By Carrie Black, Former Urban Forestry Community Service Aide II
Looking around at the extensive green canopy above Portland, it’s easy to forget that cities are not the natural environment for trees. Trees provide many benefits to the urban residents including environmental services, (such as improved air and water quality), enhanced livability, and aesthetic benefits. These benefits can add up—the value of Portland’s urban forest is estimated at $5 billion! Trees are an integral element to a healthy, livable city.
However, daily life in a city can be downright dangerous for trees – branches are hit by passing vehicles and trunks get maimed by weed eaters. One of the biggest challenges to a tree’s success actually occurs below the ground. To stay healthy and grow to their full potential, trees need growing space with access to water, oxygen, and nutrients. Just as power lines can limit a tree’s growth above ground, urban infrastructure can limit growth below ground as well.
Street trees in particular are located very close to sidewalks, often planted between the sidewalk and the street or in cutouts. Curbs and sidewalks limit growing space directly around the tree, and the compacted soil underneath traditional sidewalks makes lateral root growth difficult. Tree trunks and roots growing in these conditions inevitably cause damage to concrete sidewalks. Cracks and raised sidewalks pose a hazard to pedestrians, a liability to property owners, and require expensive maintenance.
Roots are the foundation of a tree, supporting the tree and holding it in place while also taking up water and nutrients from the soil. For a more detailed look at root physiology and how roots affect tree health, see Part 1 of this blog post – Tree Physiology Primer.
Traditional responses to sidewalk damage include modifications of the sidewalk, such as sidewalk shaving, ramping, and cutout expansion. Often, these actions require the cutting of tree roots. These methods can be costly for property owners while also having negative impacts on the tree, which loses access to water and nuturients and may become destabilized. These negative impacts can eventually lead to failure or premature removal of the tree. Because trees provide the majority of their benefits later in life when they are large and mature, removing a tree due to sidewalk conflict is a lost opportunity—resetting a process that takes decades. Alternative materials and designs for sidewalks could actually help prevent tree/sidewalk conflicts in the first place, and are a positive investment for Portland’s urban canopy.
Many alternatives to traditional sidewalks exist, ranging from short-term responsive measures to proactive installations that allow for long-term tree growth. Additionally, alternative repair and replacement designs can help expand planting site size and limit further conflict from existing trees. Utilizing such alternatives could reduce costly maintenance and repairs while helping preserve trees in Portland.
Offset sidewalks are a responsive measure where the sidewalk is re-routed around an existing large tree, giving the tree roots more space to grow and reduces future tree/sidewalk conflicts. They can also be used proactively, to make space for the planting of a new tree.
One problem with traditional paving is that it cracks and raises when roots grow under the surface. Alternatives include rubber pavers, composite plastic pavers, bricks, or pervious concrete. Pavers and bricks are more flexible than concrete, allowing roots to grow underneath without cracking the surface. They are also easier to repair. Pervious concrete allows air and water to pass through the surface of the sidewalk more easily, encouraging roots to grow deeper under the surface.
Tree roots cannot grow well in the compacted soils under traditional sidewalks. Structural soils are designed to create pore space while also providing enough compaction for sidewalks to be laid down. This lets roots grow deep beneath sidewalks instead of right at the surface. Structural soil can be put down under new sidewalks or repaired sections of old sidewalks.
Root paths are essentially trenches that pass underneath the sidewalk that are filled with loose soil. The pavement over the trench must be engineered to be self-supporting (as the soil underneath is not compacted). These trenches act as pathways for roots to access larger growing spaces on the other side of the sidewalk.
Silva Cells are another way to support paved areas while preserving uncompacted soil for tree roots to access beneath them. These and other similar products are used proactively before trees are planted when new sidewalks are laid down. With this method, interlocking frames create a support system for the sidewalk while leaving space for roots beneath.
The next time you are out for a stroll, take a break from looking up into the canopy, and consider the roots of the trees you meet. How much space do they have to grow? Do they appear to be damaging infrastructure? Is the lack of growing space affecting the tree? Applying alternative materials for sidewalks or re-routing sidewalks in these situations could not only reduce costly maintenance and repairs, it could save a tree!
Tree Canopy and Forest Management Reports from Portland Parks & Recreation Urban Forestry. https://www.portland.gov/trees/tree-canopy-forest-management
Seattle Dept. of Transportation. 2015. Trees and Sidewalks Operations Plan. https://www.seattle.gov/Documents/Departments/SDOT/Trees/TreeSidewalksOperationsPlan_final215.pdf
Urban, James. 2008. Up By Roots: Healthy Soils In the Built Environment. Champaign, IL: International Society of Arboriculture.