What Is an Urban Forest?

Urban forest is a term used to describe a collection of woody plants (trees, shrubs and bushes) growing in and around urban areas like cities (Figure 1) [1]. The woody plants found in urban forests are important to cities because they provide ecosystem services. An ecosystem service is a good or a service from the natural environment that benefits humans—from something as simple as making an area beautiful, to something as complex as filtering out air pollutants to clean the air [1]. Fifty-six percent of humans live in urban forests ecosystems and, worldwide, the number of people living in or near cities is steadily increasing. Therefore, we should put more effort into protecting and maintaining urban forests now than we ever have before.

Measuring an Urban Forest

A common way to measure a city’s urban forest is by observing the urban tree canopy (UTC) [1]. The UTC is a measure of how much land in the area is covered by trees. One way to calculate the UTC is to do on-the-ground tree counts and then estimate the tree’s crown size. The crown is made up of the branches, leaves, and reproductive structures extending from the trunk or main stems. Another method often used is to analyze aerial images taken from satellites to determine the tree coverage in an area [1]. Once tree coverage is determined a map is created to show an areas UTC (Figure 2). Knowing a city’s UTC is important because it helps scientists and city planners track the changes happening to that city’s urban forests. A city with a low UTC could set urban management plans to increase its UTC, or a city with a high UTC could set plans to maintain its canopy coverage.

Environmental Benefits of Urban Forests

One ecosystem service provided by urban forests is an improvement in air quality. This benefit happens because trees can filter out air pollutants. A study done in 2006 found that urban forests in the United States filter out 7,11,000 metric tons of pollution each year [2]. The pollutant with the greatest rate of removal was a gas called ozone, followed by tiny particles called particulate matter, and other potentially dangerous chemicals called nitrogen dioxide, sulfur dioxide, and carbon monoxide [2]. All these air pollutants can cause harm to humans and to the environment. Also, carbon dioxide (CO₂), a greenhouse gas, is stored in trees. It is estimated that 700 million metric tons of CO₂ are removed from the atmosphere by urban trees each year. Reducing greenhouse gases helps to combat global climate change and directly benefits the local community by improving air quality.

Another ecosystem service provided by urban forests is that they help to improve water quality. During rainstorms, rainwater can be stored on tree leaves. This is beneficial because excessive water runoff from cities can cause erosion in nearby streams and rivers and can transport pollution into those bodies of water. Reducing runoff also helps to reduce flooding by limiting the amount of excess water in an area. Trees can also be planted in stormwater-management systems called bioretention basins, which act as living filters that can capture pollutants.

The urban forest can also help to conserve the biodiversity in an urban area. Trees can be important sources of food and shelter for city-dwelling animals (Figure 3). Some cities also actively protect critically endangered trees and other plants and animals, which further helps with biodiversity [3]. Clearly, urban trees benefit both the environment and the people who live in urban areas.

Human Health Benefits of Urban Forests

As we have explained, trees and their leaves serve as filters, absorbing dust, small particles, and other air pollutants. These pollutants negatively affect human health and can cause diseases, particularly diseases that affect the respiratory system. People living in areas with better air quality are less likely to develop respiratory diseases like emphysema or lung cancer [4]. Better air quality has also been linked to fewer birth defects, and it can reduce children’s chances of developing asthma.

A higher UTC in cities is associated with increased exercise for city residents. This is because cities with higher UTCs generally have neighborhoods that are better for taking walks [5]. Neighborhoods that are more walkable encourage people to walk places instead of taking cars or buses. Walking daily has known health benefits, including lower cholesterol, decreased risk of heart disease, and a healthier resting heart rate. Overall, walkable cities promote a healthier lifestyle. The more green spaces people have available, the more they will take advantage of such spaces.

Green spaces also reduce stress and anxiety. Studies have linked longer levels of exposure to nature with lower levels of stress [5]. Lower stress levels increase a person’s overall wellbeing, because we are more likely to feel refreshed after enjoying outdoor space. In fact, doctors sometimes prescribe time in forests or other natural spaces for patients coping with stress! Cities with high UTCs give residents more green spaces to enjoy.

Societal Benefits of Urban Forests

When people enjoy urban forests, communities see benefits. A higher UTC creates a stronger sense of community and safety. Studies show that improving UTC leads to increased self-esteem, increasing a community’s overall social identity. Residents feel empowered when they realize they can control their surrounding environment [4].

Also, cities with higher UTCs provide more opportunities for kids to play outside. A study in Chicago found that urban forests increase the amount of time that children spend playing outside, and that these children were healthier than children who do not. This is because higher levels of outdoor play provide health benefits for children [4]. Some studies have found that more green space around schools can improve student grades and test scores, though more research is needed to better understand the relationship between urban forests and school performance [6].

The Chicago study mentioned above also found a relationship between children spending more time playing outside and a decrease in the city’s overall crime rate. The crimes that decreased ranged from disorderly conduct all the way to violent crimes [4]. Lower crime rates improve the safety of neighborhoods, which makes residents want to be more involved in the community. These benefits have been proven to increase property values in the area as well, because people want to live in safe areas [4]. This connects back to the stronger sense of community.

The Benefits Are Clear

Urban forests positively impact residents by providing ecosystem services like cleaner air and water. Cleaner air and water can improve the health of city residents. Not only that, people who live in well-developed urban forests live an overall healthier lifestyle, and they even have lower levels of stress. Furthermore, high-UTC areas are safer and have a stronger sense of community, which increases property values. The higher a city’s UTC, the more these benefits are amplified. These benefits clearly demonstrate that urban forests are extremely valuable ecosystems that should be protected and enhanced whenever possible.

Glossary

Urban Forest: ↑ A densely wooded area located in a city.

Ecosystem Services: ↑ Benefits humans receive from nature.

Urban Tree Canopy: ↑ the layer of leaves, branches, and stems of trees that cover the ground when viewed from above.

Greenhouse Gas: ↑ A gas that contributes to global warming by trapping the sun’s heat near the Earth.

Runoff: ↑ The water or other material that drains freely off the surface of something.

Bioretention Basins: ↑ Shallow areas, often containing trees, used to slow and treat stormwater runoff.

Biodiversity: ↑ The variety of species on Earth.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.