Sidewalk repairs doomed some of the 3,000 Minneapolis trees that were toppled by strong winds and storms last June, prompting recommendations to prevent such a massive tree fall during future storms.

In looking at 400 of those trees, a study said the odds of a tree falling increased if the tree was large, next to recent sidewalk replacement work, or on a narrow boulevard.

The reason when sidewalks or street repairs were to blame? Those actions can damage some of the tree's strongest roots.

The findings are part of a $30,000 study commissioned by the Minneapolis Park and Recreation Board after observing most of the tree loss appeared to have occurred on boulevards, the grassy corridors between sidewalks and streets, and not in yards.

The city is taking a strong interest in the health and longevity of its trees because so many have been lost in recent years and more are coming down. A 2011 tornado wiped out 3,200 trees. And this year, the city begins a $9 million project to take down 40,000 ash trees in the city's parks and on its boulevards and golf courses to blunt the impact of the deadly emerald ash borer.

For five months, Gary Johnson, the primary investigator and a professor at the University of Minnesota, and his team of researchers examined the fallen trees, tested the soil and poured through hundreds of public works records to determine when the repairs were made. In addition to sidewalk repairs, Johnson also found the more dense and tall the tree was, the more likely it was to topple over, in particular linden trees.

Johnson recommends Minneapolis Public Works and Park Board staff should better collaborate when planning sidewalk repairs or other street work, like curb repairs. For future street planning, Johnson also suggested making boulevards at least 8 feet wide. Trees in boulevards that were less than 4 feet wide were also most likely to fall where sidewalk work made them more susceptible, Johnson found.

Heidi Hamilton, deputy director of Minneapolis Public Works, said her department is reviewing Johnson's recommendations and does not know how much it would cost the city to implement them. She said some of his recommendations are more realistic than others.

For example, Johnson recommends building sidewalks that are above tree roots. Hamilton said that would require the city to remove and replace large amounts of pavement panels. As for the 8-foot boulevards, Hamilton said that would depend on many factors like the addition of bike lanes and other street features.

Nonetheless, she said the city plans to work with Park Board forestry staff.

"I think the Public Works Department and the Park Board will work together and come up with the best way to preserve our urban tree force," Hamilton said.

Alejandra Matos • 612-673-4028

Twitter: @amatos12