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Tree Canopy Coverage Increases Eight Percent in a Decade

Oct 17, 2018 - A recent report showed an 8% combined increase in Mercer Island’s urban tree canopy across all land use categories (click here for 2-page factsheet). Plan-It Geo, a consultant for King Conservation District, analyzed high-resolution aerial imagery and LiDAR data from 2017 and 2007 to assess current canopy and canopy change over the 10-year period.  Canopy cover on single-family residential parcels, which account for three quarters of the island’s land area, increased 7% to 47% coverage; parks and open spaces canopy increased by 6%; and the business district (Town Center) saw a 10% jump in canopy cover since 2007. The report also highlights areas that could benefit from increased tree canopy and future tree plantings.

Many factors contributed to this growth, including tree protection and replanting requirements in the City’s development code; a robust restoration program that plants thousands of trees each year in City parks and rights-of way; and voluntary planting efforts. Evan Maxim, the Interim Development Services Director commented, “We are very pleased to see that the City’s tree protection and replanting requirements have been effective in expanding our tree canopy in residential neighborhoods as well as Town Center, even with substantial redevelopment and growth on the Island over the last 10 years.  This is incredibly important in maintaining quality of life as our community grows.”

Alaine Sommargren, Parks Natural Resources Manager, was excited and encouraged by the results of the assessment. “Tree canopy measurements are an incredibly useful tool in assessing how well we care for our natural resources,” Sommargren said. “Tree canopy expansion does not happen quickly, so this 8% increase in canopy city-wide speaks to the island’s long-term commitment to stewarding the natural environment.” 

The City expanded its commitment to tree protection last fall when the City Council adopted stronger tree protection regulations in the development code and increased the city arborist position from half- to full-time. Mercer Island was one of 15 cities in King County to participate in King Conservation District’s 2018 Urban Forestry Initiative. This ongoing initiative supports regional projects that bridge stewardship across private and public lands and focuses on forest health and community stewardship.  In 2018, KCD offered partner cities a full tree canopy assessment. 

The current canopy coverage is high, even compared to surrounding Puget Sound cities. Sommargren commented, “This doesn’t mean the work is over.  Maintaining tree canopy and continuing to plant young trees to take the place of declining canopy is imperative to preserving the benefits that trees provide the city.”  Trees filter pollutants and sediment from stormwater entering Lake Washington, lower air temperatures, reduce air pollution, and provide myriad aesthetic, health and community benefits. The City will use the results of the 2018 analysis and planning software to develop a continued strategy for conserving and maintaining Mercer Island’s urban forest.

-- Read a 2-page summary factsheet
-- Read the full report

Become a Forest Steward! For more information on upcoming volunteer opportunities for planting and protecting Mercer Island’s urban forest, please visit


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