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Healthy Parks, Healthy Portland

Phone: 503-823-PLAY (7529)

Fax: 503-823-6007

1120 SW Fifth Ave., Suite 1302, Portland, OR 97204

Portland Parks & Recreation Urban Forestry News and Activities 

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Trees and Sidewalks -- Healthy roots, healthy tree

By Carrie Black, 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. 

buckling sidewalk

Tree roots break apart this sidewalk in Buckman.

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.

sidewalk removed, roots exposed

Damaged sidewalk is removed and the roots beneath await inspection.

Sidewalk alternatives

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

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.

Alternative Paving

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.  

re-routed sidewalk for tree retention

A sidewalk is re-routed to make space for this large Douglas-fir in Mill Park.

Structural Soil

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

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

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. 

Silva cells increase soil volume

Silva Cells are installed on Sandy Boulevard and SE 9th Avenue.

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!

Helpful Links

Portland Parks & Recreation. 2007. Portland’s urban forest canopy: Assessment and public tree evaluation.

Seattle Dept. of Transportation. 2015. Trees and Sidewalks Operations Plan.

Urban, James. 2008. Up by roots: Healthy soils and trees in the built environment. Champaign, IL: International Society of Arboriculture.

Portland's street tree inventory project now complete!

Submitted by Julie Fukuda, Community Service Aide II

One of largest inventories of urban trees in the U.S.


Goose Hollow oaks and hornbeams 

Oaks and hornbeams line a street in Goose Hollow.

Nearly every planted street tree in Portland – almost 220,000 – has been mapped, measured, identified, and its health rated as part of the City’s first comprehensive inventory of street trees. Findings from the inventory, which was completed in September, will be presented at a free public event in Southeast Portland:

What:  2016 Tree Inventory Summit

When: Saturday, Nov. 5th, from 8:30 a.m. – 3:30 p.m.

Where: Mt. Scott Community Center 5530 SE 72nd Ave.

 Register for the event here

All of Portland’s 96 neighborhoods were inventoried

More than 1,300 Portland-area volunteers trained by Portland Parks & Recreation Urban Forestry staff participated in the inventory. Beginning in 2010 and ending in September of this year, volunteers and Urban Forestry staff systematically gathered information on street trees in all of Portland’s 96 neighborhoods, compiling the most comprehensive and in-depth look ever at Portland’s street trees.

Reports on the last 11 neighborhoods will be available at the Nov. 5th event, where the public will also get its first look at citywide results. However, detailed reports about street trees in individual neighborhoods have been shared with interested groups as the inventory finished in each area. Those reports are posted and available here.

 Volunteers collect data on street trees in Creston-Kenilworth  Volunteers collect data on street trees in Irvington

Volunteers collect data on street trees in Creston-Kenilworth (L) and Irvington (R).

Inventory Key findings

Valuable resource

Street trees are providing $28.6 million worth of environmental, real estate value, human health, and other benefits in Portland annually. The overall value of Portland’s street trees is estimated at nearly $753 million.

A large variety of trees in Portland

A huge variety of tree species thrive in our climate. The inventory documented 162 tree types, most identified only to the genus level. This list would be even longer if trees were identified to the species level. Most trees are in good or fair condition.

Most common trees

Maples and trees in the rose family - mainly cherries, plums, hawthorns, and crabapples – dominate streetside plantings at levels well above what is considered healthy in community forestry. Over 50% of all street trees belong to only two families. There are nearly 20,000 Norway maple street trees in Portland, the most common tree, and a species on the nuisance list.  Cherries are also common, and one in five is in poor condition. Increasing diversity can reduce risk and expense due to introduction of pests and disease.


 maples and plums in Beaumont-Wilshire

These Norway maple (L) and ornamental plum trees (R) in Beaumont-Wilshire

are among Portland’s most common street trees.


Larger trees for more community benefits

Small form trees, which offer a fraction of the environmental and community benefits of large form trees, are increasing within the street tree population. This is true even for planting sites that could accommodate large trees. Dogwoods and snowbells are among the smaller trees showing a sharp rise in popularity in recent years.

More year-round green needed

Broadleaf deciduous trees, which shed their leaves each winter, are the dominant tree in all neighborhoods, particularly east of the Willamette River where they represent between 80% to 98% of street trees. Opportunities to diversify plantings, including more evergreen species to capture winter rains and reduce storm water volume, exist in all areas of the city.


