Canopy Assessment

A tree canopy assessment was recently completed by the University of Vermont, and will be shared publicly at the Select Board meeting on April 24th, 2023. You can download it here.

Overall, this survey showed a 3% gain in Lexington's total tree canopy from 2014-2021. This is good news for the town, and reflects well on conservation efforts.

However, The analysis in the UVM report can in no way be used to evaluate the impact of the Town’s tree planting efforts, efficacy of the Tree Bylaw in protecting setback trees during construction, or motivate changes to the Tree Bylaw or other policy relating to trees in or adjacent to public spaces. The trees which fall under the purview of the Bylaw and DPW constitute far too small a fraction of Lexington’s trees and are not segregated in the UVM analysis. Further analysis of the raw data could extract the relevant canopy fraction.

Tree canopy along streets, sidewalks, and near homes has the greatest impact on quality of life.
The UVM Canopy Assessment does not discriminate between these trees and those in deep backyards and conservation land. All trees are lumped together in 50 acre hexagons.
Trees in those deep reservoirs are generally not subject to the Tree Bylaw or under the purview of the DPW.

In the following analysis, we use 3D data from Google Earth to extract the canopy fraction in areas that are relevant to the Tree Bylaw and contribute most to quality of life. The results are very different when compared to the aggregated data.

Milk Street, Lexington

An average neighborhood, as indicated in the UVM report, with a canopy fraction of 54.8%-67.6%.

The actual canopy fraction along this street is 25.6%.

The total canopy fraction in a hexagon is not a good indicator of the canopy near the public right-of-way.

Woodcliffe Road, Lexington

An above average neighborhood, as indicated in the UVM report, with a canopy fraction of 67.7%-82.7%.

The actual canopy fraction along this street is 40.1%.

The total canopy fraction in a hexagon is not a good indicator of the canopy near the public right-of-way.

Urban Areas

Cambridge, MA relied on a series of canopy assessments from UVM when crafting their tree bylaw. This type of analysis makes sense in such an environment where nearly all the trees are very close to the public right-of-way and subject to the bylaw. The loss/gain trends of "important" trees (those addressed by public policy) track the totaled numbers. In the following analysis, you can see how a large fraction of Cambridge trees are in close proximity to the public right-of-way, leading to the opposite bias compared to what we saw in Lexington: the actual tree canopy along the street can be much higher than the average in the area.

Chalk Street, Cambridge

An average neighborhood, as indicated in the UVM report, with a canopy fraction of 25%-40%.

The actual canopy fraction along this street is 63.5%.

Trees impacted by the Tree Bylaw (1.2%)

Only 0.2 sq. mi., or 1.2 % of Lexington's land area, has been subject to the Tree Bylaw's rules for major construction over the 2014-2021 period of the UVM report (estimated).

273 New Construction permits filed in 2020-2022, equates to 637 over 7 years.
Average lot is 0.42 acre.
Average setback area is 8,800 sq. f.t.

Contribution of Street Tree Planting

Let's assume that Lexington plants 150 trees per year, and that we plant 4-year-old, 3" diameter Red Maples. (These are very generous assumptions)

When we account for growth, the total crown area of all trees planted over that 7-year period would be 175,350 sq. ft., or 0.038% of Lexington's land area.

Reference: “Urban tree database and allometric equations” (2016)

E. Gregory McPherson, Natalie S. van Doorn, Paula J. Peper

Source: Gen. Tech. Rep. PSW-GTR-253. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 86 p.

Contribution of All Street Trees

The Lexington Tree Inventory lists 22,049 living street trees. If we assume they are all in good health and use Google Earth to sample representative tree canopy sizes, we find that the entirety of Lexington's street tree canopy accounts for 2.4-4.8% of the Town's land area.

A Better Analysis

Trees along streets contribute the most to heat island effects and quality of life in public spaces.
We suggest specifically calling out trees within a 90-ft.-wide corridor along our streets.
The Town has 137 miles of paved roads, meaning trees on an estimated 14% of the Town's land area are most relevant to public policy.

Summary

As a crude analogy, imagine you want to know if the diet you are on is helping you lose weight, but every time you get on the scale 20 other people get on with you. It is impossible to separate the meaningful data from the background.

This is the situation with the UVM Canopy Assessment. Only a small fraction of our land area is subject to Town policy or contributes to public spaces, but all the data is binned together.

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