On February 3rd, the Minneapolis City Council’s Transportation and Public Works committee received a very important, somewhat disturbing report from Professor Gary Johnson in the University of Minnesota’s Department of Forest Resources. The most striking finding: boulevard trees near recent sidewalk replacement work are more than twice as likely to fail.
The report is based on extensive research Johnson’s team did, with help from City and Park Board staff, after the powerful storm of June 21, 2013. That storm tipped 367 trees in Minneapolis. Many of them landed on people’s houses and cars, many of them (somewhat ironically) destroyed nearby sidewalks, and all of them had to be killed and removed.
The study found that there were some striking correlations for tree failure. The most dramatic, again, is that trees near a recently-replaced sidewalk panel were 2.24 times more likely to fail. Some species were more sensitive – they tend to be the ones that get the biggest, and therefore provide the most canopy. Bigger trees were more likely to fail. And trees in narrow boulevards were more likely to fail than trees in wide boulevards, but only where sidewalk replacement work was done.
The takeaway? The way we repair sidewalks is killing trees.
This can’t continue. Our urban tree canopy is too important to undermine in this way. Canopy has many benefits – to human health, stormwater management, heat island effect, climate, air quality, property values and more. Trees are infrastructure. And urban trees are already facing too many threats and stresses – Emerald Ash Borer, climate change, etc. – for us to do things that make them likelier to die.
This is an issue for pedestrians. When I walk down a street, I want a reasonably flat sidewalk, but I also want shade from trees in the summer. I want to walk down a street that is beautiful. I want to walk down a street that signals to drivers that this is a place where there are people, a place to drive slowly and carefully. Street trees provide these benefits to pedestrians.
So what do we do?
The staff report lays out some good first steps that staff have already taken. This mostly includes a commitment to working better with the Park Board’s Forestry division, and their new Forestry Preservation Coordinator. But more can and should be done.
For example, the City’s Urban Forest Policy, which was adopted in 2004, should possibly be updated. It includes statements like: “[t]he contractor may remove all roots within the area defined as six and one half (6-1/2) inches below the top of the new finished sidewalk grade, by severing them off cleanly with a sharp axe, or by grinding them off using a root grinding machine, instead of breaking them off with a backhoe or similar equipment.” Is it really a good idea to remove all roots within 6.5 inches of an entire sidewalk panel? I think this report says it’s not.
We may have to be a little more flexible on sidewalk grades. Here’s a good example:
The sidewalk panels in front of this house were heaved by the tree on the right. The homeowner was (justifiably, we now know) worried that the City’s standard practice would end up killing her boulevard tree, so she worked with Public Works to get a very slight grade change built into her sidewalk, rather than cutting or grinding the tree’s roots. I apologize for the unexciting picture; the grade change is so subtle that it’s really hard to photograph.
We may have to get more comfortable with grinding sidewalks where they’re heaved by roots, rather than grinding roots when they heave sidewalks. Just as we’ve become more accustomed to including cut-outs for trees that narrow sidewalks a little, for short distances, maybe we need to see our way clear to leaving cracked but still flat panels in place where the replacement work would be disruptive to the roots beneath. Maybe there are creative new solutions others have tried that we can copy here. Whatever the solutions, we need to avoid more of this:
After the report was presented, the committee voted unanimously for a motion made by Cam Gordon (my boss, for the record), directing Public Works staff to work with both the Tree Advisory Commission and Pedestrian Advisory Committee on recommendations for changes to policy and practice, and report back in July.