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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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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

A Guide to Tree Care (Part 1)

Selecting and Planting Your Tree

by Matthew Downs, Urban Forestry AmeriCorps Member

urban trees

Trees along a street in Portland (photo by Julie Fukuda)

Join Urban Forestry and the Powelhurst-Gilbert Tree Team for a free workshop (presentation and treewalk) to learn about general tree care:

When: Saturday, May 7, 2016, 1-3pm

Where: Holgate Baptist Church (11242 SE Holgate Blvd)

How: Register here:

Why do we need trees?

Trees help city-dwellers in multiple ways. They filter the air we breathe, calm our nerves, shade our streets, and cool our neighborhoods in the summer. Some studies have correlated trees with reduced crime and better birth outcomes. And in Portland’s stormy winters, trees help to mitigate flooding by intercepting rainfall. So yes, trees are necessary to our health in the cityscape – bringing us back to nature – but their management is essential before we receive any such benefits. In Portland, we’ve implemented Title 11 Trees to help do so.

Right Tree, Right Place

Two main things to keep in mind here: site width and overhead wires. Between our utility and communication lines, buildings, streets and sidewalks, there wouldn’t seem to be a lot of space left for trees. Yet, there is! And it becomes a matter of choosing the right species for the available planting site.

Here at Urban Forestry, we like to use the helpful motto Right tree, Right place. By this we mean that trees vary greatly across genera and species. Say you’ve got an empty 6 ft planting strip in front of your house without high-voltage wires overhead. How about an evergreen like a live oak to intercept that rainfall!

evergreen oak

Leaves of an evergreen oak (photo by Nik Desai)

Species Selection

Before setting your heart on any one tree, visit Urban Forestry’s Approved Street Tree Planting page for species lists based on site width and overhead wire obstructions. These lists account for size and form, but you’ll have to dig deeper to ensure your selection is getting the proper sunlight or soil conditions. Visit Friends of Trees to browse tree options with helpful characteristics reports for each.

If you’ve got the ability to make your selection from nursery stock, ask yourself the following questions about form when actually selecting the tree:

Is there a proper central leader?

Nurseries often prune for presentation. Be wary of trees that have been topped to encourage lush lateral growth, but lack dominant central leaders. Keep in mind the majority, if not all of the branches on young trees are temporary.

Central Leader

An evergreen magnolia with a strong central leader (photo by Nik Desai)

Are the roots girdling?

Seedlings are grown in pots, transferred to bigger pots as they grow, and later bigger pots as needed. Sometimes that doesn’t happen as timely as we’d hope, and the roots can encircle and constrict their own optimal form. While root pruning is possible, some girdling roots make for an absolute lost cause. That is to say be choosy, because not all nursery stock is fit for planting.

girdling roots

This poor tree is in for some trouble due to girdling roots. Pay attention to girdling roots before you plant your tree! (photo by Gina Dake)



Obtain a permit before planting in the right-of-way. This is an essential part of Title 11 Trees that enforces the “Right Place” end of Right Tree, Right Place. An Urban Forestry Tree Inspector will mark your curb at the exact spot that the tree should be planted. This ensures the tree won’t interfere with drivers’ vision, utility lines, or create any other potential hazards.


The roots have many variables to consider. Are they wrapped in a ball with burlap and soil? Is the tree in a container? Are they roots completely exposed? If the tree is in a container or has a root ball, the roots will be tightly bound by soil. Use a knife or a spade and rough up the exterior of the soil wad to free some roots for lateral growth. Often times the root flare tree in a container will be covered by soil as well, so be sure to dig it out so the flare is visible. This will help with the planting depth.

planting depth

Debra Kneeshaw, Portland Parks Horticulturalist, demonstrates proper planting (photo by Jim Gersbach). 


Way too many trees die because they’ve been planted too low or too high. The root flare must be visible above the soil surface, but not so high that the roots are exposed. If the tree sits too low, add compacted soil under the roots to bring it higher. The proper depth is right at that Goldilocks point.

Fill the rest of the hole in with the soil you’ve dug out for the hole. Do not use fertilizer or potting soil. This will train the tree to expect nutrients in its immediate vicinity rather than growing vigorous, supportive roots via the search of water and nutrients.       

Pull away competing plant roots, and use the excess soil to build a berm in the form of a ring around the tree. This will lock in water for the first stage of establishment.

planting diagram

(Diagram courtesy of Urban Forestry)

Cross section of a tree planting. Note the berm built up around the tree. The diameter of this circle should be twice the diameter of the root ball.

For questions or further information, please contact:  

Celebrate Arbor Day on April 30, 2016

...and Share Your Tree Story!

2016 Arbor Day poster illustrated by local artist, John Maddin.

Join us at Arbor Day! There will be opportunities for visitors to add their tree stories to our story tree, arts and crafts for kids and adults, information about trees and tree care, tree seedling giveaways, historical and heritage tree tours led by a local historian, music, storytelling and much more!    

When: Saturday, April 30, 2016 from 8:30am - 2:00pm

Where: Portland Farmers Market at the PSU South Park Blocks (near SW Park Ave and Montgomery St

Music begins at 10:30am, followed by the Naito Awards, the Maynard Drawson Award, Arbor Day Foundation Awards and the Mayoral Proclamation. 

This year's Individual Naito Community Trees Award will be presented to Jim Labbe. For over two decades, Jim has carried forth a tireless passion for urban trees and the richness they offer both people and wildlife, alike. He has contributed to the conservation and management of Portland’s urban forest as a student, watershed council coordinator, consultant, citizen advocate and as Urban Conservationist with the Audubon Society of Portland.

This year's Group Naito Community Trees Award will be presented to the Albina Neighborhood Tree Team. In their short tenure, this group has illustrated what an equity-based approach to working with neighbors and trees in low income neighborhoods can accomplish. Their work could be replicated across Portland, bringing people together and lessening the financial burden that hazardous trees can place on low-income households.

The second annual Maynard Drawson Award, established by the Oregon Travel Experience and the Oregon Heritage Tree Program, will be presented to Urban Forestry Commissioner, Catherine Mushel. Maynard “The Tree Man” Drawson, founded the state’s Heritage Tree Program, making Oregon the first state to have such a program. This annual award honors individuals who have championed Oregon’s trees with the same fervor and success as Drawson. Catherine exemplifies Maynard Drawson’s commitment, and zest for Oregon trees, through over 20 years of collective volunteer work with Friends of Trees, Urban Forestry’s Neighborhood Tree Steward Program, and the Urban Forestry Commission- working to increase protection and community awareness of heritage trees, Portland's street trees, and trees threatened by Dutch Elm Disease.

Arbor Day Foundation Awards will be presented to Portland State University, as a Tree Campus USA and for the 39th consecutive year, the City of Portland, will receive recognition as a Tree City USA. Award ceremonies for all of the above will run from 11am-noon.  

Arbor Day 2015

Some of the characters that you might encounter on Arbor Day!

The theme for this year's celebration is Share Your Tree Story!

Our story project focuses on Our Storied Connections with Trees. We want to hear from you, so send your tree story to: In 500 words or less, please describe a tree that has been significant to you and why. Be sure to include information about type of tree (if you know) or names by which the tree was known. Also, where the tree was located. Feel free to send photos, too!

We plan to share these stories with the general public on our Urban Forestry website and as part of our greater art project, The Year in Tree Stories. By submitting your stories and/or photos, you consent to use of these by the City of Portland for any lawful purpose, including, for example, promotion of Urban Forestry programs, publicity, illustration, advertising and web content. 

To learn more about the event and the launch of A Year in Tree Stories, please visit: or contact: