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Urban forests make megacities more environmentally sustainable

Just 20% more trees could double the benefits of urban forests

Central Park New York City at sunset © istock.com/HenrikAMeyer
Central Park New York City at sunset © istock.com/HenrikAMeyer

Each month the Elsevier Atlas Award recognizes research that could significantly impact people's lives around the world.

Implementing and managing urban forests: A much needed conservation strategy to increase ecosystem services and urban wellbeing

T. Endreny, R. Santagata, A. Perna, C. De Stefano, R.F. Rallo, S. Ulgiati
Ecological Modelling, Volume 360, September 2017, Pages 328-335

A tree on the sidewalk shelters a young woman as she walks under the canopy. Across the street, a group of children play on the floor in the shade of a cluster of trees on the green. It’s a hot day in the city and residents are looking to their urban forest for relief from the sun.

It’s a familiar picture – about 20 percent of the area of each of the world’s megacities is urban forest. But according to a new Atlas award-winning study in Ecological Modelling, a further 20 percent could be transformed into forest – something that would change residents’ lives for the better. Dr. Theodore Endreny , lead author of the paper and now professor of the Department of Environmental Resources Engineering at the College of Environmental Science and Forestry (ESF), explained:

I was born in Manhattan, and I lived there as a young parent with my children. I remember going to the parks in New York City, and in the morning finding a layer of soot on the swings – black carbon from the diesel fuel being combusted by lorries (trucks) bringing food into the city. I was aware at that point that a canopy of tree leaves can filter out and protect that play space from that soot. By cultivating the trees within the city, residents and visitors get direct benefits. They're getting an immediate cleansing of the air that's around them. They're getting that direct cooling from the tree, and even food and other products.

Nearly 10 percent of the world’s population live in megacities – cities of at least 10 million people. While those people often rely on nature outside of the city for their food and recreation, nature within the city in the form of urban forests can provide enormous benefits. An urban forest contains the single tree in someone’s back yard, the row of trees along a street or a wooded area in a public park; joining these areas together with other trees extends the size of the urban forest.

As an interdisciplinary group of ecological engineers, including biologists, technicians, economists and engineers, the team led by Dr. Endreny, then at Parthenope University of Naples, in Italy, realized that the world’s megacities were probably sharing the same stresses, so looking for similarities could help highlight ways to improve sustainability in the cities.

In the study, the team used a tool called i-Tree Canopy to estimate the current tree coverage in cities and the potential for more urban forest cover, and worked out the benefits that would bring. They estimated the current tree cover in ten megacities in five continents, looked at the benefits of urban forests – including removing pollution from the air, saving energy and providing food – and estimated the current value of those benefits at over $500 million per year.

Creating a model for each megacity, they estimated benefits like reductions in air pollution, stormwater, building energy and carbon emissions, and assessed how those benefits changed as the tree cover was increased. The model took into account the local megacity tree cover, human population, air pollution, climate, energy use, and purchasing power. Dr. Endreny continued:

There are many famous examples of urban forests in the megacities we studied, from Central Park in New York City to St. James’ Park in London and Bosque de Chapultepec in Mexico City. All have urban forest areas, ranging from 10 percent coverage in the desert city Cairo to 35 percent in temperate Moscow. What really surprised us was that each city has the potential to host a further 20 percent coverage of forest canopy, which could have a big impact on the city’s residents and visitors.

What’s even more striking is that planting those additional trees could bring nearly double the benefits. So why aren’t we seeing an increase in urban forests yet? City planners and authorities still consider the city’s natural resources to be largely outside the metropolitan area. Changing that perception and conserving nature within the city, says Dr. Endreny, will make megacities healthier, more enjoyable places to be.

I think the future of the urban forest is very exciting – it should represent the diversity that our cities bring through immigration, and mixing of perspectives. We may find an increased amount of provisioning services, foods that are being gathered, and edible forest. And that future will likely include many more vertical forests, as we bring the benefits of the tree and the collection of trees into downtown spaces.

Everyone can take action to increase the urban forest areas in our cities. You can start by visiting the free resource www.itreetools.org to find out how much coverage there is in your city now, and how the benefits of the urban forest increase as more trees are planted.

Sergio Ulgiati, Prof. Massimiliano Lega, Prof. Theodore Endreny, Dr. Claudio Colaiacomo of Elsevier, Dr. Renato Rallo and Dr. Remo Santagata at the Atlas Award Ceremony on 13 December 2017 at the University of Naples Parthenope

A conversation with Dr. Theodore Endreny

In this podcast, Dr. Theodore Endreny talks about the benefits of urban forests, and why it’s time to plant more trees in our megacities.

