10.5 Greenspace

Most of the ecosystem services we focus on in urban areas are regulating services. Trees, grass, and other plants can mitigate heat islands, air pollution, water pollution, and flooding. Urban greenspaces can also provide wildlife habitat and support biodiversity, which is essential to the continued health of urban ecosystems and the services they provide. Each location where a city now exists originally had a certain landscape based on the biome it’s part of, like a forest, prairie, or desert. Some cities still have patches of that original landscape within them, and these areas are called remnants (Fig 1).

Remnants can be extremely valuable habitat for plants and animals. For example, a study done in Italy found that landscape remnants made up 5% of one city’s land area but hosted over 85% of all the plant species in the city, including some that were not found anywhere outside the remnants.^[1]

A study in Australia surveyed bee species in bushland remnants and in residential gardens and found that the remnants were home to more rare and unique bee species than the gardens, as well as hosting higher overall bee diversity.^[2] This section addresses many ecosystem services that newly-planted areas of vegetation can provide. However, if there’s a choice between preserving original habitat or creating new habitat, preserving the original is almost certainly the more environmentally responsible option (and will likely require less time and money).

Soil, the foundation of terrestrial ecosystems, is dramatically different in urban areas than in undisturbed natural areas. Soil in a forest, for example, is made of small pieces of clay, sand, and organic matter, loosely piled together so that there are tiny air pockets between them. That kind of soil has high porosity, which means water can easily infiltrate those air pockets and plant roots can easily work their way in between the soil particles. Constructing buildings and roads involves compressing the soil both intentionally, to ensure it stays stable, and unintentionally, as heavy equipment travels to and from the construction site. In some areas of cities, people and vehicles then move around on the soil and compact it further. This compaction dramatically decreases the soil’s porosity, so that some urban soils can be almost as dense as concrete.

When it rains, most of the water runs off the compacted soil just like it would off of pavement. It’s also difficult for roots of trees and other plants to grow into this dense soil. Even when they manage it, the low-porosity soil can’t hold much water, so those plants are more vulnerable to drought than they would be in more porous soil.^[3] Urban soil is polluted by traffic, industry, waste incineration, and more. Contaminants like heavy metals, microplastics, pesticides, and antibiotic resistance genes are found at high levels both within cities and in the surrounding natural areas.^[4]

Brownfields are polluted sites, such as former industrial areas, gas stations, or landfills, where soil contamination can make it unsafe to use the land for housing or public space. In some cases, developers simply excavate the soil or put a cap over the top, but plants, fungi, and microorganisms can also be used to break down, neutralize, or absorb pollutants, an approach known as bioremediation. Planting former brownfields can simultaneously mitigate pollution and provide public green spaces for city residents. You can read more about cities turning brownfields into parks here.

Trees in cities provide a variety of ecosystem services and economic benefits (Fig 2). They mitigate urban heat islands, first by shading surfaces, then through evapotranspiration. They intercept 10-40% of rainwater on its way to the ground, and their roots help absorb the rest. They filter air pollution and carbon dioxide, remove heavy metals, excess nutrients, and other pollutants from the soil, and shade buildings, which reduces the energy needed for cooling.^[5] ‘Green screens,’ lines of trees along highways, not only mitigate air pollution but can reduce traffic noise and improve the view for people living nearby.

Unfortunately, in the United States, another effect of policies like redlining means that urban street trees are inequitably distributed. The temperature difference between white and POC neighborhoods, due to white neighborhoods having higher tree canopy cover, leads to significantly more deaths, more doctors’ visits, and higher electricity consumption in POC neighborhoods.^[6]

Overall, urban tree cover in the United States is decreasing by tens of millions of trees every year.^[7] This is a problem in cities around the world, as urbanization replaces forested areas with human structures. In Freetown, Sierra Leone, deforestation around the city combined with heavy rains led to huge landslides, prompting a large-scale reforestation effort. The program has planted hundreds of thousands of trees, including many in disadvantaged areas with low canopy cover, creating over 1,000 jobs and decreasing flooding (more details click here). Urban reforestation can give a huge return on investment because trees provide such valuable benefits to people living near them.

