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Bloor-Annex BIA Parkettes - DTAH, Toronto, ON

Research Corner: Assessing stress to salty street trees

An installation of street trees in Etobicoke is helping to reshape what we understand about street tree health and road salting


The trees within both types of soil cell are notably taller than those in the standard tree pits; maples are represented here in yellow, and elms in pink. IMAGE/ Jen Hill

The benefits of large shade trees in public spaces are well documented, from habitat provision, carbon sequestration, and public health through to shading and positive psychological effects. For these reasons, many cities have set targets to increase their urban canopies.

In terms of stormwater management, trees can be thought of as part umbrella, part drinking straw. They suck water out of the ground and transpire it as an invisible gas—emitted from pores in the leaves—to the atmosphere. They also trap a little rainwater on every leaf, which evaporates directly back into the atmosphere, as from any wet surface. These two separate processes are often described by the single term evapotranspiration.

“Soil cells” or “supported pavement systems” are increasingly being used by municipalities to optimize the water stored in the soil and transpired by trees. They work by reducing soil compaction and permitting more water to be available for the trees to drink and soak away into the ground. However, using this type of system to treat roadway runoff year- round poses the risk of damaging the trees due to high levels of salt from winter road maintenance. Salt damage can occur when salt enters the soil or when it is sprayed by moving vehicles onto trees. When salt damage occurs via the soil and roots, it can cause leaves to not open, or to be stunted in size; in the long term, it can cause dieback and a reduction in crown size.

So, my research question was:

Do trees that are irrigated year-round with salty street runoff display significant signs of stress or stunted growth?

These two trees, planted within a stormwater soil cell, receive untreated, salty, road-runoff water throughout the winter. IMAGE/ Jen Hill

My research site is the Queensway, a major arterial road in Etobicoke. In winter, it receives priority plowing and salting, second only to the local expressways. The street trees in my study are Freeman’s maples and American elms planted on the Queensway in 2009 within a supported soil cell system (Silva Cell). The stormwater runs from the road into a catchbasin (an iron grate in the road with a concrete chamber below) and from there into the soil cell; beneath the proprietary sandy soil mixture, perforated pipes drain excess water into the storm sewer. (The stormwater control provided by the supported soil cell system has been monitored for several years by researchers at Ryerson University in a separate research project.)

These two trees, planted within a stormwater soil cell, receive untreated, salty, road-runoff water throughout the winter. IMAGE/ Jen Hill

My study compares two different soil cell plantings, each of which contains an American elm and a Freeman’s maple. Both of these soil cells have a catchbasin connection, but only one continues to receive stormwater (referred to in this article as the “stormwater soil cell”), whereas the other (referred to here as the “dry soil cell”) was disconnected in 2011. As a control group, I have located an area to the west where neighbourhood street trees—five American elms and three Freeman’s maples—were planted in 2009 to standard City of Toronto specifications—that is, in regular soil without soil cells.

This year, I visited the project to see if several years of salty road runoff has significantly stressed the trees. Using a variety of free and paid digital applications (see below), I’ve recorded the trees’ height, trunk diameter (d130), crown volume, and leaf area index. The crown volume is calculated by measuring the outside edge of the tree’s crown in all directions, accounting for the density of the leaves within that outline. Leaf area index is a measure of how many overlapping leaves the tree has on average between the very top and the ground.

The stormwater soil cell trees, which have been receiving salty runoff for many years, are significantly taller than comparable nearby street trees—the five elms and three maples—in a standard planting configuration. They have much thicker trucks, larger crowns, and very similar density of leaves within their crowns. Overall, the trees receiving stormwater have a greater leaf surface area for capturing falling rain, for sequestering CO2, for shading, and for insect habitat.

The stormwater soil cell trees are very similar in crown volume to the dry soil cell installation, which has not received stormwater runoff since 2011. The disconnected, dry soil cell trees are also notably larger than the five elms and three maples in the control group, but their crowns are smaller than those that receive stormwater in the stormwater soil cell.

Given that excess salt can be toxic to trees, it is perhaps surprising that the trees receiving street-salt runoff in the stormwater soil cell are doing comparatively well. The stormwater soil cell trees may be thriving due to the enhanced drainage provided by the soil mix used within the cells and the pipe connection to the sewer. Fresh water being flushed through the cells for much of the year might be preventing the damaging salt ions from accumulating. The monitoring of tree health is continuing into the winter of 2018/2019, and the data is currently being prepared for publication. For further information, contact: [email protected].