How do you decide whether or not to save a tree on a construction project (Part 3 – risk assessment)?

In recent posts, we have talked about some of the factors that go into deciding whether to retain or remove a tree on a construction site. We have talked about signs and symptoms on the tree that may indicate how healthy and stable it is, and we talked about identifying and assessing potential targets – things that could get hurt or broken if a tree or part of a tree fell on them. Today we’re going to touch on industry standards and best practices for tree risk assessment.

The national voluntary consensus standard for tree risk assessment is ANSI A300 (Part 9)-2011 Tree Risk Assessment. The Secretariat for this document is the Tree Care Industry Association.

Image

It identifies three levels of assessment:

  1. A Level 1 assessment is a limited visual assessment of a tree or trees near specified targets. Level 1 assessments include: an aerial patrol of a utility transmission line; a windshield survey of street trees; an assessment made using Google Street View; or, an assessment of your client’s neighbor’s tree from your client’s property.
  2. A Level 2 assessment is a 360-degree, ground-based visual inspection of the tree crown, trunk, trunk flare, above-ground roots, and site conditions around the tree in relation to targets.
  3. A Level 3 assessment is anything more involved than a Level 2 and may include things like evaluation of target risk; aerial assessment with climbers; collection and processing of samples; use of ground-penetrating radar or sonic tomography; etc.

These are not mutually exclusive. For a large population of trees, it may be more efficient to specify a Level 1 to identify trees with open and obvious defects and then create a subset to perform Level 2 assessments on rather than performing Level 2 assessments on all trees as an initial step. Depending on the tree(s) and site, there may be need for Level 3 assessments for a subset of that group.

There is also a Best Management Practices guide for tree risk assessment. It is published by the International Society of Arboriculture.

TRA BMP

The industry has moved in recent years from quantitative tree risk assessment to qualitative tree risk assessment as shown in this matrix.

probability x consequences table

It based on the general risk assessment formula: Risk = Probability x Consequences.

In our first post in the series, we touched on some of the factors in visual assessment of probability of failure. In the second, we look at visual assessment of consequences of failure. This brings us to the standard table shown here.

I would like to note that only trees very likely to fail with severe consequences in the event of failure pose an extreme risk. Only trees likely to fail with significant consequences in the event of failure pose a high risk.

Under severe conditions (high winds, etc.) intact trees may fail; this is intended to identify trees that may fail in less than severe conditions.

We have covered assessing tree condition (probability of failure) and potential targets (consequences of failure), as well as industry standards and best practices for tree risk assessment. Registered Consulting Arborists can provide you with industry-compliant tree risk assessments for your project. In our final post of this series, we’ll talk about the national standard for management of treesduring construction and how what we have discussed to this points fits in.