Neighbourhood renewal projects involve machines tearing up big chunks of old roadways and sidewalks, followed by heavy dump trucks, packers and pavers – the scale is BIG!
But in parallel with the highly visible construction activity, a team of City of Edmonton urban foresters – with the eager assistance of Transportation Services engineers – are focused on the latticework of tiny root hairs and the structural roots that are so necessary to preserve the health of trees in the renewal area.
The City has a long-term plan to double its urban tree cover (to about 20% of the city’s area), so when neighbourhoods are renewed, great care is given to protecting the trees from construction damage that can often take several years to show up in dying branches.
“People don’t understand how much detail goes into maintaining healthy trees,” says Bonnie Fermanuik, Community Services’ senior urban forester.
She explains that, over many years, settling airborne dust combines with sand and gravel from winter road grading, building up the soil level and forming a ‘soil cone’ around the base of trees that can raise the original soil level by several inches.
When people walk on this area, or when maintenance machines drive over it, the soil is compacted. The combination of the raised soil level covering more of the structural roots at the tree base, and the more compact soil, reduces the amount of air and water that penetrate to nourish the tree.
Bonnie says that effect is compounded by the fact that grass out-competes fine tree feeder roots for water in dry years.
“So when we re-sod boulevards in a renewal neighbourhood, we pay very close attention to the immediate base of the tree.
“We forbid contractors from removing and replacing sod within two meters of the tree, which protects that soil from machine compaction. It also protects roots in what we call the ‘sacred zone’ from mechanical damage.”
At this stage, a technique that’s unique in North America is used to non-mechanically remove the soil from around the feeder and structural (buttress) roots in that critical six meters. A special compressed air wand is used to literally blow soil away from the roots.
“It does no mechanical damage while it completely exposes all of the roots,” says Bonnie. “Then we re-fill the area with loose soil that will absorb far more air and water than the soil we blow away ever did. Fresh sod is sometimes then laid over the new soil layer, however we urban foresters prefer gentler hydro-seeding.”
Bonnie says that’s only one part of how trees are protected in renewal projects.
“Neighbourhood renewal engineers have been great in working with us to protect large buttress roots from damage in the laying of new sidewalks and the rebuilding of curbs.
“Normally, in order to build a form for a new curb, they’d cut six inches of root where the root had reached the old curb and stopped. Now, wherever it’s possible, and where there are trees that could be damaged by root cutting, they narrow the road by six inches instead of cutting the roots on the boulevard side of the old curb.
“And in many specific locations, where a buttress root is growing toward or under a sidewalk, the engineers mitigate future damage both to the sidewalk and the root system by building a curve into the sidewalk (see photo).
“In those locations, they use more reinforcing bar in the concrete to make it more flexible, and they score the concrete more often to limit future root damage to a smaller part of the sidewalk,” says Bonnie.
Foresters and engineers have also required contractors to use lighter equipment with wider tracks to strip sod outside the ‘sacred zone’ and when pulling up old sidewalks.
“This limits the compaction of the soil in those areas because their weight is distributed over a greater area, and it tends to protect some roots that are right under the sidewalk,” she says.