Tree Maintenance, Pruning, and Risk Management
This domain covers pruning standards, maintenance scheduling, and the risk assessment process that drives municipal liability management.
Core rules
- ANSI A300 Part 1 governs pruning: final cuts just outside the branch collar and branch bark ridge, no flush cuts, no topping; recognize cleaning, thinning, raising, reduction, and structural pruning.
- Structural pruning of young trees prevents most future failures and is far cheaper than corrective work on mature trees.
- ANSI Z133 sets the safety requirements that any crew, in-house or contract, must follow.
- Risk assessment follows ISA practice at three levels: limited visual (Level 1), basic (Level 2), and advanced (Level 3).
- Risk combines likelihood of failure, likelihood of impact on a target, and consequences. No target, no risk.
- Mitigation options include pruning, cabling and bracing under A300 Part 3, target relocation, monitoring, and removal.
Worked example
A 24-inch silver maple with a codominant union and decay sits beside a bus shelter. Likelihood of failure is probable given the union and decay; likelihood of impact is high because the shelter is occupied daily; consequences are severe. The combination rates High. Mitigation: reduce the more exposed leader, install a cable per A300 Part 3, relocate the shelter while work proceeds, and reinspect in 12 months. Removal becomes defensible if decay exceeds the amount of reduction that can be done safely.
Traps
Rating risk by tree size alone. Calling a tree hazardous when nothing can be struck. Using cabling as a substitute for removal. Pruning oaks or elms in spring in regions with oak wilt or Dutch elm disease. Treating a risk rating as permanent.