STEP 01 INDICATORS
Through research and co-evaluation, six main urban challenges have been derived. These challenges are presented as indicators that will help in finding urban, semi-urban or rural areas where trees are needed in municipalities. The indicators can be prioritised according to your urban profile. It could be said that the more urban your city is, the more prominent heat stress and water retention are, and the more rural your city is, the more important biodiversity is. This information can be used in the following steps, where prioritisation of the indicators plays a big part.
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INDICATORS
The list below further explains the severity of each indicator and why they are a key problem within cities.
LACK OF BIODIVERSITY
Approximately 14% of flora and fauna on the European Red List of Species are present in the Netherlands, with high percentages of endangered mammals, amphibians and invertebrates. Current urban development has further pushed native species of importance to the periphery of cities, consequently leading to a deficit in ecosystem services provided by biotas such as urban cooling and pollination.
URBAN HEAT ISLAND
The excess amount of impervious surfaces in cities has led to intensified urban temperatures in summer months. Otherwise known as the Urban Heat Island effect, this rise in temperature can greatly impact citizens by increasing energy demand and costs, air pollution, heat-related illnesses and mortality and water quality.
CONCENTRATION OF POLLUTANTS
Urban pollutants include other emitted gases from anthropogenic activity, such as particulate matter, nitrogen dioxide and carbon monoxide. While dutch wildlife populations have halved in the last 30 years largely due to nitrogen pollution. Fine particulates are considered the most challenging as have profound effects on human health compared to any other pollutant.
CO2 PRODUCTION
Cities are responsible for more than 80% of global greenhouse emissions. The most common gas being Carbon Dioxide (CO2), in which emissions have increased more than 2% annually across the Netherlands due to transport, deforestation and population increase. This increase in CO2 in turn contributes to the greenhouse effect, which has greater effects globally on weather patterns, sea levels, food and water supplies.
RAINWATER FLOODINGS
Precipitation levels have increased by more than 20% in the Netherlands in the past century. With rainstorms majorly falling in heavy, short bursts that produce excessive puddles and overwhelm old drainage systems in cities like Amsterdam. This, along with the lack of green space retaining water runoff, causes small-scale flooding and drastic impacts to mental wellbeing and modes of transport across the city.
CITIZEN WELL-BEING
Environmental factors and urban design have a more profound effect on city-goers than meets the eye, with urban dwellers at 40% more risk of depression than people living in rural areas. Urban neighbourhoods that considerably lack green tend to have higher crime and lack social cohesion compared to more leafy parts of the city.
BENEFITS
Trees have significant benefits for urban areas and their surroundings. Here the socio-ecological advantages of planting trees in cities are stated in correspondence to the six urban challenges indicated above.
HUBS FOR BIODIVERSITY
Trees provide substantial sources of food, shelter and habitat for both flora and fauna. Increasing shrub vegetation cover by 20% can allow for a 75% higher occupancy of endemic bats, birds and beetles in cities. The planting of mature trees can therefore bring in ecologically-important fauna, such as pollinators, as well as culturally-rich species. Oak trees are considered keystone species due to attracting 284 organisms.
COOLING EFFECT
Both the shading and evapotranspiration function of trees can have impressive outcomes in reducing heat stress created by climate change and the Urban Heat Island effect. If placed appropriately within urban areas, trees can cool the air between 2 and 8°C; the equivalent of 10 air conditioners. This in turn can reduce energy consumption, reduce UV rays radiating off both manmade and organic surfaces during the summer.
POLLUTANT CATCHMENT
Additionally, trees improve air quality by capturing and filtering damaging fine particulates like dirt, dust and smoke, as well as absorbing urban pollutants like carbon monoxide, ozone and dangerous oxides. By this organic method of urban air purification, effects on health could thus be combated with the planting of trees and shrubs that are particularly good at capturing urban pollutants, such as Sycamore.
CO2 SEQUESTRATION
On average, a mature Plane tree can absorb up to 150kg of CO2 per year through photosynthesis. This process of sequestering carbon from the atmosphere undertaken by flora makes trees and shrubs fundamental mitigators in climate change and rising emissions in cities on a global scale.
RAINWATER RETENTION
Urban trees regulate water flow and improve water quality in cities by capturing and storing rainfall, which is then released into the air through evaporation from their leaves. Capturing water in this way combats stormwater runoff and therefore can help prevent urban floods as well as their roots and leaf litter help infiltrate water into the soil, keeping the ground moist during dryer months.
IMPROVED CITIZEN WELL-BEING
Research has shown that living adjacent or with direct access to green space in urban areas can boost mental, cultural and physical wellbeing. This idea of Biophilia - that humans have a strong psychological bond with nature and living things - fosters social connections, increases physical activity and decreases aggression and violence.