Irvington evergreen silverleaf oaks     Evergreen magnolia leaves and flowers

Evergreen silverleaf oaks provide benefits year round (L). Glossy evergreen leaves of a southern magnolia (R).

Setting Urban Forest Policy and Priorities

Preliminary findings from the survey have already been influencing the city’s tree-related policies. For example:

  • Seeing how vulnerable Portland is to catastrophic tree losses from pests and diseases as a result of our over abundance of maples, Urban Forestry now requires property owners to plant alternative species as street trees.
  • Given the low percentage of evergreen trees being planted, Urban Forestry has increased the number of evergreen trees on its approved planting lists from a handful to more than 20.

Empowering Portlanders and growing forest stewardship

Angie DiSalvo, Urban Forestry Outreach and Science Supervisor, has been involved with the inventory since the beginning. She organized the initial counting of trees in the Concordia neighborhood in Northeast Portland back in 2010. She says the inventory was an opportunity to partner with Portlanders from diverse backgrounds to gain a greater understanding of issues facing the urban forest and their neighborhood street trees in particular.

“Together we were able to quantify the state of street trees everywhere in the city. The data produced is empowering residents to take science-based actions to improve the urban forest right where they live.” Neighborhood Tree Teams were formed by volunteers as part of the inventory process. These volunteers will use the results to prioritize neighborhood-based tree projects. “These can range from helping low-income residents remove and replace dead street trees to conducting educational tree walks and providing pruning and care for young trees,” says DiSalvo.                                    

 Brentwood-Darlington inventory volunteers   

 2016 Street Tree Inventory volunteers at a Brentwood-Darlington inventory work day

To learn more detail about the inventory project results and how they will be used, please join us at the 2016 Tree Inventory Summit on Saturday, Nov. 5th. Pre-registration is appreciated and can be submitted here.

Tree physiology primer – all about roots!

By Carrie Black, Urban Forestry Community Service Aide II

Trees - like all other living things - need food, water, and nutrients to survive. Trees make their own food through photosynthesis, using energy from sunlight, water (from the roots), and carbon dioxide (from the air) to create sugar that is used to fuel the rest of the tree. Water is carried from the roots to the leaves through xylem cells. Sugar is transported back to the rest of the tree through phloem cells (see diagram below). Sugar is converted into starch, and can be stored in the trunk or the roots, or used for immediate growth.

 xylem and phloem diagram

Cross section diagram of a tree trunk.

The amount of photosynthesis that can occur in the leaves is directly related to the amount of water that the roots can absorb. A small root system can only support the photosynthesis of a small canopy, and roots can only grow larger if they have enough energy (sugar) provided by the leaves. In this way, the tree must balance its above ground and below ground growth. Topping a tree will stunt its growth by limiting the amount of photosynthesis that can occur in the leaves (by the way, NEVER top trees- it is very dangerous and bad for the tree). Likewise, cutting roots or restricting the roots’ growing space will limit the amount of water that can be transported to the leaves for photosynthesis.

topped tree

Topping a tree can limit its growth in the short-term, but is very bad for the tree.

What happens after photosynthesis?

It is well known that trees act as carbon sinks, taking in carbon dioxide from the air during photosynthesis and releasing the oxygen that humans breathe. While trees use carbon dioxide to make their own food, they actually need oxygen (much like humans do) to process that food into energy.

In order to use stored starch for growth, trees must convert the sugars back into energy through a process called respiration. Respiration requires oxygen. During respiration, sugar and oxygen are combined to produce energy, with water and carbon dioxide created as byproducts. The energy that is released can then be used to make new tissues. Humans do the same thing when they process stored sugars. While trees take in oxygen from their surroundings, humans breathe it in with their lungs. Just as a person who is exercising needs to breathe deeply, a tree that is actively growing needs an immediate source of oxygen.

Roots need oxygen, too!

Most tree growth occurs at the tips of branches and the tips of roots. However, while the crown of a tree is usually surrounded by open air, roots need a source of oxygen in the soil in order to grow. In the ground, air and water are held in little pockets called soil pores. If the soil is dense and compacted (with no soil pores), there will not be enough oxygen available for respiration. Too much water in the soil will also limit the amount of oxygen the roots can take in.

Tree roots grow best when they have sufficient growing space and well-drained soil with enough oxygen and water (but not too much water). The depth that oxygen can reach depends on the type of soil and amount of compaction, and the most oxygen will be found near the surface of the soil. For this reason, roots tend to grow right under the surface.

