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Urban forests: challenges and benefits

Climat et Biodiversité

According to the IPCC, 70% of global greenhouse gas emissions come from urban areas. With pollution, artificialization, heat waves and shrinking biodiversity, cities have become the symptom of a world that is running out of steam, reflecting development that is disconnected from the living world. While more than half of the world's population live in cities, it is becoming urgent to rethink urban planning, to (re)integrate plants, to recreate a natural balance. Therefore, the urban forest represents a tremendous opportunity to go beyond our conception of today's city to build the city of tomorrow.

The impacts of cities without forests

Cities and forests have almost always been seen as two opposites: a wild world, governed by natural laws, versus a universe shaped and controlled by mankind. As cities have become one of the main drivers of climate change, this antagonistic vision is reaching its limits.

When urbanization means artificialization

By 2050, there will be between 6 and 7 billion* city-dwellers on the planet, i.e. nearly 70% of the world's population. In France, this percentage will soon reach 80%**. Little by little, the size of the cities is increasing and metropolises with more than one million inhabitants are multiplying. Such a volume and such a concentration of people imply managing space and the environment in a way that is incompatible with nature. In most cases, the rapid expansion of cities has taken place at the expense of ecosystems. At the same time, urban sprawl, via the construction of concrete infrastructure, has gone hand in hand with the artificialization - and therefore the sealing - of massive swaths of soils, which is one of the proven causes of climate change and the erosion of the biodiversity.

*World Urbanization Prospects, UN, 2018 **Insee, 2020

Cities: what impacts on the climate?

The IPCC is very clear. Urban areas are the source of 70%* of global greenhouse gas emissions, which are responsible for current global warming.

In cities, the effects of global warming are exacerbated. Global warming manifests itself in the form of heat islands, which correspond to a rise in maximum temperatures in urban areas when compared with the surrounding natural environments. Contrary to popular belief, this phenomenon is not due to pollution but to the concentration of buildings that capture heat during the day and release it into the atmosphere at night. According to a recent study** published in January 2023 and carried out in 93 European cities, 75% of the urban population experiences 1°C more than rural inhabitants, and 20% of them are facing up to 2°C more.

The FAO estimates that 60% of urban people live in areas at high risk of exposure to at least one natural disaster. In the city, the phenomena of soil erosion and compaction coming from rainfall are the cause of increasingly frequent and intense floods and landslides, which can be fatal in certain regions of the globe.

*3rd part of the 6th assessment report of the IPCC, 2022 **Cooling cities through urban green infrastructure: a health impact assessment of European cities, study published in the medical journal The Lancet, 2023

Biodiversity, the missing part of cities

The construction of cities came along with a significant anthropogenic change in landscapes, with a degradation or complete loss of the natural ecosystems that were there in the first place. For centuries, urban sprawl has been at the root of biodiversity collapse, caused by the destruction of native fauna and flora habitats. This uncontrolled development has paradoxically triggered an uptick in certain animal populations, sometimes considered harmful, or in exotic plant species. Other species have had to adapt to the urban context by becoming “urban-adapted”, such as swallows whose wings have gradually shortened to fly between urban buildings.

A matter of public health

In addition to being harmful to the climate and biodiversity, the city has undesirable effects on its own residents. Inhabitants of urban areas are exposed to air pollution. According to a recent study, the particles produced by the combustion of fossil fuels - due to cars and industries - would be responsible for more than 8 million* premature deaths in 2018, i.e. just under one in five deaths worldwide. In addition to the diseases caused by urban pollution, there is a decline in the average life expectancy of people living in cities. In Paris and the Greater Paris area, where the air is particularly polluted, residents can lose up to 2 years of their lifespan.

Urban pollution is not only about air pollution, but also noise pollution. Noise, omnipresent in the city, can cause disturbances or a feeling of discomfort for many people, and is considered a long-term risk factor.

The lack of plants in the city is as much a public health problem as a question of well-being. While the presence of green or wooded spaces in the city is beneficial for mental health and stress reduction, their absence generates a host of mood-altering effects. This is why, in 2020, after difficult periods of lockdowns, 8 out of 10 French people said that making cities greener should be a priority***.

*Harvard study, published in Environmental Research, 2021 **France Public Health Authority, 2016 ***Study conducted as part of the Green Cities Observatory, 2020

EN CHALLENGES Infographics Urban Forest.jpg

Transforming urbanity with forests

Faced with such a reality, making cities greener represents a leading solution for initiating the socio-environmental transition of cities. FAO, IPCC, WRI and Cerema are all unanimous: improving the urban forest is an excellent way to mitigate the environmental and social impacts of urbanization. Furthermore, developing cities’ tree resources contributes to creating a sustainable and resilient urban model.

Urban cooling

The urban forest acts directly on the climate of cities by refreshing the surrounding air. Placed in strategic locations, trees can lower the temperature by 2°C to 8°C* and reduce air conditioning needs by 30%* when planted around buildings. This can be explained by several phenomena. First, a tree can absorb up to 80% of the sun’s rays. The shade provided by its foliage prevents concrete infrastructures from storing too much heat during the day. Second, thanks to evapotranspiration, a mature tree produces up to 450 liters of water in the form of water vapor, i.e. the equivalent of 5 air conditioners running for 20 hours**, which creates a cooler microclimate around it.

According to the study published in The Lancet at the beginning of 2023, doubling the plant cover of European cities - to reach 30% of surface area being tree-covered - would make it possible to cool city summers by 0.4°C on average and to avoid heat islands. It is important to remember that when it comes to global warming, every tenth of a degree counts.

