Sunday, June 16, 2024

A tree in Cambridge’s Larchwood section in June 2022. (Photo: Marc Levy)

After the shock four years ago that 18 percent of Cambridge’s tree canopy had been lost since 2009, it seemed ominous when a tree canopy progress report was delayed. The announcement of its release in a Jan. 4 city email was largely lost below news that a Cambridge police officer had killed a man in Cambridgeport and a statement from city officials about that shooting.

The only official presentation so far of the tree canopy update was at a little-attended meeting Jan. 11 of the Committee on Public Planting, while stories of trees being cut down or dying off lingered in public discourse. On Wednesday, though, the update gets more attention at a City Council Health & Environment Committee hearing.

And the news is good: Between 2009 and 2020 there has been only 1.2 percent loss of tree canopy crucial to counter the effects of climate change, and the public is hearing from Andrew Putnam, the city’s superintendent of urban forestry and landscapes, that “the steep decline we saw between 2000 to 2014 is starting to trend in the right direction.”

The report shows that in the 11 years since 2009, the city has lost 474 acres of overall tree canopy but gained 427, for a net loss of 47 acres. “Implementation of the city’s Urban Forestry Master Plan is improving Cambridge’s tree canopy,” the city said in its report press release.

The figures benefit from a recalculation of the loss reported in 2018 by the University of Vermont Spatial Analysis Lab using imaging from Department of Homeland Security flyovers – that is, the starting point for the initial calculations was wrong because it used a low resolution that could detect tree canopy only in solid chunks. That ignored gaps that overestimated the starting point on greenery, according to UVM, while the higher resolutions now being used can identify smaller patches of green and capture growth at trees’ edges.

“That’s where we see a lot of canopy growth, at the edge of large shade trees,” Putnam told the Committee on Public Planting.

The data people saw and relied on in 2018 has been revised.

“The flyover data uses the same set of [Light Detection and Ranging] data that they’ve always processed. But they’ve gone through and reprocessed it to match pixel sizes and smoothed it out using higher-resolution technology,” Putnam said. “It was actually less canopy to start with, is what they were finding once they had a higher resolution.”

Staff knew the “harmonization” would be confusing. When they got a draft of the report dated Oct. 12, they sent it back to the University of Vermont lab so it could add a better, more visual explanation to its report, as well as a mapped breakdown of tree loss or gain by neighborhood. The report came back around Dec. 20, but that’s when people’s minds were on the holidays, said Kathy Watkins, commissioner of Public Works, speaking by phone Jan 12. Putnam was also on the call. 

When the tree report was ready and the announcement went out, it happened to be on the day of a fatal shooting.

Seeing shadiness

Wile the UVM lab added clarifying information about its new approach, it didn’t run the data to show Cambridge’s tree canopy status without the revised figures. “The goal is to use the best available information,” Watkins said, “and they’ve improved their analyzing tools.”

There is some public skepticism. Tree activist Charles Teague expressed some in an essay Jan. 11:

It seems that high-resolution data was simulated from low-resolution data by writing some software to “smooth” the “coarse” data. Counting high- and low-resolution pixels in the report’s example yields only a 1 percent difference in area. This is expected if the goal is to simulate high-resolution data and not to create new data. But there is a 22 percent difference in 2014. The new report fails to remark on this dramatic difference, saying simply that “numbers  … may differ.” There is also no explanation why the numbers were not revised in the prior report, which had the identical mix of resolutions.

With mature trees coming down at construction sites and replaced with saplings that are slow to grow but quick to fail, Teague wrote that he finds it hard to believe the rosier picture of the new report: “My high school science teachers always asked, ‘Does this make sense?’”

Teague worried that by declaring current tree programs successful – whether through revising old data or not – the city would have a pretext for smaller tree-program budgets.

Sunnier view

His skepticism isn’t shared by every citizen activist, including Green Cambridge executive director Steven Nutter.

“It’s an evolving science,” Nutter said Jan. 11 when asked about the technological reasoning behind the retroactive change in canopy assessment. “I’m not a data scientist, but I do trust that it’s the University of Vermont and they have sound reasonings behind any changes.”

Nutter said it was important for the public “to not necessarily get hung up on the specific numbers, but just look at the overall trajectory and how the city is prioritizing tree planting and tree protection.”

That was a perspective endorsed by Watkins and Putnam, who say they are not resting on canopy improvements. “It’s just encouraging that as long as we continue taking up the action steps of the Urban Forestry Master Plan, we can make really tangible changes,” Putnam said. “There have been some high-profile removals that stick in your mind, and there’s an emotional attachment to those trees. But when we look citywide, we start to see things trending in the right direction.”

Digging into other concerns

To concerns that tree budgets didn’t seem to be rising for the next fiscal year, Watkins pointed out that more work is being done in-house than in the past, which lowers operating costs. And capital funding is still being looked at. “There’s absolutely a commitment in terms of continuing to fund the program both on the operating side and on the capital side,” she said.

The Urban Forest Master Plan deserved credit for guiding staff in part because they started following it long before a final version was released publicly, Watkins said.

There are more reports to come, and Putnam’s timeline doesn’t include more delays. (The current report was expected the winter of 2021-2022, making it a year late with the lab declining to give a public explanation.) The city expected to commission a canopy report with 2023 data to capture the effects of the most recent drought, then see a more in-depth study in 2025, Putnam told the Committee on Public Planting.

  • The Health & Environment Committee run by city councillor Patty Nolan gets an update on the Urban Forest Master Plan and the tree canopy at 12:30 p.m. Wednesday at City Hall, 795 Massachusetts Ave., Central Square. Televised and watchable by Zoom video conferencing. (Update on Feb. 28, 2023: This meeting has been canceled.)