The roots of sustainability: 5 reasons why cities need trees

As most cities and countries continue to report hotter summer days that are breaking 100-year records, indoor cooling can offer only little respite and to the privileged few. The majority outdoors – humans and other beings – continue to struggle in the heat, finding shelter in the shade of trees. And yet these very same trees are being displaced by physical urban infrastructure: buildings, roads, bridges, flyovers. For sustainable, inclusive development, urban trees need to be protected.

If the current trend of urban development continues, buildings and people may soon crowd out the majority of existing trees. Poor urban planning often leads to indiscriminate and sometimes illegal felling. Another key reason why cities are losing their tree cover lies in how trees are viewed: as an amenity at best, and at worst, as a resource that can be easily surrendered.

Unlike air and water, cities pay relatively little attention to how trees are treated and protected. This is evident from a steady decline in urban tree cover. Over a five-year period, average global urban tree cover decreased by nearly 40,000 hectares (100,000 acres) per year.

Yet cities like Oslo, Sydney and Vancouver continue to retain a minimum tree cover, and others like Seattle are about to successfully achieve their goal of increased tree cover. The shared understanding in these cities with a healthy, growing tree cover spans environmental, health, social, biodiversity and economic benefits. Let us examine these:

Physical infrastructure absorbs ambient heat, exuding it back slowly into the environment, further raising ambient temperatures and creating an “urban heat island effect”. Densely populated cities experience this effect more severely. Heat maps indicate that general temperatures in large cities can be at least 1-3°C higher than rural temperatures.

With trees, the area under shade, total moisture in environment, cooling of air by evapotranspiration and sun radiation deflected are all greater. This is crucial in summers when shaded surfaces can be cooler than unshaded surfaces by 11-25°C, and evapotranspiration can lower peak temperatures by 5-10°C.

Combating the heat island effect can curtail cooling demand. Space cooling on extremely hot days in United States and the Middle East can contribute to 70% of peak residential electrical demand. A surge in cooling demand burdens the electricity grid often causing power outages and a hike in energy bills.

With the ability to improve microclimate in cities, a single tree within five years of planting can bring 3% energy savings for one household, and 12% within 15 years. Multiplied by millions of households, energy savings of this magnitude will reflect in a reduction in overall energy consumption and related GHG emissions.

Trees utilize carbon dioxide from the environment for photosynthesis, to produce hydrocarbons that aid growth. Trees can also store carbon dioxide for immediate and long term growth. Depending on type and age, a single tree can store anywhere between one to 22 tonnes of CO2 over the course of its life; a lot of carbon dioxide. (For context, a typical passenger vehicle releases about 50 tons of CO2 in 10 years.) Once a tree is cut, this stored CO2 is released back into the environment, increasing the total amount of CO2 and adversely affecting air quality. Trees that capture and store CO2 contribute to negative emissions and remove two-thirds of all human-related emissions in the atmosphere.

Trees can reduce, block or buffer air, noise and water pollution that are considerably higher in cities. The World Health Organisation (WHO) states that: “The global population (99%) breathe air that exceeds WHO guideline limits and contains high levels of pollutants.” By 2060, premature deaths due to air pollution could be 6-9 million deaths a year. Further, the sick days, medical expenses, diminished agricultural output resulting from air pollution could cost 1% of global GDP or $2.6 trillion annually.

The WHO states that making cities greener through urban planning can tackle air pollution, the deadliest form of pollution. Covering at least 30% of total urban land area may prevent 400 premature deaths annually.

In addition to air pollution, urban trees scale down noise and water pollution. By acting as physical barriers, they buffer noise and filter stormwater to improve water quality in local waterways. Moreover, people living in cities with a substantial tree cover possess better health immunity. Trees boost blood circulation, oxygen levels and lessen blood pressure and anxiety; biochemicals released from trees as an aerosol mist contain antibiotic, antifungal and anti-rheumatic properties.

Citizens with easier access to green spaces or nature report better mood and higher motivation to exercise outdoors and socialize within their communities. Subsequently, this improves quality of life that in turn can attract greater business opportunities and even raise real estate prices by 3-15%.

Importantly, trees can help to address social equality and inclusion. Low-income populations often reside in parts that experience disproportionately higher levels of heat and pollution. Tree cover here can be lower by at least 30% compared to the affluent neighbourhoods. By addressing tree equity, the environmental, health and socioeconomic benefits offered by trees are made available to low-income groups, thus elevating social equality and inclusion.

A diverse tree cover protects overall biodiversity – animals, insects and natural vegetation – further supporting urban tree health. Trees also are a refuge for native, threatened or endangered wildlife and plants. For instance, more than 20% of world’s avian biodiversity resides in cities. Birds live off the insects, sap, nuts and fruits on trees and in return scatter tree seeds.

Preservation of high functional diversity can provide essential resources to city-dwellers. For example, 88% of tree species in New York City are forageable for medicine and food. Finally, well-maintained trees can minimize soil erosion during heavy rainfall, which wards off damage to the natural, built environment. Preventing soil erosion ensures that soil retains necessary minerals that augment growth of trees.

Given such benefits, it is not surprising that urban trees can help cities achieve 15 of the 17 UN SDGs. While cities and countries have undertaken initiatives to plant trees, continuous efforts are required to ensure urban trees attain maturity and full life. Cities can start by amending laws to increase minimum urban tree cover area, avoid concretization of public space allowing saplings to flourish, and relocate mature trees to open spaces instead of felling. The future of sustainable and inclusive cities will surely depend on how we tend to and protect our urban trees.