Cracked codominant union over a school sidewalk before a forecast wind event
A 14 m tall Norway maple (Acer platanoides), 45 cm DBH, stands in a 1.2 m turf strip between a 2.4 m public sidewalk and the street, directly in front of an elementary school. At 2.5 m the trunk forks into two codominant stems of nearly equal diameter, with a pronounced bark ridge and included bark running into the union. Two days ago, after a 70 km/h windstorm, a crack about 45 cm long appeared just below the union on the side opposite the stems' lean; it is open roughly 3 mm, and the bark along it is freshly split. No other storm damage is visible in the crown. About 120 students use that sidewalk between 08:00 and 08:45 and again between 14:30 and 15:15; dog walkers use it throughout the day. The forecast calls for another front with gusts to 80 km/h within 36 hours. The school asks whether it must close the sidewalk.
1. Rate the likelihood of failure and explain what the crack contributes to your rating compared with the included bark alone.
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The included bark and the crack are two different kinds of evidence and should not be averaged.
- Included bark is a structural defect that has existed for decades. It means the two stems have little or no wood connecting them across the union and behave as a wedge. On its own it justifies a possible likelihood of failure and, historically, prophylactic reduction or cabling on high-value trees.
- A fresh open crack below the union is different: it is evidence that displacement has already begun. The union is not a defect to monitor, it is a failure in progress. The crack is on the tension side, which is where a splitting fork typically initiates, and the freshly split bark shows it is recent rather than an old frost rib or seam.
- Rating: for the near term, with 80 km/h gusts forecast, the likelihood of failure is likely, not merely possible. I would not try to refine it further with climbing; the useful confirmation is external: does the crack length change when the crown is loaded, does the gap open and close, is there crushed or split bark at the union.
- Resist the temptation to drill. An increment core or resistance drill through the union on a cracking tree tells you about wood, not about the displacement that is already visible, and it does not change the decision in the next 36 hours.
2. Explain how likelihood of impact and consequence combine here, and why the answer differs at 08:20 and at 07:00.
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Risk is the product of three things: likelihood of failure, likelihood of impact, and consequence. The tree's likelihood of failure is constant across the day; the other two are not.
- Likelihood of impact depends on whether the target is present when the tree fails. At 08:20 the sidewalk carries a dense stream of children in a narrow window; occupancy is high. At 07:00 the same sidewalk is nearly empty, so the likelihood of impact is low even though the tree is identical. Overnight it approaches very low.
- Consequence is set by what is hit and how vulnerable it is. A child struck by a 45 cm stem from 2.5 m is a fatality scenario: severe consequence, whether that child is the only person present or one of fifty.
- Combining them gives a risk that is high at drop-off and pick-up and low at 07:00. That is not a technicality; it is the basis for a defensible management choice. A time-based control (close and barricade the walk during peak hours and during the wind event, reroute through the school grounds) is legitimate where the occupancy pattern is known and enforceable.
- What I would avoid: using the low overnight risk to justify leaving the walk open, or rating the tree once for the whole day. Rate it for the exposure that actually occurs, and say so in the report.
3. Set out your recommendation, timeline, and what you would put in writing to the school. Address whether pruning or cabling is an acceptable alternative.
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Recommendation: remove the tree. A union that has begun to split cannot be made reliable by pruning or cabling, and there is no assessment or treatment that restores a cracked fork in 36 hours.
- Timeline. If removal can be completed before the forecast front, do it. If not, close and physically barricade the sidewalk immediately and reroute pedestrians, then remove as soon as the weather allows. Do not wait for the crack to grow to justify action.
- Why not prune or cable. Reduction pruning lowers load but leaves the split union; the crack continues to open under the new geometry. Cables are a prophylactic measure for a union that has not failed, not a repair for one that has. Installing hardware from a bucket or by climbing a cracking tree also puts the arborist in the fall zone for no risk reduction. Neither belongs in a 36-hour emergency response.
- What goes to the school, in writing: the defect found and its date, the rating and its basis, the fall zone (roughly the tree's height plus clearance for the stems), the specific area to close, the removal date, and who is authorized to reopen. Copy the municipal or campus authority responsible for the right-of-way so the closure is not undone by someone who was not in the conversation.
- Documentation of the decision. If the school declines immediate closure, record the recommendation, the date, the person informed, and their decision. The technical judgement does not change; the responsibility for acting on it does.