Reasons for cities to invest in green infrastructure
Urban planners are often passionate advocates for street trees. Though cities have strapped budgets, an urban forest offers an excellent return on investment. Yes, trees drop leaves and fruit, produce pollen, and have roots that can become invasive or heave sidewalks. Yet trees make such a big difference to quality of life and urban management that these are small problems to overcome. The benefits of street trees far outpace their challenges.
Why should my city plant street trees?
Much of a city’s built environment is impermeable to water. Hardscape—made of traditional concrete, glass, stone, steel, and asphalt—prevents groundwater replenishment and produces run-off.
Trees decrease the flooding these circumstances create. In light showers, leaves and branches capture or absorb water on their surfaces, where it can evaporate rather than running to ground. Water that runs down branches, bark, and roots is channeled to soil and groundwater. Roots also help build the water holding capacity of soil. Tendrils both big and small create pockets that increase the permeability of soil. Roots also prevent soil erosion by binding it. Greater soil depth means more strata to absorb water.
Finally, trees use water in their life processes. Rather than simply returning water to the ground, a tree also returns moisture to the air in a process called transpiration. A mature oak tree can return 40,000 gallons of water to the atmosphere per year.
Traffic calming—and driver calming
Trees help with safe road design. They’re proven to slow average driving speeds. On medians, they reduce the chance of head-on crashes. A row of trees can also provide clear demarcation of pedestrian zones, creating a visual wall that helps keep drivers on the roadway.
Trees also make a significant impact on the lives on drivers, even those just passing through an area. An interesting study from 2003, “Restorative Effects of Roadside Vegetation: Implications for Automobile Driver Anger and Frustration,” showed that trees helped increase people’s resilience to frustration. The authors suggest tree lined streets might help mitigate the aggravations of commuting.
Trees create walk appeal. Where space is beautiful and safe, people are more likely to be active, including walking or riding a bicycle. In this way, trees encourage healthy lifestyles. “Active design,” including trees, is an environmental way to people moving and support their health outcomes. Increased activity may be why better heart health and longevity are correlated with green-space rich neighborhoods. Lower air pollution also is a likely contributor.
Several studies show that tree canopy creates lower rates of ‘psychological distress’. People are happier and less likely to depression when they’ve got connection to nature. This lower stress is evident in decreases in blood pressure and cortisol. There's no doubt relaxation helps support physical health benefits of tree canopy.
People who walk or ride through their community spend more time on the street. Perhaps this explains why pedestrians and cyclists spend more at local businesses. They’re not quickly moving past, and have more reasons to be drawn in.
Additionally, people who ride and walk as a primary means of transportation spend less overall on getting around. In some neighborhoods, this savings may be offset by housing charges but in others it leads to increased local retail profitability.
Many neighborhoods, buildings, or developments are named for their trees. Common place names such as Oakridge, Elmvale, Maple Grove celebrate the effect of local trees on creating a sense of place.
Placemaking is an important goal of urban planning. A vital place is one that engages the community in unique, local ways. There are many important ways to foster places that encourage neighbors to engage: errands and work close to home, the Power of 10+, art, community events, gathering spaces, and safety. Trees support this engagement by making it easier and more relaxing to be outside. It is much more appealing to do errands on foot or meet outside under the shade of a mature tree than to walk through shimmering hot concrete and city grit.
Healthy trees are aesthetically pleasing. They create variations in color, texture, and height in the visual landscape. Their beauty can be a tourist draw, whether it’s bright autumnal foliage in Vermont or the gorgeous spring cherry blossoms in the Pacific Northwest.
Trees also help promote regional biodiversity. Birds, butterflies, squirrels, chipmunks, and other local fauna require the habitat and sanctuary provided by trees. The tree’s ability to help prevent soil erosion can also help maintain understory vegetation.
Trees lower the temperature in urban areas. They offer shade, large canopies like a parasol. They also release water vapor into the air, which is why the shade beneath a tree is often fresher than shade beneath a patio umbrella. In studies of “physiologically equivalent temperature,” or how cool we feel we are, trees can moderate temperatures between 9–27°F.
Cities are up to 12°F hotter than surrounding countryside. Hardscape like concrete, brick, and asphalt absorbs sunlight, creating what’s known as an urban heat sink. Trees combat urban heat sinks in part by capturing sunlight so that it does not hit the pavement and absorb.
Trees help manage air quality by reducing particulate and absorbing gases. A large healthy tree can remove 3.5 lbs of pollutants in one year. A single tree can have 5 acres worth of surfaces when calculating all the leaves and branches. The tree’s surfaces capture and settle particulate moving by on wind currents. When it rains, the channelization of water along branches and in bark helping wash this particulate to the ground, where it is filtered by soil and other plants.
