Seeing green

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May 23, 2011
By: Dylan Thomas
Dylan Thomas
Minneapolis uses satellite imagery to track a valuable resource: trees

From QuickBird’s perch — in an orbit roughly 280 miles above the earth’s surface — Minneapolis in June looks awfully green.

That high-resolution imaging satellite was just one of the high-tech tools a team from the University of Minnesota used to produce the most detailed map ever of the city’s urban canopy. We now know trees shade about 31.5 percent of the city, and that number will serve as the new benchmark for Minneapolis as it plans new investments in its urban forest and devises strategies to protect it.

“That’s the whole goal here: Can we grow the canopy?” said project coordinator June Mathiowetz.

Beside their obvious aesthetic benefits, Mathiowetz said cities value trees for their contributions to the environment and property values. Analysis of the canopy map already suggests where the city might fill in gaps along streets, or where homeowners might add some shade to cut summer heating bills.

And with the recent arrival of emerald ash borer, an invasive pest that preys on one of the city’s most common trees, Minneapolis must also prepare to face the most significant threat to its urban forest since Dutch elm disease ravaged its stately boulevard trees.

“There’s a huge injection of technology that’s making this work potentially much easier, especially in years to follow,” Mathiowetz said.

The canopy map’s digital data can be broken down neighborhood-by-neighborhood and even plot-by-plot.

The tree canopy covers 41–60 percent of Southwest’s most forested neighborhoods, like Bryn Mawr, Kenwood and Linden Hills. Fulton, Lynnhurst and Tangletown, in a belt of green following Minnehaha Creek, join them in that highest canopy category.

West Calhoun, with its tangle of streets and expansive surface parking lots, is Southwest’s bald spot.

A digital bird’s-eye view

Most of the cost of the $33,000 mapping project was funded through a $29,000 Metro Greenways Grant from the Minnesota Department of Natural Resources. The city and Minneapolis Park and Recreation Board kicked in $2,000 each.

For their money, they got a surprise: Tree canopy covers more of the city than previously thought.

A 2004 U.S. Forest Service study found Minneapolis had about 979,000 trees — mostly green ash, American elm and boxelder — and estimated the city’s tree cover at 26.4 percent. But that study extrapolated from on-the-ground surveys, a method both more labor-intensive and less exact than the use of satellite imagery, Mathiowetz said.

Professor Marvin Bauer’s University of Minnesota team combined June 2009 imagery from QuickBird with LiDAR (light detection and ranging) data collected two years earlier to create their Minneapolis map. QuickBird’s color and near-infrared cameras aid in telling green trees apart from black streets and blue lakes, while the LiDAR — which scans for height and elevation — separates tall trees from similarly colored, but shorter, grass or bushes.

The digital bird’s-eye view of Minneapolis also tabulates other land-cover types, like water, buildings or other impervious surfaces. Combined with the city’s zoning or land-use maps, it indicates where there is the greatest potential to plant new trees, such as industrial areas.

Bauer, who completed similar studies for St. Paul and Woodbury, said it he thought it was the first time satellite imaging was used to study urban tree canopies in Minnesota. Compared to other cities that have done similar studies, Minneapolis’s tree canopy trails Baltimore (49 percent) and Washington, D.C. (35 percent), but beats New York City (24 percent) and Providence, R.I. (23 percent).

Setting goals

In Minneapolis, the Park Board manages tree care on city right-of-way and
in parks. But for Park Board Forestry Director Ralph Sievert, the canopy map offers a valuable peek into city resident’s back yards just one year after the first confirmed sightings of emerald ash borer.

Infestations are almost certainly fatal for ash, which make up nearly one-fifth of the city’s boulevard trees. The Park Board is proactively removing sick or malformed ash and replacing them with a variety of hardy trees, Sievert said.

Before it saw the results of Bauer’s study, the city had set a goal of growing the tree canopy to 30 percent by 2030. Even though it’s beating that “sustainability indicator” — one of a number tracked in the annual Minneapolis Greenprint report — it’s a goal may need to be recalibrated.

“To me, it’s going to be a challenge to keep that, because you’re going to lose a lot of trees at some point,” he said. “Even if everybody planted a tree in their backyard, there’s lag time in a young tree growing the kind of canopy as the tree that’s been lost.”

Sievert said the Park Board already knows some parts of the city are disproportionately planted with ash trees, like the Harrison neighborhood just north of Bryn Mawr, and stand to be hit particularly hard by emerald ash borer. But both Sievert and Mathiowetz suggested there might be way to target tree-planting programs, such as the annual spring tree giveaway, to counteract heavy losses of ash.

At the neighborhood level

Work by one of Bauer’s graduate students also identified about 3,000 locations on the west side of Minneapolis homes where a bit of shade could help shave summer heating bills, a finding Mathiowetz called “potentially a huge step forward in energy conservation.”

“I’m actually an example of that,” Sievert added. “I planted a little Japanese tree lilac in my backyard on the west side just to keep the sun from beating on the back window.

“That was about 1998, and now it has an 8-inch diameter on it and it works really well.”

Access to the canopy map is limited, for now, but Mathiowetz said she was fulfilling all requests to get a look at the data. There could be strong interest in Southwest, where neighborhoods like Bryn Mawr and ECCO are developing their own plans to get ahead of emerald ash borer.

Said Mathiowetz: “What this does, potentially, is give neighborhoods some tools to really work on some neighborhood
tree plans.”

Reach Dylan Thomas at dthomas@mnpubs.com.