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Streets That Reflect a Sustainable Urban Forest

Providing a variety of ecological, aesthetic, and economic benefits, street trees are a significant component of the urban infrastructure. Leaders in the design, planning, and urban forestry professions have increasingly recognized the role that street trees play in the overall health and connectivity of urban forests. The evolving awareness of professionals has led to significant street tree and street design initiatives around the country.

Seattle: An Urban Landscape Carved from the Forest

Seattle was forged from an expansive old growth forest that stretched along the Pacific coast from Northern California to Alaska. Where modern skyscrapers and the landmark Space Needle stand today, large conifers stood on steep slopes with their associated understory plants. Before settlers of European decent arrived in Seattle in 1851, the native peoples of the region enjoyed a special relationship with the lush temperate rainforests of Western Washington. At the hands of the extraction-based economy, the driving force behind America’s westward expansion, forested lands around the then small settlement of Seattle began to quickly disappear, a trend that continues to this day.

Seattle’s urban forest, the dynamic collection of trees and other vegetation found in an urban area that collectively provides ecological, aesthetic, economic, and social benefit, is the link between our past and future. In 1996, when American Forests took inventory of Seattle’s urban forest, we learned that the average tree cover for the city was only 13% of the total land area. The study also found that areas of natural tree cover declined from 10% to 5% of the total land area over a 24 year period.

The challenge for ecologically minded designers in Seattle is simple in scope: transform the memory of a lush temperate rain forest into a sustainable urban forest. Protecting and restoring the few remaining patches of native forest habitat in the city is one component for achieving this vision. Planning and designing for streets that provide optimal conditions for growing large street trees is another.

An Urban Patch

This past spring, while observing Seattle’s landscape from the air on approach to Seatac Airport, I noticed a patch of brilliant green among an otherwise muted urban grid below. What had caught my attention was the tree canopy of Pioneer Square’s Occidental Park and surrounding streets. As I admired the area from above, I began to think about the significance of having such a substantial green space in the core of the city and the challenge to maintain and expand on this important public resource.

The district, where Seattle was born, has witnessed drastic change during its 150-year history. Pioneer Square’s initial growth in the 1850s was prompted by the timber extraction industry and in particular, the location of Yesler’s Mill in the heart of the district. In 1889, a devastating fire that consumed the Pioneer Square district quickly spurred a massive rebuilding campaign. In 1897, with the discovery of gold in the Yukon, Seattle became America's principal site for travel to and from the Klondike and thus rapidly expanded. When Seattle's business nucleus began moving northward prior to World War II, Pioneer Square suffered and decayed. Beginning in the late 1960s many of district’s buildings were renovated and reoccupied. Pioneer Square is now home to a dynamic community of residents, retailers, diverse small businesses, and a large collection of entertainment venues, cafés and restaurants.

Streets with continuous tree canopies can create visual and ecological linkages between patches of parks, natural areas, and treed residential and commercial landscapes. To support the vision of a healthy urban forest, planting large growing trees is worthy of expansion to adjacent streets and repetition in other neighborhoods.

Over the past 30 years, a slow yet noticeable change has been taking place: the growth of London Plane Trees in Pioneer Square’s streets and public parks. The trees planted in the early 1970s as per the Pioneer Square Historic District Open Space Plan authored by Jones & Jones, are one of the most important and recognizable streetscape design features of the district. The Pioneer Square National Historic District offers perhaps an excellent example of how a community enjoys the benefits of street trees in Seattle.

The Street Trees Define the Character of the Streets

On the ground, at the human scale, the distinct character of Pioneer Square is just as captivating as it was from above. This is due in large part to the powerful presence and spatial qualities of the large maturing urban street trees.

In the morning, my bus drops me off about one block from the edge of Pioneer Square. Each time I enter the district, I am greeted warmly by the trees. I always anticipate reaching them as I begin the several block walk. Once among them, my morning commute is transformed into a pleasant experience, and the bus ride through the busy city streets is a distant memory.

