City Trees Give More in Ecosystem Services Than They Cost
City planners need to plant more trees in urban areas, but the benefits go beyond beautifying pavement.
Urban areas occupy 4% of the land surface of the planet, says Theodore A. Endreny in a Comment article in Nature Communications. That may strike readers as surprising, given most people’s attention to cities for work, travel and home. But since so many spend the majority of their time there, it’s incumbent upon city planners to bring some of the natural world into urban environments for what Endreny calls the “ecosystem services” they provide.
Growth in urban populations creates opportunities for urban forests to deliver ecosystem services critical to human wellbeing and biodiversity. Our challenge is to strategically expand urban forests and provide our international communities, particularly the vulnerable, with healthier, happier, and enriched lives.
Trees are too often removed for urbanization, well captured by Joni Mitchell’s lyrics “They paved paradise. And put up a parking lot.” Urban areas globally will expand to accommodate population growth and migration trends. Yet, urban denizens benefit greatly with trees in their habitat, and that is the theme of the 2018 International Day of Forests; Forests and Sustainable Cities. Urban areas can concentrate poverty and sickness, and trees can help alleviate these ills through their ecosystem services. Our global challenge is to grow urban forests and sustain human wellbeing and biodiversity.
What does he mean by “human… biodiversity”? Endreny finds it in “shared spaces that enhance mixing of community across ages, cultures, and incomes.” Trees produce a modicum of peace in the hearts of those who live among them. At the very least, it may be harder to throw Molotov cocktails with tree limbs in the way.
He offers some statistics to show what city dwellers are missing. Even though urban areas make up only 4% of the land surface, if planted at global average tree density, they could contain 121 billion trees. The actual tree count in urban forests is about <10 billion trees, just 8% of their land area’s tree-carrying potential. What can be done? City parks certainly help, but the “urban forest” could expand to rooftops, pocket parks, street trees, nurseries and riparian corridors.
Speaking of river courses, Los Angeles has made some effort to restore parts of the L.A. River with trees and bike trails, but much of it still consists of long, ugly, graffiti-laden concrete channels. Urban planners should consider whether the same services of flood control could have been provided by a tree-lined natural watercourse. Almost every year, people or animals fall into the concrete washes during floods and drown. Concrete, too, channels all the pollutants from millions of homes and office buildings out to the sea. Wouldn’t a botanical ecosystem do a better job of filtration? If mangrove forests can tame the violence of tsunamis in the far east, couldn’t the right kind of riparian forests tame the occasional urban floods?
What exactly are the “ecosystem services” that trees can provide to denizens of the urban forest? Endreny classifies them as:
- Cultural (e.g., spiritual, recreational)
- Provisioning (e.g., food, fiber, water)
- Regulating (e.g., climate and flood control)
- Supporting (e.g., pollination, soil formation)
Trees can reduce the temperature of inner cities with their shade and transpiration. They can reduce noise. They can filter water. Think of all the ways that a tree can be a blessing to a city dweller walking in a concrete jungle, or looking out an office window. Endreny estimates $2.25 return for every $1 spent on planting trees. The poor may benefit the most: “Urban forest services are invaluable for the vulnerable and low capacity residents without food, water, and energy security, who can find in these forests nutrition, clean water, wood fuel, and shelter, as well as jobs and a sense of purpose.”
With all the government money going to healthcare and other priorities, why not invest more in items that give back more than they consume? He gives some figures on what tree-planting has done to benefit some major cities. Direct results, he figures, are double the expense. “With money effectively growing on trees, what could slow growth of the urban forest?”
Not to overstate his argument for trees, Endreny considers potential “ecosystem disservices” they might engender: accidents, allergens, obstructing views, inviting pests or exotic species, and just the initial investment costs of planting and maintaining them. “Yet where there are threats,” he says, “there are opportunities.” Each of these threats can be managed with appropriate design. He sees a bright future in engineering the urban forest.
A virtuous cycle is possible for extending urban forests, with benefits paying for management, and new forests advancing research to maximize services and minimize disservices. The field of urban forestry will grow with that of urban science, which is poised to grow rapidly, generating discoveries at the social-ecological system nexus critical to sustainability. Linking urban forestry to ecological engineering provides an opportunity to focus on building with nature to achieve renewably powered and systems-based self-designs that satisfy human needs and advance ecosystem conservation…. Exciting discoveries will emerge in urban forestry as we manage these threats and pursue these opportunities.
This doesn’t have to be a top-down program. Individual citizens can take part, making it a “leaderless movement” to improve the urban ecosystem. Citizens can inventory the trees in their urban forests, learn about the benefits of various tree species, and locate new places for planting. Individuals can also be quite vocal in protecting their favorite trees.
We need to tell the stories of success from communities across the globe, where fruit harvests supply food banks in Seattle USA, greenbelts treat wastewater and combat desertification in Ouarzazate, Morocco, toxic soils are cleansed by trees in Guangxi, China, urban temperatures and poverty are reduced with forest stands in Bobo-Dioulasso, Burkina Faso, tree nurseries purify drinking water and generate wood fuel for the needy in Dhaka, Bangladesh; and children in an impoverished school are nurtured by a tree garden irrigated with gray-water within the desert landscape of Lima, Peru.
Does Endreny overstate the case in his last sentence? “These achievements are grassroots, wholesome, and empowering, and ensure that trees will not be relegated to a museum.” Unquestionably, if urban areas only take up 4% of the land areas. But then, who doesn’t love the appearance of a healthy, tall, green tree along a city boulevard?
Update 3/26/18: For another article on the urban forest, see “How tree bonds can help preserve the urban forest” on The Conversation. Four authors consider ways to fund more tree planting in cities.
Update 3/26/18: In his daily Breakpoint commentary for March 22, John Stonestreet wrote about “talking trees,” concluding that “naturalism has no language of wonder.”
Exercise: Take notice of the “urban forest” in your city on your way to work. Imagine the trees erased from the scene. Would it make a difference to your sense of well-being? Could your city do better with tree planting?
Isn’t it just like God to make things that give more than they receive? I recently marveled at a tree I regretfully had to remove (I did get some firewood and mulch from it). All those strong limbs, leaves, and light-harvesting machines arose from dirt! A little seed that had started that tree growing years ago contained all the program code to take ingredients from dirt and transform them into a massive, glorious living thing, operated on by machines at the cellular level more complex than biochemists can fathom. Plant a pebble in the dirt and it won’t do that. What amazing and beautiful objects God has given us in trees. They provide object lessons instructing us that we, too, should contribute more than we consume.
Do human beings need trees? It’s noteworthy that one of the most joyful holidays in Israel from the time of Moses that continues to this day is to build outdoor booths with leafy branches of trees and live in them for a week. It’s as if the Creator wanted to help fallen humanity get out of their artificial worlds and take a look at the glories outside at least once a year. Better yet, leave the city once in awhile and actually walk or drive through a natural forest.
An urban forest had historical significance in one particular city. It provided palm branches for people to welcome the Lord Jesus Christ into Jerusalem as they shouted, “Hosanna! Blessed is He who comes in the name of the Lord” (Matthew 21). Happy Palm Sunday to all our CEH readers!