What is an urban forest?

An urban forest includes a single tree. It can also extend out to rural forest areas. You can imagine a park in any megacity where the community gathers and enjoys the shade and biodiversity of a set of trees a park; the trees are included in the urban forest. You might also imagine a street in any one of the 10 megacities we studied lined with trees, and those street trees are included in the urban forest. In some of the cities you have cemeteries, plazas and rooftops with trees that are part of the forest. You may have rivers and waterfronts lined with trees, and each of those areas would be part of the urban forest. Connecting each one of these elements together continues the forest.

Why are you interested in the benefits of urban forests?

I was born in Manhattan, and I lived there as a young parent with my children. I remember going to the parks in New York City, and in the morning finding a layer of soot on the swings and on the slide that I was putting my daughter on. That soot was coming as a black carbon from diesel fuel that was being combusted within diesel engines by lorries bringing food into the city. I was aware at that point that a canopy of tree leaves can filter out and protect that play space from that soot.

I'm also a runner and an avid fisherman. To me, it just makes my heart lift and my soul sing to think that there could be children and adults in these urban areas that could provide a forest, and then have that forest provide them with a fishable, swimmable, and drinkable water resource.

Can you tell us about your study?

I was working with students and faculty at Parthenope University of Naples, in Italy, and we chose to look at the urban system and ask, ‘how can we make this more sustainable?’ We realized that all cities around the world were likely sharing the same stresses, and that it would be really useful to look at what similarities there were between these cities, and see if by sharing those similarities and approaches toward a solution, we might be able to engender more collaboration and cooperation between governments and continents to address a way of providing a sustainability within these cities that can benefit all humans on our earth.

What most surprised you?

The general uniform distribution of existing tree cover surprised me. There was Cairo, a desert community that had almost less than 10 percent tree cover, and then Moscow, with almost 35 percent tree cover. But within that variation, those are the two extremes, and the rest of the cities averaged almost close to 20 percent existing tree cover.

Also, the potential tree canopy cover almost averaged 20 percent, but urban residents and city planners think of the large, external space around cities as providing the ecosystem services that we need – the water supply, and the food. The perspective that the city is also a natural space, and that we can have a conservation plan within the city, is something that we hope the readers are surprised by, and embrace.

Who can take action to help residents of megacities benefit from your findings, and what should they do?

Everyone can take action. City and regional planners can continue to incorporate the planning for urban forests, and those who are elected to office can continue to share a vision that the urban forest is an important part of the community, and they can advocate and support groups that are looking to increase it. Individuals who rent or own a plot can also take action by planting a tree or a shrub, which is smaller than a tree but has a bushy, leafy canopy that can provide many of the benefits we’re talking about.

What do you think the city of the future looks like?

The future of the urban forest is very exciting. It should be extremely diverse to represent the diversity that our cities bring through immigration, and mixing of perspectives. We may find an increased amount of provisioning services foods that are being gathered and edible forest. We may also see an increase in the amount of cultural and spiritual interaction birding painting outdoor exercise and meditation. That future will likely include many more vertical forests as we bring the benefits of the tree and the collection of trees into the downtown spaces that don't have enough land area to provide the forest cover needed to benefit the large number of people in the city center.

Get involved

You can look at the urban forest in your city, see where trees could be planted and what benefits they could bring, using free i-Tree tools. i-Tree is currently available in several countries, including the US, Canada, Australia and many countries in Europe, and it is being extended to more.

  • i-Tree Eco – quantify the structure of, threats to, and benefits and values provided by forest
  • i-Tree Landscape – prioritize areas for tree planting and protection
  • i-Tree Canopy – estimate tree canopy and benefits using aerial photographs
  • i-Tree Design – analyze current and future tree benefits
  • i-Tree MyTree – assess the value of one to several trees
  • i-Tree Hydro – quantify the effects of tree canopy on water quantity and quality
  • i-Tree Species – determine the best species to plant
  • i-Tree Planting – estimate long-term environmental benefits and values of a tree planting project
  • i-Tree Harvest – estimate the amount of carbon stored in harvested wood products

About Ecological Modelling

Ecological Modelling publishes studies that use mathematical models and systems analysis to describe ecological processes and support the sustainable management of resources. Human activity and well-being are dependent on and integrated with the functioning of ecosystems and the services they provide. The journal aims to understand this through modelling, systems analysis, thermodynamics, computer simulations and ecological theory. Ecological Modelling supports the activities of the International Society of Ecological Modelling (ISEM).

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