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researcher of the Biodiversity and Environment Institute and professor at the School of Sciences from the University of Navarra.
Ecosystem Services are "benefits that nature brings to society". Ecological systems - and the natural capital on which they depend - are fundamental to the functioning of the Earth's life support system, on which humankind and all other living things depend. Urbanisation means a clear loss of services. In this text, we try to relate these services to the everyday life of people in cities.
Vegetation in cities
There are seven natural ecosystems in cities: trees, grasslands, urban forests, crops, wetlands, contiguous lakes or sea, and rivers or streams. The current design of cities does not favour their presence. The population values non-uniform pavements, with spontaneous vegetation or that present on walls, linear meadows, tree pits or planters. It is valued more if it is associated with management human, and less if it is spontaneous. And it is more highly valued by people who maintain contact regular contact with nature outside the city.
Air pollution, associated with millions of deaths per year worldwide, is the most serious environmental health problem. It is caused by particulate matter, molecules of anthropogenic origin, or molecules present in nature, but in higher than natural proportions (ozone). Urban or peri-urban reforestation can help to improve air quality.
Benefits of vegetation in cities
1. Provision of habitats
It is surprising how many plants and animals are able to live in the habitats "created" by humans in cities, thanks to their ability to adapt (non-anthropisation). The benefits of increased habitat diversity are less obvious, but very important.
Traffic noise can be reduced by up to 50% with appropriate vegetation and design. A 5 metre deep barrier can reduce traffic noise by 9-11 decibels. Tree trunks also have a reduction effect. Synthetic barriers are less effective (psychologically and in absolute values).
3. Deposition and dispersion of pollution
Vegetation reduces the concentration of pollutants in parks, especially if they are large. Large particles are reduced the most. In addition, the design of vegetation encourages users to avoid polluted boundaries. Trees and shrubs can achieve ozone reductions of up to 2 %. Green roofs contribute less than trees.
4. Effect on the background climate: cooling
The use of vegetation in cities mitigates the urban heat island effect in non-tropical cities, but in order to have an effect with some impact, plots should be at least average hectare. The associated reduction in energy consumption, and reduction of the greenhouse effect (partly responsible for Global Warming) can be significant.
5. mobility and changes in physical activity
If sustainable mobility infrastructures are attractive (green) they are considered to be spaces of high environmental quality, and more used to the detriment of motorised transport. Vegetation is of at least moderate importance in improving air quality and heating sensation. The core topic - reclaiming space for people - has a lot to do with vegetation.
6. Soil permeability
Trees and shrubs intercept rainwater, and permeable soils are crucial to reduce pressure on the drainage system, reducing the risk of flooding. As a result of urbanisation, with 50-90 % impervious soil, 80 % of rainfall is lost to surface runoff (forest: 10 %). The associated filtration can be considered as another "service": that of natural water treatment.
7. Carbon sequestration
It occurs in two scenarios: by accumulation of carbon in the trunks and branches of trees and shrubs; and by accumulation in the soil due to biological activity (fungi and bacteria). If the soil is "sealed" it does not accumulate water and loses the capacity to sustain the webs of life.
8. Cultural services
There are proven health and psychological benefits of green spaces, especially if they have high plant and animal biodiversity. Property value increases in areas close to green spaces. It should not be forgotten that they are recreational spaces for citizens.
Seemingly negative aspects of vegetation in cities
Wild ecosystems can be perceived negatively, and maintenance costs (e.g. green roofs and seepage) must be taken into account. Some actions to increase Biodiversity may be criticised for apparent neglect of maintenance services, so they should be carried out to good quality standards and their purpose should be communicated, promoting the correct environmental Education . Avoid "planting any species anywhere" and avoid actions that cause, for example, an excess of pollen or other products. Some designs (e.g. trees with wide canopies in narrow streets) may cause a ceiling effect: pollutants may not dissipate and may concentrate.
Access to urban vegetation: Environmental Justice
In most cities there is a positive correlation between urban vegetation and the upper Education , and a negative correlation in neighbourhoods or cities with a minority presence. In urban environments it is important to design the presence of urban vegetation to achieve urban green equity as part of environmental justice.
How to improve environmental quality in cities?
By increasing biodiversity, but without protecting specific species, but rather habitats close to natural habitats, which are greatly simplified by human pressure for reasons of public safety or pollution. The options are: increase the density of native trees, and they need to be large; increase undergrowth vegetation. This alone increases the presence of bats, native birds and insects, important links in many ecosystems. Not too costly management has great effects on the environmental quality of cities.