*FAO, infographic Benefits of urban trees, 2016 **Study published by Ademe, 2019

Biodiversity and green network

Urban trees and shrubs are important biodiversity support systems for the fauna and flora. Flowers, fruits, twigs, dead wood or even tree hollows represent a source of food for pollinating insects, birds, amphibians and small mammals that populate our cities, while providing them with numerous shelters.

Improving the urban forest makes it possible to create more biological corridors, forming what is called the "green network". This “network” concept, defined by the French Environmental Code, aims to link different reservoirs of biodiversity together in an uninterrupted area. Animals need to circulate to ensure their biological cycles and to protect themselves from predators; the urban forest thus facilitates their movements in cities, which are a very hostile environment for wildlife.

The presence of trees in the city also contributes to recreate a living soil, namely thanks to the dead leaves which decompose into humus and thereby strengthen the life of micro-organisms in the soil. Faced with growing artificialization, the urban forest is an excellent way to promote the renaturation of urban soils.

Improving air and water quality

Urban trees help improve air quality. According to the NGO The Nature Conservancy*, they can reduce the concentration of fine particles in the air by 20 to 50% . The leaves of trees, covered with hairs or wax, have the ability to intercept airborne pollutants while the gaseous exchanges made with the atmosphere allow them to absorb certain atmospheric pollutants**. However, the enormous amount of pollutants emitted in urban areas cannot be compensated by trees alone, which must be integrated into an overarching policy for improving air quality. Similarly, tree roots help protect drinking water resources by filtering water pollution, and well-designed green infrastructure can support wastewater treatment.

*The Nature Conservancy, Planting Healthy Air, a global analysis of the role of urban trees in addressing particulate matter pollution and extreme heat, 2016 **Such as nitrogen dioxide or ozone

Sustainable water management

Thanks to trees’ incredible root systems that help to fixate the ground, revegetation protects cities from soil erosion and landslides. In addition, urban forest is particularly useful for combatting water runoff from soil sealing, with one tree absorbing up to 150L of water per day*. How? Some of the rainwater is directly intercepted by the foliage of trees and shrubs, what is called the interception mechanism, and the remaining rainwater infiltrates the soil via the tree roots which maintain its porosity. In addition, plants consume water from the soil to grow, thereby avoiding soil saturation problems. Rainwater infiltrates very little, if at all, into soil already saturated with water.

*Nicholas Martin, 2019

Physical and psychological benefits

The One Health concept, developed in the 2000s, highlights the relationships between ecosystems and human health. This global approach is particularly well illustrated in urban areas, where the health of people living in cities is strongly linked to the existence of wooded areas.

Based on data from 2015, a year of exceptional heat waves, scientists have estimated that a third* of heat-related deaths could be avoided by planting trees in cities. Trees, with their ability to reduce pollution, limit the risk of contracting various pathologies caused by air pollution, such as asthma. Urban forest also provides us with increased immune system protection: it has been shown that during a day of hiking under the tree canopy, the human body produces 30%** more immune cells than during a day of walking in the city. Having access to green spaces reduces stress and anxiety, which goes hand in hand with a decrease in heart rate and blood pressure. Ultimately, the improvement of the physical health of people living in cities, associated with the presence of an urban forest, could increase their life expectancy by 7 years***.

The well-being of residents is also positively affected by trees. Quite naturally, urban parks, gardens and micro-forests promote social fabrics and outdoor physical activities, which lead to healthier habits. In addition, trees form a real barrier which, by absorbing acoustic energy, dissipates ambient noise.

*Cooling cities through urban green infrastructure: a health impact assessment of European cities, study published in the medical journal The Lancet, 2023 **Clemens G. A., The healing effect of the tree, the emotional, cognitive and physical benefits of biophilia, Le Courrier du livre, 2016 ***Kardan O. et al., Neighborhood greenspace and health in a large urban center, 2015

Economic benefits and attractiveness

The many benefits listed above are difficult to quantify, although some of them have been the subject of further studies. In particular, it has been proven that improving the urban living environment using trees can increase the value of a property by up to 20%. Moreover, by reducing the pressure on the healthcare system, green areas would generate up to several million euros* in savings. Finally, developing and maintaining the urban forest leads to the creation of a large number of jobs in a growing economic sector.

Wooded areas greatly contribute to the diversity and quality of urban landscapes, thereby enhancing their attractiveness to residents and visitors. The different categories of afforestation make it possible to remedy the monotony of “all concrete” while structuring the multiple spaces that make up the city. With its variety of colors, scents and shapes, the beauty of trees attracts and captivates.

*FAO, Urban Forestry **Asterès report on behalf of UNEP, Urban green spaces. Places of public health, vectors of economic activity, 2016

Combatting climate change

By storing carbon, urban forests are one of the strategic means for mitigating and adapting to climate change in cities. In fact, a small tree can sequester up to 16kg* of CO2 per year, and a mature tree can absorb up to 360kg*. In addition to the carbon sequestration generated by the aerial part of the trees, the reconstitution of the soil by their roots leads to an even greater storage capacity for CO2.


EN SOLUTIONS Infographics Urban Forest.jpg

While the consequences of urbanization can no longer be concealed, the urban forest is a window to the city of tomorrow. Urban forests provide us with the opportunity to rethink urban planning, to create alliances between mineral and green spaces, to build bridges that connect the city with the countryside. However, investments in urban forests currently accounts for only 1.3%* of major French cities budget. To increase this figure and reduce the environmental footprint of cities, public and private stakeholders need to work together. In partnership with many municipalities and companies in France and Europe, Reforest'Action is playing a key role in developing more sustainable and liveable cities. Discover our offer for planting urban micro-forests.

*Unep and Hortis, Gardens & health: towards healthy cities?, Green Cities Observatory, September 2017