9.5 Mitigation During Construction

Matching Mitigation to the Damage Pathway

Mitigation is not one treatment applied to every site. It is a targeted response to a predicted impact. If roots may be cut, consider route changes, boring, root-sensitive excavation, clean selective root pruning, and inspection. If soil may be compacted, exclude traffic first, then add ground protection only where limited access is unavoidable. If grade may change, preserve original grade near the tree or design transitions that do not bury the root collar.

The Mitigation Hierarchy

Avoidance is usually the best mitigation, and the exam favors it. Moving a trench outside the TPZ beats repairing cut roots; keeping equipment off rooting soil beats trying to decompact it later. But real projects have constraints, and the test often asks what to recommend when conflict remains. The answer should reduce injury while staying practical.

Root-Sensitive Excavation

Use air excavation (air-spade) or hand tools to expose roots near the tree so you can see them rather than tearing them blindly. Compressed-air tools loosen and remove soil without cutting roots, which lets you reroute around a structural root or make a single clean cut where pruning is unavoidable. If roots must be cut, cuts should be clean and judged by size and location: a large buttress root within five times the trunk diameter is not equivalent to a pencil-thin absorbing root well out in the zone. The arborist should be on site for critical operations when root decisions determine whether preservation succeeds.

Ground protection can include mulch layers, timber mats, or steel plates that spread load. Install it before traffic, remove it when no longer needed, and treat it as a controlled exception, not a license. Vehicle weight, route width, duration, and soil moisture all matter; wet soil compacts far more readily than dry soil, so timing access to dry conditions is itself a mitigation.

Water Management

Water management is mitigation too. Construction can interrupt irrigation, compact soil so water runs off, expose roots to drying, or change drainage. Base watering on soil-moisture checks and tree need, not the calendar. Overwatering compacted or poorly drained soil worsens oxygen stress; underwatering exposed roots speeds decline.

Field Response Steps for Root Encounters

1. Stop work in the immediate area long enough to assess the root.
2. Identify root size, location, condition, and likely function.
3. Keep exposed roots moist and shaded while deciding.
4. Adjust excavation method or alignment if feasible.
5. Make clean cuts only when root pruning is necessary and appropriate.
6. Document what was found, what was done, and who approved it.

Scenario: A crew finds roots while excavating a sign footing. Yanking them with a backhoe is the wrong answer because tearing enlarges wounds and removes more tissue than needed. Pause, expose the area carefully (ideally with an air-spade), evaluate size and location, and decide whether the footing can move or whether controlled root pruning is acceptable.

Scenario: A crane must cross near a preserved tree for one day. Do not allow access just because it is brief. Soil moisture, machine weight, path width, root location, and ground protection all govern the risk. A defined route with load-spreading mats and inspection may be acceptable when avoidance is impossible, but open traffic in the TPZ is not.

Trunk, Branch, and Soil-Volume Mitigation

Not all mitigation is below grade. Where equipment must pass close, protect the trunk with spaced vertical planks strapped over a cushioning layer (never nailed into the bark) so glancing contact does not gouge cambium. Where overhead clearance is needed, specify objective-based crown raising or reduction performed to ANSI A300 Part 1 by a qualified arborist before the project, rather than letting equipment operators tear limbs.

Where a tree will lose rooting soil permanently to new pavement, mitigation can include structural soil, suspended pavement (silva cells), or root paths that give roots usable volume under hardscape instead of compacted base.

Timing is a mitigation lever the exam rewards. Scheduling heavy access during dry-soil periods, doing root pruning during the dormant season when feasible, and sequencing demolition so fencing is installed first all reduce harm at little cost. For the exam, choose mitigation that addresses the actual pathway of harm: fertilizer does not repair torn structural roots, mulch does not justify burying the flare, and watering does not fix a severed root plate. Control the source of injury first, then support recovery.