last resort.

I have noticed a disturbing trend where certified arborists, especially those who have recently passed the ISA certification exam, are condemning large veteran trees for removal because of a defect. Such a defect-phobic trend does not serve the community or the trees we should protect. Tree removal should be the treatment of last resort only after considering a full range of mitigation treatments.

For a Tree City USA bulletin on How to Recognize - and Prevent - Hazard Trees, click here.

For a good webpage showing various hazardous tree conditions by Steve Nix of About.com:Forestry, click here.

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Detecting Decay

Sometimes decay in a tree trunk or branch is readily visible. Other times it is not. And even when decay is visible through an opening or old pruning wound, it is difficult to determine how extensive the decay is.

I use the latest technology, the Resistograph, that can accurately measure decay. Using the Resistograph is only slightly invasive to the tree but the procedure is important if substantial decay or potentially hazardous conditions exist.

It is important to remember that all mature trees have some decay. It is the location of the decay, the extent of the decay and the tree species that determine the risk factor. One must also consider the size of the part of the tree that can fail and the target-- people or property that can be hit if the tree fails.

 

 

 

 

 

 

 

 

 

 

 

 

Signs That a Tree MAY Be Hazardous

A single sign or symptom listed below does not necessarily mean a tree is at high risk of failure. Sometimes a tree can have multiple defects and still not be at high risk of failure. It depends on many factors. If you see any of the signs and symptoms, have the tree checked by an ISA Certified Arborist. If you want an independent opinion on risk, let the arborist know up front he or she will not be hired to remove or prune the tree or hire an arborist who works only as a consultant and does not have any connection with a tree servicie.

• Numerous Dead limbs
• Thin crown (low vigor)
• Forked trunks
• Fungus on or at the base of the trunk or large limb
• A cavity or other sign of decay
• Wounds or cracks
• Cankers
• Sudden leaning
• Construction activity nearby in the last five to 20 years
• History of failure such as previous limb drop
• Species prone to failure -- laurel oak, water oak, maples, for example
• Severe pruning history -- has the tree been topped, lion-tailed or otherwise heavily pruned
• Adjacent trees removed -- has there been clearing of other trees nearby

Not all large trees are good neighbors
This heavily decayed laurel oak had a double leader and leaned over a corner bedroom. The tree was over 70 ft. tall and the trunk was nearly 6 ft. across at the widest part. The trunk exhibited no outward signs of decay. But testing with a mallet followed by drilling with the Resistograph quickly showed the extent of the problem. A tree service was called and the tree was quickly removed.

Large, old laurel oaks and water oaks tend to have this decay problem. So if you have a large over-mature laurel oak or water oak within falling range of your home, have it checked regularly.

It is important to note that the amount of decay shown in the above photo but in a live oak could be acceptable considering target and other structural factors. Tree species must be considered when evaluating risk.

How to recognize when help is needed

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(This Chuck Lippi article was published July 10, 2004 in the Home & Garden Section of the Daytona News-Journal)

Trees add many benefits to our yards, neighborhoods and cities. Trees provide cooling shade, beauty, habitat for birds and animals and a softening visual break from the straight lines of urban buildings and roads. There are also psychological and social benefits of living in an area with trees. And many realtors understand a well-placed, healthy tree or a wooded lot can significantly increase property values.

But trees are living things that occasionally break or topple over especially during wind storms which are common during Florida’s summer months. When a tree or tree branch lands on people or property, that can mean trouble. Every tree has the potential to fail but only a few do each year. It is important for property owners to regularly check their trees for defects and problems. Even cities and counties are responsible for the safety of the many trees on public property. Sometimes the arborist can lessen the potential of the tree to fail. A property owner can check trees in the yard to determine if an arborist should be called.

If a tree has a history of dropping limbs, it should be examined closely. Another sign of trouble is a large dead or leafless branch. Often dead branches are symptoms of root problems or construction damage years earlier. Smaller dead branches, however, are common on a mature tree. The tree simply shuts down and sheds smaller unproductive branches that are shaded too much to produce sufficient carbohydrates for the tree. In both cases dead branches should be removed because they will eventually fall.

