Sunday, May 19, 2024

Tree canopy change in Cambridge from 2009 to 2020 in a graphic from an October report.

After months of stonewalling, the city released a tree canopy report Jan. 4 that adds data from 2020. The city claims this report “shows that the implementation of the Urban Forest Master Plan (UFMP) is improving our tree canopy.” This is not true. The plan wasn’t complete until just before the new data was collected.

The key reported improvement is a 7 percent canopy gain for 2014-2018, before the Urban Forest Master Plan Task Force even started. This gain contradicts the task force reporting of a dramatic 10 percent loss for that time.

The implications of these contradictions are profound.

Claiming that the tree canopy recovered without the plan means that trees can be cut down as they always have been, that the plan does not need more funding and that zoning need not change to encourage tree planting or preservation. It means that the millions spent on the plan were wasted.

Pullback has already started. Forestry workers have said they plan not to ask for a budget increase despite the only partial implementation of the plan. This is bad.

Recovery created by changing previous data

The graphs below show how the new report creates recovery by changing previous data, dramatically revising the 2014 citywide tree canopy area down 22 percent. Revising the 2009 area reduces the canopy needed to get back to coverage at that time.

Originally, 2014-2018 was reported as a 10 percent loss of tree canopy area. The new report flips that loss to a 7 percent gain. This is a remarkable 17 percent error when the 2018 report claimed accuracy so great that “trees as small as eight feet in height [are] mapped.”

“Does this make sense?”

The original 2020 report said “Tree canopy loss is typically … nearly instantaneous. … Tree canopy gains, whether due to plantings or natural growth, are slow and occur over decades.”

Some losses during 2014-2018 include the historic, tree-killing 2016 drought that exceeded this past summer’s drought in some metrics. It also includes the die-off of more than 100 trees in the North Cambridge Linear Park, the mass tree cuttings at Jerry’s Pit and along the passenger rail lines throughout the city.

Some gains were from the city planting saplings. Deducting failures, an average 454 saplings were planted each year – less than half the street tree plantings the Urban Forest Master Plan recommends for canopy recovery. With only an average of 2.5 years to grow, it’s obvious these saplings were but a tiny part of canopy gain.

My high school science teachers always asked, “Does this make sense?”

It does not seem credible that the slow-growing gains kept up with the mass tree cuttings – especially as the UFMP Task Force experts created many detailed tree canopy recovery scenarios and none demonstrated the dramatic recovery shown in the new report. We are also asked to believe that all 20 of the plan’s experts missed that the canopy was experiencing stunning gains rather than alarming losses.

To be sure, the new report admits to revising the data:

“Due to improved accuracy achieved through the harmonization process, tree canopy numbers in this assessment may differ from previous analyses. Original tree canopy outputs for 2009 and 2014 were much coarser in resolution and required smoothing prior to harmonization.”

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.

Cambridge is one of many customers of the fee-based report production facility affiliated with the University of Vermont, which uses undergraduates to “correct” data manually. Pandemic staffing issues caused the report’s delay. Perhaps accuracy was also affected.

Given the profound climate, monetary and public health policy implications, it’s critical that the new report be credible. Unfortunately, it is not.

What can you do?

Your city management and City Council need to hear from you, repeatedly. Tell them that the city needs to commit to fully funding and implementing the Urban Forest Master Plan with the original tree canopy goal. Tell them to create a new, independent parks and forestry department with new, professional management to produce a credible schedule and budget for the plan.

Finally, you need to show up for public budget and tree meetings. 

For notification, sign up at Cambridge4Trees.org.


Charles Teague has been advocating for preserving the tree canopy and Linear Park since 2016.


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