As street trees die, cities search for ways to stay green
Toronto residents are upset. Through-out the 2.6-million population Ontario city and especially in its center, thousands of street trees are dying prematurely, many within a year or two of being planted. Public concern is so strong that last fall the government organized a conference in which 200 municipal personnel spent an entire day discussing the problem with tree experts from throughout North America.
Seen from the air, only about 16 percent of the “old” section of Toronto (the part developed before 1945) is under tree cover, down from 22 percent in 1992. And Toronto is not unusual. American Forests, a nonprofit conservation organization, has found that tree cover is declining in many North American urban areas.
Based on satellite images of 40 US cities, American Forests reported in 2003 that “urban areas have 21 percent less tree canopy today than they did 10 years earlier.” Tree canopy covers only 12 percent of Buffalo and Lackawanna, New York. Trees shelter less than 20 percent of metropolitan San Diego. (At the other end of the spectrum, one of the lushest cities is Savannah, Georgia, where trees shelter more than 60 percent of the land and buildings.)
As tree cover declines, cities suffer in many ways. Fewer trees are left to counter the urban “heat island” effect. Planning consultant Jonathan Barnett notes that in a city like Omaha, trees sheltering parking areas can reduce peak summer temperatures by nine degrees — enough to affect people’s comfort. Trees are useful for removing pollutants from the air and for absorbing carbon dioxide, giving off oxygen, and relieving human stress. If a metropolitan area has enough tree cover, it is less vulnerable to flooding and sewer overflows and less in need of expensive civil engineering projects.
For new urbanists, a chief concern is the viability of street trees. Urban designers depend on rows of trees to narrow the perceived width of thoroughfares, to generate a sense of enclosure for outdoor areas, and to make public areas more inviting for pedestrians.
Toronto urban design director Robert Freedman says tree die-off has become a pressing issue particularly for cities in northern climates, where ice-melting salt on streets and sidewalks seeps into the soil and poisons curbside trees. Early tree death could intensify as global trade spreads diseases and insects from distant parts of the world. At the Toronto conference, specialists said the inadvertent importation of troublesome insects and the rise of new pathologies such as “sudden oak death” will require more intelligent planning of urban landscapes and more systematic maintenance.
Killer tree pits
Some common procedures exacerbate tree problems. For decades, it’s been common to plant street trees in “tree pits.” But if these excavations are too small, the root system cannot support the tree for more than a few years, according to James Urban, an authority on trees in built-up areas. The lack of room for roots stunts the tree’s growth, and soon the tree begins to die, says Urban, principal of Urban Trees and Soils in Annapolis, Maryland.
The Toronto conference also identified the following as causes of tree death:
• Poor soil. Developers sometimes scrape the topsoil off of properties, leaving dirt that has little feeding value for trees.
• Compaction. Trucks and heavy equipment unintentionally compress the dirt, turning it into a practically solid mass, which the roots cannot easily penetrate.
• Inadequate drainage. If the soil around and below the tree is clay, water has a hard time dispersing. The tree may, in effect, drown.
• Utility trenches. Contractors and municipalities often dig trenches for the many wires and pipes serving contemporary developments. The trenches frequently go deep enough to destroy root systems, causing trees to topple or die within a year or two.
• Tree grates. Many cities install decorative metal grates around newly planted trees. As the trunk grows, it may end up fighting the encircling obstacle. Though some tree grates are designed so that the innermost section can be removed as the trunk expands, rarely do municipalities remove them promptly enough. The grate girdles the trunk, stopping the flow of water and chemicals between the top and bottom of the tree. If the tree doesn’t die first, it may lift the grate and create a hazard for pedestrians.
• Excessive paving. Covering the tree pit with bricks or paving stones may injure the growing trunk and roots and may prevent needed water from reaching the roots. Sidewalks become problems when they compact the soil, overly confine the roots, and prevent the tree from getting enough rainwater.
When threats such as those are compounded by saltwater runoff from wintertime streets, it’s not surprising that a large proportion of city trees last only 5 to 12 years. In Toronto, where many young dead trees have been chopped off at waist or chest height, “citizens talk quite a lot about trees dying,” says Freedman. Mayor David Miller is leading a “Clean and Beautiful City” initiative, trying to improve Toronto’s appearance. Several sectors of municipal government, including forestry, parks, policy planning, urban design, public works, and emergency services are working together to devise new standards this year for tree planting and maintenance. Says Freedman: “Our aim is to fundamentally change the way we plant and maintain street trees.”
The fundamental solution to most city tree problems is simple: Give each tree access to more and better soil. Instead of allowing utility crews to dig trenches through the root area, municipalities could in some instances require tunneling for utilities. Paul Ostergaard of Pittsburgh-based Urban Design Associates says the best approach is to put the utilities in alleys, allowing trees along the street frontage to grow with fewer impediments.
Instead of installing tree grates, municipalities could leave the soil exposed, covered with mulch, stone dust, or other substances, or planted with flowers. New urbanists insist that the surface should be appropriate to its context. The higher density the setting, the more formal the treatment. “Exposed soil areas are fine for certain parts of the Transect, but you wouldn’t want that in T5 [urban center] or T6 [urban core],” says Kevin Klinkenberg of 180 Degree Design Studio in Kansas City.