However, a high number of trees is not enough, on its own, to ensure long-term ecosystem services; taxonomic and age diversity of trees are also essential. The more diverse an ecosystem is, the healthier and more resilient it can be. Diseases and pests can be specialized to target one species, or they can affect a group of related species. If every tree on a particular street belongs to the same family, they could all get wiped out at the same time, as happened when Dutch elm disease spread across the US in the mid-1900s. eliminating elm trees – very popular street trees – in many cities.

Planting a wide range of species that can each tolerate different conditions and threats maximizes the strength of the ecosystem as a whole. It is also important to have a range of ages so that the trees in an area do not all get old and die at the same time. In addition to planting a variety of trees, it’s essential to have plans and resources in place for maintaining them so they can live as long as possible in the hostile urban environment. You can click here for a 10-minute video on tree equity and efforts to increase and maintain urban canopy cover in the United States.

As mentioned in section 3, plants are a major part of the solution to flooding and water pollution issues in cities. They can be used on roofs (which make up 20-25% of total urban area^[8]), on slopes and in ditches, and in rain gardens (Fig 3). You can click here to view a Polish company that creates green bus stops (planters and trellises that eventually cover bus shelters in plants); their website lists the many benefits of this infrastructure. Watch this video click here TED talk to learn about green flood-mitigation infrastructure that doubles as beautiful public space in Bangkok, Thailand.

In temperate regions, the most common type of vegetation in cities is short-mown turf grass lawn, which is maintained both in parks and on private land. Lawn makes up 25% of city land area in the UK, 23% in the US, and 22.5% in Sweden. ^[9] Lawns are deeply entrenched in Western culture and valued for their neat appearance and suitability for recreation. Lawns provide some ecosystem services, including erosion control and carbon storage, but they are time-intensive and expensive to maintain, requiring frequent mowing as well as fertilizers, herbicides, and/or irrigation. Mowing produces GHG emissions, chemical inputs can pollute the soil and water, and irrigation can contribute to the depletion of water resources. In addition, lawns are often maintained as monocultures, which decreases the biodiversity they can support. ^[10] Even when lawns include multiple plant species, they are low-quality habitat for invertebrates and other animals. Mowing prevents plants from flowering and going to seed, limiting the availability of nesting opportunities for insects and foraging opportunities for many animal species.

An increasingly popular alternative to lawns is urban meadows, or ‘pocket prairies’: infrequently-mowed open areas of native grasses and wildflowers, created in urban areas to mimic original ecosystems like North American prairie or Eurasian steppe. In dry biomes, xeriscape meadows can include succulents and drought-tolerant bushes and grasses.^[11] Meadows with a variety of species growing to their full height can support impressive biodiversity, including microbes in the soil, insects that nest in different parts of plants, bees and other pollinators that rely on flowers for food, and birds and mammals that forage on and around plants. ^[12] Pollinators provide a huge service to human agriculture, so maintaining pollinator habitat is essential to protecting our own food supply.

Native plants have much deeper roots than non-natives and turf grass, so they do much more to absorb rainwater, stabilize soil, capture heavy metals and nutrients, and prevent flooding. They can filter more particulate air pollution than lawns can, and because they require so much less mowing, they also reduce GHG emissions and noise pollution.^[13] In some urban areas, homeowners are not allowed to create meadows because of regulations against tall plants that are seen as weeds. However, even in these places, native plants can be used in landscaped gardens and still provide valuable ecosystem services and habitat (Fig 4).

