Maintaining the urban forest

Large trees need large spaces with vast expanses of soil to grow, something that is in short supply in urban areas. Large trees are also seen as inconvenient in cityscapes: they get in the way of utility lines and sidewalks (roots can upend sidewalks and roads), and are a hazard if the trees get diseased or old and start to lose branches. They are expensive to maintain since they need to be pruned, cleaned of leaf litter and sometimes fertilized.

Smaller trees, many urban planners believe, are more manageable and serve much the same function as larger trees.

That is not so, say tree experts. While large trees—those 35 to 50 feet tall at maturity—would not naturally choose to grow in an urban environment, urban areas need these large trees for many reasons: their canopies absorb carbon dioxide and pull particulate matter from the air; their leaves and trunks help with storm water drainage; and their sheer size to keep the heat island effect to bearable levels. Research has also shown that people prefer large trees in their communities, and large trees have the effect of slowing down traffic and increasing property values.

“Small trees are often the most logical choice for urban areas, but large trees are the best choice for community values,” said Michael Kuhns, professor of forestry and forestry extension specialist at Utah State University.

Omaha’s Old Market shopping area is lined with hackberries.

Despite the benefits of planting large trees in cities, data does support that large trees are more expensive to maintain, said Jeff Kirwan, extension specialist and professor, department of forestry at Virginia Tech. What is often not understood is the cost-to-benefit ratio. “Large trees do pay for themselves, and more, in environmental and other benefits. Most municipalities don’t understand the benefits as much as the cost, so they dwell on the cost.”

One hot issue in cities now is managing storm water. Only the big trees have the capacity to intercept about 80 percent of a 1-inch rainfall, compared to small trees, which intercept only 16 percent. “To a storm water engineer, a large tree is the perfect management system,” said Kirwan.

Canopy cover on the decline

Large trees in urban areas, however, are on the decline—about 25 percent of city tree canopy has disappeared in the past 30 years according to a satellite image analysis conducted by American Forests (, a Washington, D.C.-based nonprofit group. The organization estimates that 634,407,719 trees are currently missing from metropolitan areas across the United States as a result of urban (and suburban) development.

In terms of pollution, big trees are sorely being missed in cities. They remove 60 to 70 percent more pollution than small trees, and they can also absorb more than 1,500 gallons of water a year. They also keep temperatures down. In Atlanta, Ga., for example, where 380,000 acres of trees have been cut down in the city and surrounding areas in the past 30 years, the average temperature is 5 to 8 degrees higher than surrounding woodlands.

In large cities, planners often create obstacles to planting large trees, said Kuhn. City restrictions from planting large trees include:

• Anywhere cars might hit them
• Anyplace where they can interfere with pavement or curbs
• Anyplace where there are power lines or overhead utilities
• Anyplace with underground utilities
• Anyplace they interfere with signs
• Anyplace where they can hang over a building or home
• Anyplace where they might pose a hazard to people.

These recommendations, noted Kuhn, come from well-meaning sources, such as city planners, attorneys and utility companies. “Even tree people have jumped on this bandwagon, calling for smaller trees,” said Kuhn. “What ends up happening with all these restrictions is that there isn’t one place where a large tree could be planted appropriately and be able to comply with these recommendations.”

Main Street in Brigham City in northern Utah is lined with London Planetrees.

While photos from early cities such as Washington, D.C., reveal that large trees were quite at home lining the city streets, Susan Day, research assistant professor of urban forestry at the department of forestry at Virginia Tech, points out that today’s cities have much more underground infrastructure than in the past, making it difficult for large trees to thrive.

“Infrastructure gets renewed, replaced all the time … that results in trees getting constantly disrupted. Sidewalks in newer cities are more compacted, again, making it difficult for trees to grow.” There were also varieties of trees that thrived in urban settings, such as the American elm, that no longer exist in large numbers. “We no longer have as many species to choose from that will grow in these tough settings,” said Day.