In the spring I notice the fresh smell of the emerging leaves. In the fall, the dying and decaying leaves provide some shelter from the rain and I listen for the soft rustle of leaves in the breeze. In the winter, the irregular branching patterns and swaying limbs hint of the connection between the city and the vast moist forests of the northwest. In the summer, I relish walking through the patches of early morning sunlight that filter through the leaves before I head indoors for the day.

Pioneer Square's trees co-exist with overhead and underground utilities, lighting features, and overhead electric transmission wires for buses and the Waterfront Street Car. A Native American Totem sits in the square as a reminder of the special relationship that native peoples had with the temperate rainforests of Western Washington before westward expansion hit Seattle in 1851.

Pioneer Square’s street trees are a dynamic element in what is a very dynamic landscape. The wind, shade, breeze, sun, rain, sky, and high and low tide are all among the natural elements that shape my daily perception of the neighborhood. Although Pioneer Square is a very urban place, the presence of trees is a constant reminder that we are connected to the greater cycles of the earth. The street trees bring perspective to and interact with these larger natural processes well.

The large irregularly formed trees compliment and seem to respond to the individually unique historic brick masonry buildings. Their mature height bridges the gap between life on the street and our lives inside buildings. In many ways the trees define our experience and memory of the district.

Along First Avenue, the main corridor through Pioneer Square, the trees bring order and consistency to the street. When you enter the district in a car or bus via First Avenue, the trees are the first element to signal that we are somewhere different than other city places. Pedestrian and vehicular spaces are well defined and delineated due to their presence in a modest median separating north and southbound traffic. The trees also signal that this area is a human, not auto scaled landscape. The trees cue passing motorists to slow down, which greatly contributes to the feeling of a safe pedestrian oriented community.

Maintaining the Vision: The First Avenue Replanting

In January 2000, a fierce Pacific storm blew down a 27-year-old London Plane tree planted in the median along First Avenue. The tree was one of seven in this particular median. Based on input from representatives of several Seattle City departments and community members, the tree was immediately removed by emergency road crews. Once the hazard tree was removed, it was discovered that a 6-inch steam pipe was ruptured. It could not be determined if the steam pipe rupture was directly related to the damaged tree. Nonetheless, Seattle Steam quickly repaired the ruptured line.

In 1996, American Forests took inventory of Seattle's urban forest, finding that average tree cover for the city was only 13 percent of the total land area. The study also found that areas of natural tree cover declined from 10 percent to five percent of the total land area over a 24 year period. For nearly 30 years London Plane Trees have been growing in Pioneer Square's streets and public parks.

As preparations began to replace the damaged tree later in the spring, it was noticed that four of the remaining 6 trees in the median were slow to leaf out and showed signs of physical stress. Soon there after, it was determined that the steam pipe rupture of had likely killed these trees.

Shane DeWald from Seattle Transportation, Woody Woodward from Seattle Steam, Nolan Rundquist the City Arborist, and Kevin Carl from Jones & Jones formed a working group in order to devise a solution to replace the trees.

Carl, an Architect / Urban Designer at Jones & Jones, who also serves on the Pioneer Square Community Council / Public Space Committee, played a critical role in maintaining the vision originally set forth by the firm in the 1970s and reinforced by the Pioneer Square Neighborhood Plan of 1999. As a committed steward of the Historic District, he recommended that the median be redesigned to improve growing conditions for new trees.

A review of Seattle Transportation’s Record Drawings showed that trees were originally planted in shallow 6x6 pits, which were cut from the original roadbed. The root balls of the trees partially rested in the pits. In-fill soil was then mounded to the top of the root ball to form the above grade median that was retained by curbs.

The regular spacing of trees in the median along First Avenue cue motorists to slow down as they pass through the district. Restaurants along First Avenue thrive due to the presence of these trees making outdoor dining more inviting for customers.

Carl recommended that the trees be replanted in a continuous trench, which would provide a significantly better growing environment than individual tree pits would. Continuous planting trenches give tree roots more room to spread horizontally, allowing for better aeration, intake of water and nutrients, and better tree stability, minimizing surface impacts such as sidewalk lift. Of the remaining two trees in the median, one was still in good condition and the other was growing at a significant angle. Based on this assessment and the prospect of improving growing conditions for the new trees, it was decided that there would be a greater long-term benefit to replacing six of the seven trees at once, in conjunction with an improved growing medium.