A surprising variety of plant and animal life can exist in cities. Over 14,000 plant species and 2,000 bird species live in cities around the world, and most of these are native to the areas they inhabit.^[14]

The first step to protecting urban biodiversity is to measure it by conducting a species inventory (like this example, click here done in Mexico City, Mexico). These efforts can engage local residents in citizen science projects, which help get people interested in the nature around their homes and motivate them to protect it. Once city planners know the locations and identities of species (pollinators, endangered species, invasives, etc.) or habitats (wetlands, coastlands, biodiversity hotspots), they can target their efforts at those species and areas. Check out this article click here for an in-depth look at how birds have adapted to urban environments and how to make cities more bird-friendly.

Cities can be nexuses for invasive species, because humans transport plant and animal species both intentionally and by accident. Exotic plants are often used in gardens and then spread to other areas, and people buy pets such as snakes and lizards and then release them into the wild. In the United States, Callery pear trees and Burmese pythons, among many other species, were brought to the country intentionally and became invasive. Seeds and small animals can also travel on ships, in luggage and packaging, and on people’s shoes. Many invasive species are well-adapted to urban environments and so can form established populations in cities. From there, they can move to nearby natural areas and threaten native species. Because many invasives are spread by people who don’t realize the effect they will have, educating the public is key to limiting this issue.

In supporting pollinators and other wildlife, the total amount of habitat in a city matters, but the connectivity of the habitat is also important. Many animal species can only travel short distances, and plant seeds can also only travel so far. Animals are often killed while crossing roads. These limitations on movement can lead to a population dying out when it gets stuck in a small patch of habitat that can’t support it. Habitat corridors address this issue by connecting patches of green space across urban areas, often along rivers, in rights-of-way next to roads, or under power lines.

City governments can protect corridors and include habitat connectivity as a priority in urban planning. In 2016, Medellín, Colombia started a green corridor program that trained citizens to plant trees around the city, and in the following three years, they planted nearly 9,000 trees in 30 urban corridors. The new trees have lowered average city temperatures, improved air quality, and increased biodiversity. ^[15]

Cities can also acquire land for habitat restoration, create incentives for private developers and landowners to create habitat on their properties, and educate the public on the value of green space, both for nature and people.

Spending time in nature (Fig 5) has both physical and mental health benefits: lowered blood pressure, heart rate, and stress hormones and improved attention, cognitive performance, and mood.^[16]

Public greenspaces can give people opportunities to build social connections and learn about the world around them. However, fewer than half of urban residents around the world have convenient access to open public spaces, including greenspaces (Fig 6). Many demographics that are marginalized in other ways (socioeconomics, race, culture, age, physical ability, etc.) also have less access to greenspace. This gives those groups worse health and quality of life and makes them more sensitive to the effects of climate change. ^[17]

In 2019, Lima, Peru started an urban regeneration program, in collaboration with local communities and targeted at vulnerable populations, that has increased tree cover, pedestrian streets, and access to greenspace. ^[18]

Many activities people enjoy in parks and greenspace, such as picnics and sports, require turf grass lawn. However, mixing patches of forest and native plants into lawn areas dramatically increases the quality of the habitat and provides beauty and variety for human visitors. Many city residents may not have opportunities to visit pristine natural areas, but healthy, vibrant ecosystems within the city can provide experiences of plants and animals. This contact with nature can help people understand the importance of conservation, so urban greenspace can have an additional, indirect benefit to the health of the planet.

Additionally, cities are in some cases even better places for conservation than rural areas. Farmers and other people whose livelihoods rely on a particular land use may be reluctant to reserve parts of their land for habitat, ^[19] whereas in urban and suburban areas, both governments and individuals pay to maintain little-used lawn in public parks and private yards. Replacing lawn with habitat can save taxpayers and urban landowners money by requiring less maintenance, as well as providing beauty, interest, and the satisfaction of having improved the city’s environmental health.

Knowledge Check

Take a moment to complete the short quiz below to assess your understanding of this section. Read each question carefully and refer to the section content as needed. This quiz is not graded – it’s simply an opportunity for you to reflect on what you’ve learned and reinforce key concepts.