Not every place is the right place

Large trees can also be planted economically if the right tree is planted in the right place. “Big” trees do not necessarily mean large and fast growing, since many of these types of trees, such as poplar or cottonwood, wound easily, require a lot of moisture and are prone to decay. Better choices would be the London Planetree, sycamore and oaks. Day recommends any bottomland species—predisposed to severe conditions such as flooding and drought—namely the silver maple, water elm and black gum trees.

“Diversity is what is important,” recommends Day, because of the fear of pests and diseases that could wipe out an entire monoculture of trees in a city, such as what happened with the American elm and chestnut trees. “Trees that do well in urban environments that get big and stay strong and sound are best,” said Kuhns. “By the way, not every big tree has to be a giant tree to get the benefits. If there is room for a small tree, put in a medium-sized tree. Push the envelope and opt for larger trees.”

While a concrete jungle may not the best place for a large towering oak tree, there are places, even in large metropolitan areas, where big trees are appropriate. “Anyplace there is room for a large tree’s canopy and root system there should be a large tree planted,” said Kuhns. Some ideal locations for a large, good-quality tree could be in residential areas and near municipal buildings, which are stable and have less of a chance of disruption because of construction. Other areas that large trees are less prone to disruption are parks or cemeteries. Parking strips and medians, provided there is enough room (at least 8 to 10 feet wide), are also good options.

Even dense urban environments can accommodate large trees if room is made for roots, including the use of:

• Tree pits—Particularly if there is access to soil volume under the pavement, there’s enough water and it is the right species. “I do not recommend grates since they are inflexible and expensive,” said Kuhns. He does recommend tree guards to prevent damage to the trunk.

• Porous pavers, gravels and other materials—Using these materials to plant trees along sidewalks or parking lots increases rooting space, improves soil aeration and water infiltration, and it can be combined with structural soils, as well.

• Special sidewalks—Meandering sidewalks (away from the roots of trees) and rubber sidewalks have also been successfully used to allow tree roots room to grow.

• Planting bays and planting strips—These can be bumped out of the curb to get more planting areas, and yet allow parking spaces on the street.

• Wider parking strips—2 to 3 feet is what is commonly used to plant trees, but 8 to 10 feet is actually needed.

Large trees can also be planted under overhead electric lines if directional pruning is used.

The worst possible place for large trees to survive, however, is a paved commercial area, like a parking lot. Yet, studies have shown that retail area profits increase when trees are planted. “The problem is not only the compacted soil, but the short-term nature of commercial development,” said Kuhns, who noted that many strip malls undergo reconstruction every decade or so, making life impossible for large trees. While it’s useful to grow high-quality large trees in these locations, particularly in helping absorb storm water, these trees must be grown properly and not disrupted for decades.

Advocacy of tree professionals

The problem with many developments, says Day, is that trees are often designed as an afterthought. “If trees are considered an important component from the beginning, and not stuck in the design like a little ornament at the end, the trees, and the environment, would benefit,” said Day.

As the world becomes more urbanized (more than half of the world’s population now live in cities), city planners need to integrate trees into land use decisions. “A lot of communication needs to be done up front of a project among the engineers, ecologists, architects, city planners and arborists,” said Day. “Tree specialists need to advocate for big trees in cities and speak to town councils, mayors and the people who make the decisions. When the decision-makers get on board with this, then the engineers, the lawyers, the architects will start to apply the technical solutions to make it happen. Because it is possible to have large trees in cities, and it is important that they be there for the sake of the environment and our health.”

Progress is being made in many cities, said Kuhns, as urban foresters and arborists are beginning to play a bigger role in helping cities understand the importance of big trees, and helping to select those species that will best thrive in this tough environment. City officials are also getting more interested in the data provided by foresters when they talk about how big trees can significantly decrease storm water flows.

“Smaller stature trees have a tremendous role, under utility lines, near buildings or in places where rooting is limited such as containers, but there is still an important role for large trees in cities,” said Kuhns.

Marcia Passos Duffy is a freelance writer based in Keene, N.H.