Making Space for Trees: Designing Tree-Friendly Infrastructure

Date of Visit: 06/06/2019
Type of event: Conference
Topic: Making space for trees
Location: World Forestry Center, Portland, Oregon
International Fellows: Richard Banda (Malawi), Temitope Dauda (Nigeria), Zhongyuan Ding (China), Will Maiden (United Kingdom), Rodolfo Vieto (Costa Rica)

The World Forestry Center recently hosted the 2019 Oregon and Community Forestry Conference on Thursday the 6th of June. The theme of the conference this year was making space for trees and designing tree-friendly infrastructure. Five of this year’s International Fellows attended the conference to gain insight into managing trees in an urban situation.

The keynote speaker, Howard Stenn, is a Seattle-based landscape gardener. His key message was “grow your soils before you grow your trees.” This is of particular importance in the urban environment where trees find themselves in a stressful environment. Space for roots is limited, water availability is often reduced, and the heat island effect all add to the pressure faced by urban trees. Howard noted that compaction is the biggest limiting factor to urban trees. Areas of grass are driven over by mowers and construction disturbs roots in the important 1-foot growing zone beneath the soil. Nevertheless, Howard stressed that this is not a reason to ignore new plantings in the urban situation.

Robert E. Vanderhoof, a veteran natural resources professional, presented the issues facing the utility arboriculture sector in the US. 2018 was a destructive year in terms of wildfires in California. The Camp fire, which led to the destruction of the town of Paradise, has caused the rethinking of trees near powerlines. The new realities of a drier, warmer climate mean that some electricity providers are starting to consider turning off the electricity supply when the risk of forest fire is high. In areas of high fire risk, the California standard of having no plants taller than 30 cm (12 inches) will be adopted, meaning trees pruned clear of cables may now need to be completely felled. Robert stressed that he believes in making a place for trees rather than finding a place for trees. Perhaps planting trees under powerlines is not the safest use of the land. So, what is to be done if we are to retain tree cover? The planting of mitigation parks was suggested as one option. These are areas designated for providing green space where tree cover is lost. Although this would maintain and perhaps build tree cover, it will not be able to compensate for the benefits of shade provision, stormwater abatement, and aesthetic value which trees in urban situations provide.

An example of closely planted, quick-growing birch in Seattle, Washington. 

This approach was discouraged by speakers at the conference.

One topic that was discussed by some of the speakers is the current trend to plant quick-growing tree cultivars in neighborhood blocks with dense development. This is something I recently noted on a trip to Seattle. The downside to this approach is the lack of growing space above and below ground. Trees will potentially have to be thinned out in the coming years, increasing maintenance costs and requiring landscape redesign in the future.

Bioswales provide stormwater remediation and planting areas for street trees

Kerry Rappold and Steve Adams discussed their experience with incorporating bioswales in the city of Wilsonville, Oregon. Bioswales are areas of vegetation designed to remove pollutants and promote water infiltration into the soil. The provision of bioswales gives the opportunity to include trees and shrubs within their design. The trees benefit from the stormwater which helps them to grow whilst also reducing the pressure on drainage systems. Kerry and Steve noted that bioswale provision is an area of success over the past 10 years in Wilsonville and that the lessons they learnt in the city can be transferred to existing infrastructure.


An International Fellow’s Thoughts and Perspectives

William Maiden,

International Fellow from the UK,

settling into life in America

An aspect I liked about the conference is the promotion of the added value that trees bring to the urban landscape. Trees are not solely considered as an aesthetic addition but also to clean stormwater runoff, sequester carbon, and bioremediate polluted air. Bioswale provision is also something that could be implemented in the UK, particularly in new developments. An area where I see similarities to the UK is the current trend for smaller, fastigiate trees in comparison to large, spreading trees. Whilst this does add trees to a landscape, smaller trees do not provide as many benefits. In the UK and the US, the reliance on shorter-lived species, such as birch, do provide value to the urban street scene, but also reflects the current fashion for instant landscapes and smaller, lower maintenance trees. Whether you are walking down the Mall in London dominated by its crown of London planes, or Portland’s South West Park Avenue lined with ascending elms, I believe a place needs to be found for large trees in our urban landscape.