Analyzing the Arborist’s Report

Using the Sunshine Act, we obtained the report from Hort Science, San Francisco Recreation and Parks’ hired arborist.
The arborist evaluated 250 trees near the Recreation Center, and another 377 trees elsewhere in the Park. They recommended felling 247 of the trees – nearly 40% of the ones they checked. Why?

Here’s the breakout:

For those who would like to go into detail, here’s how the Hazard Rating system works.


The method Hort uses for rating trees is this. It considers three factors:

  • Failure potential (How likely is it that the tree would fall or drop a limb);
  • Size of the part that would fail (the size of the tree or the branch);
  • Value of the target area (if it fell, what would it damage?).

Each is scored on a scale of 1-4, and the scores are added together. This gives a tree a  rating of 3 (least problematic) to 12 (most problematic).  Only a small young tree far from any road or building or playground would be a 3. The RPD action threshold is 9, and most trees with a 9 rating (and some with an 8 rating) would be removed.

This methodology is strongly biased against large trees in busy areas. For instance, a big tree near a roadway would get a score of 4 for size, and 4 for “value of target.”  This means it is an automatic 9, because the score for “failure potential” cannot be less than 1.

We have a problem with this, but that is an issue for another day. For now, we would like to point out that only 39 trees of the 627 evaluated in Glen Canyon were rated 9 or higher.

The immediate threat is to the 70 trees that have been contracted already – only one of which was rated “hazardous.” We based the map below on the one from the bid documents for the project contract. We looked at each tree that is contracted to be cut down, and checked it against the Hort Science report. Here’s what we found: One hazardous tree. Fourteen trees dead or dying (and there are reasons to save those for wildlife). Ten trees that are being felled to move the tennis courts and make way for the grand entrance. And 42 trees that have “poor suitability.”

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3 Responses to Analyzing the Arborist’s Report

  1. Pingback: Glen Canyon Trees: Who Knew? « Save the Trees of Glen Canyon Park, San Francisco

  2. Pingback: Glen Canyon: The Good, The Bad, and the Ugly « San Francisco Forest Alliance

  3. Toby Crane says:

    Thanks for sharing this information and what we need to know about the arborists because in Edmonton they are serving to the max. Keep up the great posts.

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