Urban says that when nearby residents or businesses agree to water flowers around trees, the trees fare better, receiving much-needed water during summer hot spells. People are more conscientious about watering flowers than trees. Some specialists at the Toronto conference said automatic irrigation is usually unnecessary as long as the trees have a reasonable quantity of high-quality, uncompacted soil from which to extract moisture. However, Barnett, of Wallace, Roberts & Todd, says, “The tree needs some kind of assistance for the first couple of years. The best answer is to provide an access pipe that can help someone water the tree roots by hand when it is clear there is a drought.” In heavily trafficked areas, it may be necessary to install a fence or other barrier to keep pedestrians from trampling on the base of the tree.
“Trees are an essential form giver in the urban environment and should be an integral part of the Transect,” Ostergaard says. “In urban residential neighborhoods, rows of trees can flourish in broad tree lawns and front yards because of the amount of exposed soil. In more dense urban areas, large concentrations of trees are most often found in squares and parks, where permeable surfaces can be created.”
Ostergaard says northern communities that are considering installing medians in their streets might follow the example of Grant Street in downtown Pittsburgh. There the city installed an elevated, tree-planted median, lined with granite. It shields trees from road salt while providing safe harbor for pedestrians.
“Any tree pit that is effectively a flower pot is a mistake,” Barnett says. “Eventually the tree will reach the limits of root expansion and start to die.” One form of “structured soil volume” recommended by Urban is a “continuous soil trench,” which runs beneath sidewalks or other pavement, linking the soil area of two or more trees together. A continuous soil trench gives each tree more room for root growth and offers an alternative to small, isolated tree pits. Most trees do not send their roots deeper than three feet, so the soil trench usually need not be deeper than that. The soil may require the addition of some organic material, such as compost. The most critical factors are how much soil is provided, whether it drains properly, and whether it is loose enough — i.e., not hardened through compaction. A base of gravel can be installed beneath sidewalks to improve drainage and give the roots access to air.
Urban has devised a system that uses a grid or cage of plastic to support the sidewalk. The supports can extend below grade, and the resulting space is filled with soft rooting soil. The sidewalk becomes, in effect, a roof over the rooting space. “By fixing soil problems, we open the door to a larger number of species,” Urban points out. With approximately 10 species dominating municipal tree planting, there is currently too much vulnerability to pests and diseases, which can quickly wipe out much of a city’s tree cover.
Some communities have resumed planting elms. Many American elms were killed by Dutch elm disease, a fungus that began to be spread by beetles in the US in the 1930s. But some elms resisted the disease, and from them the Elm Research Institute in Keene, New Hampshire, has cultivated what it describes as a disease-resistant “American Liberty” elm, which looks like the tree that graced tens of thousands of streets. Nearly 300,000 have been planted in more than 1,000 communities. The Institute says Liberty elms send their strongest roots downward rather than laterally, can grow as high as 100 feet, and “are tolerant of salt conditions and soil compaction,” thus serving well as street trees. Other organizations, too, have cultivated disease-resistant elms, including the Princeton, New Harmony, and Valley Forge varieties.
The economic equation
Transforming municipal tree-planting will not be cheap. Urban says it will cost $5,000 to $10,000 per tree to get all the factors of soil, drainage, and pavement design right for downtown commercial sites. The figure would be much lower for residential areas. Chicago, which has made great strides at greening the city under Mayor Richard M. Daley, allocates $14 million a year to its Bureau of Forestry.
Some money can be saved by not purchasing tree grates or installing irrigation. Additional money may be saved by not having to remove so many young, dead trees. Speakers in Toronto said trees should be regarded as an investment. Trees produce measurable economic benefits — for property owners and for communities.
Some studies have found that homebuyers will pay 3 to 7 percent more for properties with ample trees, says Kathleen Wolf of the University of Washington Center for Urban Horticulture. Businesses have been willing to pay 7 percent higher rents for office buildings that are well-landscaped, Wolf says. At maturity, a tree may add tens of thousands of dollars to a property’s value — some of which goes back to the municipality in property tax revenue. Tree advocates argue that governments should spend more on trees, which gain value, and less on other infrastructure elements such as streetlights, which can be even more expensive and which decrease in value over the years.
Chicago assigns a value to trees, based on their diameter, and requires departments such as Transportation to repay the value if they’re removed for street widening or other projects, says Joe McCarthy of the Bureau of Forestry. That policy discourages unnecessary tree removals for projects such as street widening. Chicago has found that planting trees and other vegetation helps to revitalize and redevelop both residential and business areas. In 1990, the year after Daley became mayo r, Chicago had an estimated 430,000 street trees. By 2003, the number grew to an estimated 538,000, many of them planted by private interests responding to the city’s investment.
“I am convinced that we can achieve very high canopy rates in very dense areas if we treat trees as infrastructure and give them what they need for soil,” says Urban, who wrote the tree section of the reference volume Architectural Graphic Standards. “If you combine ‘new urban trees’ with green roofs and other ways of softening our cities, along with large urban parks, I am fairly confident we can make cities very viable from an ecological standpoint. I certainly can envision a 40 percent canopy coverage at typical New Urbanism density.” u