NetZeroCities

From the land use planning perspective, urban green spaces or urban green areas are defined by Corine Land Cover classification[1] as “Areas with vegetation within or partly embraced by urban fabric. This class is assigned for urban greenery, which usually has recreational or ornamental character and is usually accessible for the public, and can also include small water bodies. It includes parks, gardens, city squares with greenery, vegetated areas, cemetery with vegetation, green inner spaces of city blocks, ponds, lakes, small water bodies”.

However, instead of considering them as individual and disconnected green spots, all these green and blue areas can together form a green infrastructure whose roles and functions should be seen as an ecosystem.

As summarised by the European Environment Agency[2], “Green infrastructure is a strategically planned network of natural and semi-natural areas with other environmental features designed and managed to deliver a wide range of ecosystem services (as defined by the EC’s GI communication – COM/2013/0249final). Green infrastructure is present in both rural and urban settings. In urban areas, many different features may be part of green infrastructure (e.g. parks, gardens, grassy verges, green walls or green roofs) as far as they are part of an interconnected network and are delivering multiple ecosystem services. These green urban elements (or blue if aquatic ecosystems are concerned) may be found within the city and in its peri-urban area” (source: EEA, glossary of green infrastructure).

They are therefore relying on nature-based solutions. IUCN defines nature-based solutions as “actions to protect, sustainably manage, and restore natural and modified ecosystems that address societal challenges effectively and adaptively, simultaneously benefiting people and nature.”

Greening through the creation of urban green spaces can therefore provide multiple ecosystem services and is a response to the challenges of biodiversity and climate change. Climate change reinforces certain hazards that are already problematic in the urban environment: overheating in summer, extreme rainfall causing flooding by runoff on impermeable surfaces, etc. The physical and physiological properties of green spaces make it possible to limit certain hazards: shading and leaf transpiration for cooling are examples. Thus, greening is now recognised as a possible response to the challenges of biodiversity loss and climate change.

The purpose of this concept sheet is to highlight the role and functions of green and blue areas in cities as part of an ecosystem.

[1]https://land.copernicus.eu/user-corner/technical-library/corine-land-cover-nomenclature-guidelines/html/index-clc-141.html

[2]https://www.eea.europa.eu/themes/sustainability-transitions/urban-environment/urban-green-infrastructure/glossary-for-urban-green-infrastructure

Climate and geography

Cities can be a hostile environment for the proper development of vegetation. There are many sources of occasional or long-term stress: air and soil pollution to which species can be more or less sensitive, poor water supply, intentional or accidental damage. In urban environment, air is drier, the sunshine is often too direct, but also sometimes too weak. Soils can be inhospitable: compacted, compacted, too filtering, poor, polluted, ... Buried networks and pavements make rooting difficult and inefficient. Older plantations, when they include only one species, can make the plant heritage particularly vulnerable to diseases and pests. Species are not equal in coping with these difficulties. They develop different strategies for adaptation:

To the local climate: a species located in its natural area will have a better resistance to local conditions. However, this must be put into perspective because of the specificities of the urban environment and the effects of climate change.
To the specificities of the urban environment: some species are more tolerant than others of compact and dry soils, pollution and injuries. They are therefore better adapted to this urban context.
Expected climate change: Climate change has multiple and varied consequences depending on the territory: increased water shortages over longer periods, increased soil salinity and frequency of storms on the coast, forest fires in areas not previously affected, etc. The species must therefore be adapted to these local effects.

It is necessary to know these adaptation strategies and to optimise them to ensure the resilience of urban green spaces.

Policy and regulatory/legal framework:

Territories are confronted with contradictory issues between the availability of land, the incentive to drastically reduce artificialization and a growing demand for green spaces by citizens. The implementation of nature projects must become a structuring element of urban planning. Nature issues should be treated on an equal footing with more traditional issues such as housing, facilities, shops and mobility. Another major challenge is to avoid building on the soils that are most suitable for vegetation or most likely to infiltrate rainwater. Indeed, when they have access to water and a living soil, vegetated developments provide optimal services. A land strategy based on the ecological potential of soils, backed up by planning, becomes an essential lever for identifying and reserving spaces for renaturing the city. Under these conditions, nature could become the basis of a new "ecological urbanism" that would structure spaces and mobility. It is therefore essential to give nature a place of its own so that it can contribute to a liveable, vibrant, resilient and sober city.

Project governance and implementation modalities:

This approach, which aims to make nature a pillar of a climate-neutral city, shall bring together the various skills and forces present in the territory. It shall be based on natural ecosystems already in place, which are the real actors to be preserved as a priority. The objective is to adopt an integrative approach that considers the city and its ecological and climatic realities (3).
The successful integration of nature as a component of a new urbanism is conditioned by the implementation, acceptance and maintenance of natural facilities, in a long-term perspective and a global strategy.

