Tree Risk Assessment - Analyzing Tree Health and Safety Through a Standard Method
Today I had the opportunity to evaluate a honeylocust (Gleditsia triacanthos) for a client that was concerned about the health and safety of the tree. The client was concerned about the tree because it seemed to have decay in its stem and was in close proximity to a driveway and parking lot.
To evaluate the tree, I performed an ANSI A300 Level 3 Advanced Risk Assessment. This assessment included a ground-level inspection of the tree's crown, base and trunk plus surrounding site conditions. Resistance-recording drilling was performed to analyze trunk decay. Such an assessment details tree characteristics and then compares the likelihood of tree failure to the likelihood of damage from failure to assess overall risk.
TREE AND SITE CHARACTERISTICS
The tree measured approximately thirty inches in diameter at a height of 4.5 feet, which is the standard height for measuring tree size.
The tree had a wound in the trunk at a height of six feet on the east side where an old stem was removed. The wound measures approximately sixteen inches by sixteen inches. There is an old pruning wound at twelve feet. There is little to no callus wood around the wounds. Callus is a type of wood that forms in response to wounds in an effort by the tree to seal off the wound. A lack of callus indicates stress in the tree. A fruiting body had been previously removed at a height of three feet on the north side of the tree, which could possibly indicate internal decay.
The tree is approximately 35-40 feet in height. The crown is somewhat one-sided to the east. The tree's leaves appeared green and healthy overall, with only a small amount of dead limbs present in the crown. A small number of leaves have damage from leaf chewing insects, but this damage does not appear severe enough to significantly affect tree health.
The tree is somewhat mature. This means that the energy cost to the tree to maintain itself is beginning to exceed the energy input the tree receives from photosynthesis. This will lead to the tree becoming less able to resist the spread of decay in the future.
I utilized a tool called an IML Resistograph to test for the presence of decay in four locations around the circumference of the trunk at a height of approximately 4.5 feet. The Resistograph uses a small drill bit to penetrate the tree stem and determine the amount of sound versus decayed wood present in a tree. The amount of sound wood is compared to stem diameter to estimate the amount of strength the tree has lost as a result of the presence of decay. The Resistograph showed very little sound wood present where I tested.
The table below shows an accepted manner within arboriculture for evaluating tree risk qualitatively. The likelihood of tree failure and impact are compared to the potential consequences of failure to determine overall tree risk.
Tree risk assessment matrix. Source: www.isa-arbor.com
If the honeylocust were to fail, the main potential targets would include the entry drive, power lines and the parking lot. This accounts for the failure of the entire tree or portions of the tree, such as individual branches. Potential damage would include property damage and injury to persons under the tree. The most likely failure route is to the east or north, which means damage would occur to power lines and potentially the entry drive. Therefore, I rate the potential consequences of tree failure as SIGNIFICANT.
I rated the tree as LIKELY to fail and impact a target, given the tree's overall condition, combined with its presence near several targets as described above.
As can be seen in the table, a combination of LIKELY failure and impact and SIGNIFICANT consequence of failure equates to a HIGH overall tree risk.
I recommended the tree be removed. There are no remedial actions other than removal that would reasonably reduce the amount of decay present in the tree, which is the main concern. Therefore, the high risk present in the tree cannot be reasonably reduced without removal.
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