Cottonwood With Included Bark Over a Picnic Shelter
A 90 cm DBH plains cottonwood in a city park has codominant stems with an included-bark union at 4 m and a vertical crack extending 1.8 m from the union. A basal cavity is 30 cm wide, 45 cm deep, and occupies about 25% of the trunk circumference. The target is a picnic shelter 8 m away, used by about 40 visitors per day. Prevailing winds are from the southwest, and the tree leans slightly toward the shelter. The park has had two drought years, and the crown has 20% dieback. The city has a limited removals budget and a policy to preserve mature trees. The shelter can be relocated at moderate cost, but the public has reserved it for an event in three weeks.
1. Describe how you would complete the risk assessment and classify the likelihood of failure and consequence. Justify your reasoning.
Read the worked answer
I would follow a structured risk assessment: define the target, inspect the tree, and evaluate likelihood of failure, likelihood of impact, and consequence. The included-bark union plus a 1.8 m vertical crack is a high-likelihood failure feature because the union has weak attachment and the crack indicates separation. The basal cavity at 25% circumference is significant but not automatically decisive; I would sound the trunk, probe the cavity, and use an advanced assessment such as resistograph or sonic tomography if available. The target is a shelter used by about 40 people per day, and the tree leans toward it, so consequence is high. With a high-likelihood feature and a high-consequence target, the initial risk rating is high. Because the event is in three weeks, I would treat the shelter as temporarily closed or relocate the event until the advanced assessment and mitigation are complete.
2. Recommend mitigation options, their expected residual risk, and whether the tree can remain during the event.
Read the worked answer
Options include removing the target by relocating or closing the shelter, reducing end weight on the codominant stems, installing cables or braces, or removing the tree. Relocating the shelter is the most reliable way to reduce risk because it removes the high-consequence target. Pruning alone will not repair an included-bark union with a crack and may not lower risk enough for a shelter target. Cabling and bracing can support a weakened union but do not restore strength; they require engineering design, annual inspection, and still leave residual risk from the basal cavity. Given the high-use target and short event timeline, I would not allow occupancy under the tree. If the tree is retained, require advanced assessment, target relocation, and written residual-risk acceptance by the city; otherwise remove it.
3. What documentation and communication steps should accompany the decision?
Read the worked answer
Document the inspection date, assessor credentials, tree dimensions, defect locations and sizes, target description and occupancy, risk rating method, mitigation options considered, residual risk, and the final recommendation with a re-inspection date. Take scaled photographs and a site sketch showing the shelter, lean direction, and drop zone. Communicate in plain language: the crack and included bark mean the union is weakened; the shelter is directly at risk; the tree will not be occupied during the event; and the city will relocate the shelter, restrict access, or remove the tree. Record who accepted the residual risk and when. Provide the public with the event relocation plan and a timeline for the final decision.