Portland losing tree canopy
Two recent studies show declines in Portland's tree canopy between 2015 and 2020. Although the losses are relatively small, they come after significant increases over the previous 15 years.
And there could be greater losses in coming years because of new residential developments policies, according to one of the researchers.
One study was conducted by Portland Parks & Recreation and presented to the Portland City Council on March 16. It found the tree canopy increased in all parts of town by 3,112 acres between 2000 and 2015. But, the study said, the canopy declined by 823 acres over the next five years. That is a net loss the size of Mount Tabor every year, the study said.
The parks study said the tree canopy fell in every zoning class of the city after 2015. The largest loss — 524 acres — was in the residential class, which combined both single- and multi-family zones. The study did not offer any reasons for the reversal, however.
Ironically, 2015 was the year the council adopted new rules to preserve and increase Portland's tree canopy, the study said.
"This important, thorough scientific study is a wake-up call for us," City Commissioner Carmen Rubio, who oversees Parks and Recreation, said when the bureau's study was released. "Trees strengthen our resilience to climate change, but they are also vulnerable to climate-change-fueled weather."
Second study agrees
The other study was conducted by researchers at Portland State University led by Professor Vivek Shandas, who has studied heat disparities across the city. It is not expected to be published in a peer-reviewed journal for several months. But Shandas summarized its findings in general terms with the Portland Tribune after the parks study was released.
The PSU study uses far more data sources than the parks one, which is primarily based on comparisons of aerial photographs of the same locations taken every five years. In contrast, the PSU study includes satellite imagery and searches of government data bases, including property records.
Shandas said the PSU study agrees that Portland's tree canopy increased between 2000 and 2015, and then either stagnated or declined sharply over the next five years. But unlike the parks study, Shandas says the changes since 2015 were concentrated in East Portland neighborhoods such as Centennial, Cully, Foster-Powell, Hazelwood, Lents and Mill Park, Powellhurst Gilbert. Outer Southwest Portland was also especially hard hit. Losses in some of the neighborhoods were as high as 5.7%, Shandas said.
The PSU study also agreed that the largest declines were in residential zones. But, unlike the city's work, the PSU study found no declines in multifamily zones. All were in single-family zones. In fact, Shandas said, the location of building permits taken out in single-family zones were most closely linked to the decline of tree canopy. Looking at where tax parcels were sold alone predicted where tree canopy was lost almost half the time, he said.
"The places that have most of the housing sales have most of the losses," Shandas said.
Although the PSU researchers did not conduct a physical lot-by-lot comparison, that reflects complaints by neighborhood activists over many years who say large trees frequently are cut down when a small, older house is demolished and replaced with a much larger one. The neighborhoods identified by Shandas are among those most likely to be gentrifying in recent years.
"The higher income, the lower the losses. The lower the income, the higher the losses," Shandas said about the income levels of the residents where the changes are occuring.
Shandas says that trend could increase in coming years now that the Oregon Legislature and Portland City Council have essentially eliminated single-family-only neighborhoods to encourage more housing. Up to six units now are allowed on practically every residential lot in the city, including multiple houses and auxiliary dwelling units, multifamily projects with up to six units, and cottage clusters. Many likely will require the removal of existing trees to make room for them.
"Building footprints correspond with losses. The bigger the footprint, the lower the tree canopy. Height is not as important as lateral density," Shandas said.
Trees considered critical
Both studies agree that expanding tree canopy is critical for several reasons. Trees are considered vital to mitigate carbon in the atmosphere because they sequester carbon. They also provide shade in hot weather, which is becoming an increasing problem in Portland because of climate change. During last summer's heat dome, which killed more than 100 people, Shandas measured temperatures as high as 125 degrees in Lents, which has relative few trees, compared to 90 degrees in Northwest Portland near Forest Park.
The City Council has repeatedly recognized the importance of trees. They have been cited as an important indictor in the Portland Urban Forest Management Plan (2004), Urban Forest Action Plan (2007), the Climate Action Plan (2015), and the 2035 Comprehensive Plan (2016).
Some have said the canopy should cover one-third of the city.
Despite the losses in recent years, the city is not that far short of meeting its goals. According to the parks study, the canopy was up to 30.7% in 2015 before falling to 29.8% in 2020. The question now is whether the losses will continue or even increase in coming years, especially in residential zones in East Portland, or whether steps can be taken to reverse the losses.
When the parks study was released, the bureau pointed to voter approval of the 2020 Parks Local Option Levy as a reason for hope. Some of its funding will be used to better protect the 1.2 million park trees in Portland parks by performing proactive maintenance, safety checks, hazard removal, and planting new trees in parks and natural areas. The City Council already approved the addition of 23 full-time employees for bureau's Urban Forestry team last fall.
"PP&R will continue to care for and maintain Portland's trees, to address hazards on city lands and public streets, and to plant more trees citywide," Parks Director Adena Long said when the study was released. "This report shows the need for continued efforts and vigilance.
The bureau will take its next measurements in 2025. its recent study can be found here.
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