Adding oxygen and removing carbon dioxide
Trees are our biggest tool in the struggle to lower atmospheric carbon dioxide and mitigate climate change. Research has shown that reforestation on non-food producing lands, both urban and rural, is a way to capture 200 billion pounds of carbon dioxide over the next 50–100 years. Trees are inexpensive for the wealth of benefits they offer.
In urban environments, the local climate control of trees lowering both heating and cooling requirements helps prevent energy use. Creating a healthier microclimate helps lower the overall energy requirement of the city.
As the trees on a street mature, property values tend to increase. Homes with trees sell for 5–10% more. Given the other benefits of trees—their beauty, health effects, flood mitigation, and ability to cool and refresh the air—trees offer a great return on this investment. Properties with trees save on cooling costs due to shade and transpiration. They also save on heating costs through windbreak and climate moderation effects. Well-placed trees can add beauty to landscaping. They also may help provide privacy to residents, depending on placement.
The history of urban tree planting
Trees were not always seen as an important asset to the health of cities. Cities were a hub of industry close surrounded by wilderness or farmland. Tenements were built up near production hubs, but wild or pastoral land did not seem to be in short supply.
Yet as cities grew, parks and trees became a clearer need for city dwellers working long hours. Most tenement dwellers had little time, money, or energy to travel to the ever-more-distant borders of the city. Additionally, the dirt, grit, and heat of citification became increasingly evident.
By the 1800s, New York City was experiencing the disease and discomfort of hot, treeless streets. In the 1870s, Dr. Stephen Smith spearheaded research and advocacy for planting trees, suggesting that adding trees—the only air conditioners available at the time—could save between 3,000 and 5,000 lives per year.
Why cities of the future still need trees
With advances in engineering and indoor climate control, are Dr. Smith’s recommendations of more than a century ago still valid? The World Economic Forum, and many other experts, argue yes. Modern engineering is, of course, important. But most infrastructure solutions provide one or two functions. A few well-placed trees can provide erosion control, flood control, climate calming, air cleaning, mental and physical health support, and beauty. Very few built solutions achieve so much in so little space.
Planting trees between the sidewalk and street
When a city, or an individual with permission from the city, plants a tree, they usually add to the overall health and well-being of their neighborhood. However, for the tree to be successful, there are important considerations before going ahead.
Street trees are spaced per municipal guidelines, tree requirements, and environmental conditions. Usually they are spaced apart far enough that they do not interfere with one another (20–25 feet is common). They are generally spaced well back from corners, to prevent being a line-of-sight hazard. They are often 5 or more feet from other site fixtures like lamp standards and signs.
Trees must be dug into the soil and will create root systems that could become a disruption. Check with utilities and the city.
Root barriers and directors:
Trees need their roots to deliver water and nutrients from the soil. Different types of trees have different common rooting patterns. However, in general, trees will adapt to local conditions. Root systems seek out the air, water, and nutrients needed. In urban environments it’s important to both help control where roots can go and to provide adequate soil and water to the root systems.
Root barrier systems prevent roots from spreading towards pipes that they could disrupt and disturb, or guide roots down so they don’t contribute to sidewalk heave. Some companies provide specialized director systems that help bring roots away from the soil through a combination of barriers and growth forms.
Preventing soil compaction:
In urban environments, the combined pressure of hundreds of feet stepping over soil can lead to severe compaction. The ground becomes extremely hard and throughout is less porous. Hard packed soil can create as much run off as concrete.
The soil around the root systems of trees should therefore be protected from being walked on using tree grates, plantings, or short swale-fences, depending on space considerations.
One trend for packed sidewalks in dry cities is to install grates in a long strip, creating a soil zone open to air, water, and small plant growth.
Trees should be picked for their zone and hardiness to local conditions and planted well-watered. They need particular nurturing over the first summer, when root systems are still establishing. Mulch should be used for nutrients and protection during the winter. Once they are well established, regular municipal maintenance is usually sufficient.
Benefits of street trees
Cities that invest in trees see benefits to economic, public, and social health. Major benefits of a well-cared for urban forest include:
- Water management
- Traffic calming
- Physical health
- Mental health
- Economic health
- Community identity
- Cooler air
- Cleaner air
- Adding oxygen, removing CO2
- Property values
There has often been a disparity in tree cover in poorer neighborhoods. Some see the addition of trees as the first step in a process called eco-gentrification. However, trees are best when widely distributed. Gentrification often follows large, splashy projects that become destinations. Yet if similar, smaller projects happen in every neighborhood in the city in the same timeframe, the net effect is of a greener city, rather than a single neighborhood becoming more attractive. It is because of the power of urban trees that makes it wise to install them carefully and intentionally.
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