What does this mean for the structure of a root system?

Many people imagine tree roots as a mirror image of the branches, but this is a common misconception. Tree roots actually grow outward horizontally from the base of the tree (picture a wine glass sitting on a dinner plate).

 wine glass analogy

This wine glass sitting on a plate represents the basic shape of a tree and its roots.

Roots become smaller as they grow outward from the root collar (base of the trunk). Buttress roots right near the trunk help stabilize the tree. In the first 3-6 feet, woody lateral roots spread out and taper down to 2-4 inches in diameter. Small, ephemeral absorbing roots grow out of lateral roots, soaking up water and oxygen and other nutrients. These roots are opportunistic – they will grow wherever the conditions are suitable, and will die back if resources are not available.

tree roots Tilia crowded roots

Top, exposed roots of this tree growing on a hill side show their horizontal structure. Bottom, crowded roots are girdling this linden tree.

In the context of urban trees, infrastructure can severely limit the growing space available to the roots. Stay tuned for Part 2 of this blog post – Trees and Sidewalks. We will discuss how sidewalks impact tree health, and give alternatives to traditional sidewalks that could prevent tree/infrastructure conflicts.


Day, S.D., P.E. Wiseman, S.B. Dickinson, & J.R. Harris. 2010. Contemporary concepts of root system architecture of urban trees. Arboriculture & Urban Forestry, 36(4): 149-159.

UC Davis, Fruit & Nut Research & Information Center. No date. Photosynthesis and respiration.

Urban, James. 2008. Up by roots: Healthy soils and trees in the built environment. Champaign, IL: International Society of Arboriculture.

Join Urban Forestry! Now recruiting AmeriCorps Tree Plan Coordinator

Passionate about education, equity and the urban forest? Portland Parks & Recreation is seeking an AmeriCorps member to work with our Urban Forestry programs to help us better serve our diverse communities.

Position term: September 7, 2016 - July 28, 2017 (11 months – 1700 hours)

Benefits: monthly living allowance ($12,530 paid over the term of service), education award of $5,775, loan forbearance, basic medical insurance, child care allowance for those who qualify.

The position: In Partnership with the Confluence Environmental Center, the Tree Plan Coordinator will be responsible for serving with community groups to plan and execute tree care and education events in support of Neighborhood Tree Plans (65%). Events may include tree pruning, planting, inventory, assessment, identification, and other topics. The position also assists with promotion and coordination of the Neighborhood Tree Stewards program, with particular focus on participation from individuals and communities in racially and economically diverse neighborhoods throughout Portland (15%).  The Member will participate in Confluence led professional development activities:  Professional Development Series, completing a Change Agent Project, team meetings, National Service events and other self-directed development opportunities (20%).

For the full description, please click here.

Applications are due midnight July 10, 2016. See full description above for application details.

Questions? Please contact:

To learn more about Confluence AmeriCorps, please visit:

For more information about Urban Forestry at Portland Parks and Recreation, please visit:




Selecting and Promoting Diverse Tree Species

Upcoming treewalk and presentation- learn about alternatives to maple and ash!

by Matthew Downs, Urban Forestry Tree Plan Coordinator and AmeriCorps Member

Quercus myrsinifolia

Quercus myrsinifolia

We city-dwellers place a high value on our urban forest. Trees provide innumerable aesthetic and ecological benefits that a concrete jungle simply can’t. Join us for Selecting and Promoting Diverse Tree Species, a tree walk and presentation highlighting the benefits of diversity as protection against pests and pathogens, as well as information on how and where to pick the right tree for your planting site.

This workshop is hosted by the Mt. Tabor and North Tabor Tree Teams, in partnership with Friends of Trees, Bureau of Environmental Services, and Urban Forestry.

When:     Saturday, June 4, 2016, 10:00am- noon (rain or shine)

Where: Western Seminary, Bueermann Hall, Room 101, 5511 SE Hawthorne Blvd, Portland, OR 97215


Halesia fruit

Halesia sp., also known as Silverbell


9:45 - 10:00 am: Registration and coffee, tea

10:00 – 10:30 am: Presentations from Friends of Trees and Bureau of Environmental Services

10:30 – 11:30 am: Tree Walk with Jim Gersbach of Urban Forestry

11:30 – noon: Individual tree help. Get your questions answered!


Matthew Downs at Portland Parks & Recreation, Urban Forestry, 503-334-0348