Narrow v-shaped crotches especially those located low on the trunk are prone to fail. The desirable and strongest shape for large shade trees is a single leader. When a tree has multiple leaders of the same size low on the trunk, some weight should be pruned off the leaders with reduction cuts (not topping cuts) to prevent the tree from splitting.

Cavities and decayed wood are other signs of potential problems. Most mature trees have some decay and cavities. The location and size of the decay determine if the tree is at risk. Some trees with rather large cavities are still structurally sound while other trees are not. Decay in a live oak, for example, is often less serious than the same size decay in a laurel oak or water oak. A consulting arborist with decay measuring equipment can best evaluate decay.

Although tree roots are below ground and invisible to most observers, there are indications of possible trouble. If you have ever noticed trees growing in a natural forest, all the trees have a widening at the base which we call the root flare. Some tree species have larger root flare than others but all trees have root flare. And the root flare should be visible. If not, fill dirt could have been placed around the tree during construction years before. The root flare is meant by nature to be above ground. When buried, the root flare can decay which will increase the possibility of tree just toppling over. That also brings up another root problem which is roots cut by construction for a house or sidewalk. Trees react very slowly to construction damage which may not show up for five or 10 years.

Excessive pruning or improper pruning can also make a tree more prone to failure rather than safer. Topping, the process of removing a large branch and leaving a stub, is an outdated pruning method that causes excessive and weakly attached sprouts to form just below the cut. Also a topping cut opens the tree up to decay. Proper cuts made back at a crotch or branch union allow the tree to form a protection layer that can slow or stop decay. Another widely used pruning technique that is bad is lion-tailing which is the removal of smaller interior and lower branches. A tuft of branches is left at the end of a larger branch much like a lion’s tail. Don’t allow an arborist to “clean out” those sprouts and smaller branches because these branches help make the branch thicker by depositing carbohydrates into the wood of the nearby larger branch. When a branch has been “cleaned out” or lion-tailed, then it will not thicken and grow stronger and most of the weight is out at the end making the branch more susceptible to breakage. Smaller branches should be evenly distributed throughout the tree canopy and not excessively “limbed up” or cleaned up.”

Also excessive crown thinning may also weaken a tree by removing too many leaves which produce carbohydrates that power the tree and by removing too much wood where the carbohydrates are stored. Usually you should not remove more than 10% to 20% of the canopy of a mature tree and not more than 30% of the canopy of a young tree.

There is some new research that indicates thinning a tree to reduce wind load during storms will actually increase the speed of the wind as it is funneled through smaller openings in the canopy much like making holes in a sail. The preliminary research indicates reducing the size of the canopy (sail) by reduction pruning (not topping) is the better way to reduce wind stress on trees.

Fungus growing at the base of a tree or on the trunk or branches is a sign that there is decay in the wood. Usually fungus at the base of a tree is in the form of brownish orange mushrooms while fungus on the trunk or branches is in the form of conks, shelf-like spongy growths that measure from two to six inches wide. If you see fungus on your tree or right at the base, you should have your tree checked. External lichens and fungus growing on the bark, however, are common and not a problem for the tree.

Large trees with limited root space can be prone to tipping over as the tree grows larger. Roots anchor a tree sideways not deep. Long roots help brace a tree. When roots are not allowed to grow outward, the tree is in effect “pot bound” and can tip over.

Finally, a leaning tree, especially a tree that has recently started leaning and has soiled heaved up near one side of the trunk, should be checked. A sudden lean indicates pending failure. Other trees such as many live oaks, however, have grown up leaning and are strong and stable.

Given the natural variability of trees, an arborist may not be able to detect every possible way a tree or part of a tree may fail. Nor is there a way to predict when a tree will fail. Nor is there any complete guarantee that any procedures done to help the tree will prevent failure in the future.

Remedial steps an arborist can take to reduce a tree hazard

• Move the target: Although you can't move the house, sometimes a landscape feature, parking site, playground equipment or picnic table can be moved out of the danger zone.
• Pruning: Remove the defective branches and take the weight off large branches.
• Cable and brace the tree: Provide physical support for weak branches.
• Removal: Some hazardous trees are best removed