The median restoration plan called for excavating the existing roadbed and compacted soils down to the pipe and replacing the materials with fill and new high grade soil mix. Seattle Steam was concerned about the future integrity of their steam line, a structural +/- 100-year-old cast iron pipe, which still rested 36-inches below grade. The working group proposed that the steam line be protected with 2-inch ridged insulation to be laid over the pipe like a tent. This treatment of the pipe would discourage roots from directly interfering with the line as they grow downward and would shed irrigation water away from the pipe.

The city estimated that the cost of a traditional replacement plan using 2 to 3-inch caliper trees would cost approximately $5,000, which the city was prepared to spend. The Pioneer Square Community Council’s proposed restoration plan was estimated to cost approximately $16,000, which included planting larger, 4 to 5-inch caliper trees. The Pioneer Square Community Council, a non-profit organization, raised $12,000 through private donations to offset the city’s budget and fund the median restoration.

With funding in place to proceed with the restoration, scheduling and construction of the project began. As part of a collaborative effort to complete the median restoration project, Seattle Transportation coordinated the various components of the work. The installation was successfully completed 15 months after the storm initially damaged the area, in time for an Arbor Day event. This event, held in Pioneer Square Park in view of the new trees, recognized the many community participants and city personnel who worked together to maintain the vision of a green and livable community.

Carl's efforts as a community council member and an associate at Jones & Jones, exemplifies the personal commitment needed to maintain a vision for the urban forest. He remarked that, “It takes 10 – 20 years to see a street come to life after the original planting. The designer’s vision and commitment to the project shouldn’t stop once the installation is complete.”

Opportunities

Planning and designing for street trees in a district like Pioneer Square is the first of many steps needed to achieve a broader vision of an ecologically sustainable city. The effort that Jones & Jones initiated in the 1970s has continually evolved and has been expanded upon, as the project on First Avenue demonstrates. Ilze Jones, one of the founding principals of Jones & Jones commented that, “The trees really put the district on the map. You can’t ignore the impact that the trees have; they are tremendous living beings.”

Although urban street trees are often considered to have short life spans, proper planting techniques and species selection significantly improve their survival rate. As Landscape Architects are presented with new information regarding the growing requirements of street trees, it is important that we adapt our planting designs. Providing continuous planting beds where possible is one very simple way to provide improved growing conditions for street trees. The design of sub-surface conditions must respond to the significant problems of improper rooting volumes and soil compaction that street tree’s are continually subjected to, just as irrigation, aeration, and fertilization throughout a tree's life greatly improves its chance of survival.

Each urban streetscape has a unique set of growing conditions. Street tree choices, planting locations, planting area details, and planting techniques need to directly respond to those conditions. The streetscape should be thought of as a place, part of the urban forest, which has the ability to support a plant community and comfort a human population.

Filtered sunlight and shadows from the street trees are among the dynamic elements that influence the daily perception of the district. Although Pioneer Square is a very urban place, the presence of trees is a constant reminder that we are connected to the greater cycles of earth.

The vision for creating a sustainable urban forest in Seattle will be achieved as designers and planners recognize that large street trees are crucial to creating livable streets. Street trees should not be included as an afterthought; rather they should be meticulously planned for, as are the other elements of the urban infrastructure. The design of a total tree environment needs careful attention to ensure its long-term survival. Green city streets do more than clean the air, reduce noise, and increase property values. They become the ecological corridors between patches of parks, natural areas, and backyards. The visual and biological connection that street trees can provide to the rest of the urban forest is critical to ensure livable and healthy cities.

In Seattle, the urban forest legacy established in the Pioneer Square Historic District is worthy of expansion to other neighborhoods. Planting more street trees citywide is our opportunity to honor the forest that was once here, respond to the ecological needs of a growing city, provide residents with the many benefits which street trees provide, and live up to our official nickname, The Emerald City.

Alex E. Schwartz is an Associate Landscape Architect with Jones & Jones Architects and Landscape Architects in Seattle, Washington.


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December 6, 2019, 1:38 pm PDT

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