This strategy could be based on three phases:

Developing a nature project: sharing ambitions and overcoming opposition
Survey the existing fauna, flora and habitats and identify the land that can be mobilised
Define development actions for and with nature by preserving the existing, adopting protection rules, favouring the systematic integration of nature, creating nature spaces and anticipating the reserve of space through planning.

Project governance and implementation modalities:

The design and implementation of urban green areas shall involve multiple stakeholders within the city administration (urban planning, landscape, water management, etc). Services involved in the maintenance and management of green public spaces shall be involved from the beginning of the project to determine the needs and constraints of the projects and the choice of species to be planted.

Climate / Geography:

Location of green areas, as well as the choice of plants and trees species shall result from an assessment and site diagnosis (environmental conditions, soil conditions, geographic and climatic conditions, landscape environment, etc). An assessment of current and future climate should be undertaken to identify appropriate local species.

Green and blue network:

Urban green areas shall not be considered and designed as isolated spots but rather as being part and connected to a larger blue and green network. Therefore, they shall contribute to the continuity of this green and blue corridor.

Policy / regulatory framework:

Land pressure for urban development can reduce the opportunities for development of urban green areas within urban settlements. Integration of urban green areas as a key parameter for resilient and sustainable urban development shall be clearly highlighted with the land-use planning documents including limitations for future development (no construction areas, buffer areas, etc). The explanation of ecosystem services provided by the urban green areas can help to demonstrate their value for city development.

Urban form and layout:

Available urban spaces and urban form (building height, setback between buildings, depth of open ground, etc) will influence the needs and constraints of the project. They shall be carefully analysed to determine the key principles for planting.

Social context:

Participation with citizens is required to design these areas as multifunctional spaces.

Adaptation to climate change:

Green spaces help to cool the surface and ambient temperatures of urbanised areas through the shade and evapotranspiration they generate. They can therefore contribute to reducing localized situations of climatic discomfort, or even limit the urban heat island effect. This service is maximised by the choice of large trees with dense foliage, capable of maintaining their transpiration for a long time on dry ground without endangering themselves. Shrubs are also relevant options: if the plantings are massive, evapotranspiration can contribute significantly to localised cooling. However, the benefits of green spaces are only possible if the vegetation is healthy and supplied with water according to its needs, so as to achieve maximum transpiration. The proper development of this vegetation and sensible pruning practices also increase these positive effects.

Flood risk management:

Plants are major consumers of water, particularly through transpiration: by consuming water in the soil, they maintain infiltration capacity. Indeed, rainwater does not infiltrate, or only slightly, into a saturated soil, which therefore causes runoff. Species that consume large quantities of water may be of interest: the more water the tree consumes and transpires, the less saturated the soil surfaces will be when it rains. However, it is prudent to choose species that are able to transpire abundantly and tolerate dry soil. Plants can also reduce runoff by retaining some of the rain directly in their foliage through interception. However, some of the water does reach the soil: plants can then facilitate infiltration via their roots. A well-developed and dense root network maintains more porosity, and therefore infiltration. Trees and shrubs can have contrasting tolerances to soil moisture, especially when the water content is so high that it prevents gas exchange between the roots and the soil. Species should therefore be chosen to suit the site conditions, especially if rainwater is to be concentrated in the site. Properly chosen, trees and shrubs can be placed directly in water management structures such as swales, where they improve performance by intercepting and maintaining infiltration potential.

Air quality improvement:

Urban green spaces contribute to improving air quality. They intercept particulate air pollutants such as nitrogen dioxide or ozone. Some plants even have the advantage of having rough, hairy or even wax-covered leaves and will retain these particles more durably, as they are otherwise quickly carried away by the wind or precipitation. It is necessary to remember that even if these effects are proven, they are in proportions far below the emissions linked to human activities. Revegetation alone will not solve the air quality problems of a city. It is important to specify that the effects of vegetation on the concentration of pollutants in the air in a very localised manner are difficult to anticipate: they depend on the shape of the trees, the shape of the surrounding buildings, the air circulation, etc. In certain configurations, the barrier effect of vegetation can increase the concentration of buildings. Finally, some species have a certain duplicity, since they are characterised, in particular in case of stress, by high emissions of volatile organic compounds, themselves precursors of ozone.

Examples of proposed indicators:

Proportion of urban green areas within the city boundaries in relation to the total area
Canopy ratio within the urban settlement
Distribution of urban green areas within the city boundaries (or average distance to the closest green area)
Average urban green area per inhabitant (i.e. total urban green areas / total population)

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