As we face increasingly high temperatures in the city, the role of street trees becomes more important as they can reduce daytime temperatures between five and 20 degrees Celsius.
Traditionally, street trees have been associated with increased residential house prices due to their aesthetic role of ornamentation and beautification.
This has changed over the past few decades as they are recognised in providing significant environmental, economic and social benefits.
Some of the trees planted more than three decades ago would possibly not be considered today. As our urban environments have become increasingly “heated” due to the amounts of cement structures/building, asphalt and pavements, the role trees play in cooling our cities is significant.
The cooling effect provided by trees is directly related to tree size, canopy cover, tree location, and planting density. As much as 80 per cent of the cooling effect of trees results directly from shading. Street trees provide energy savings through their shading and cooling effects in summer and the wind-chill protection they offer in winter.
Apart from cooling our cities, street trees reduce stormwater run-off, store carbon and increase air quality. Street trees are effective at capturing airborne pollutants including nitrogen oxides, sulphur dioxides, carbon monoxide, carbon dioxide (CO2).
It has been estimated that large healthy trees can remove between 60 times more air pollution than smaller trees. They can also enhance biodiversity by providing food, habitat and wildlife corridors or connectivity for urban fauna. Street trees provide increased privacy and noise reduction, and a study has shown how streets with more trees reduce crime.
As we ask more from our trees in the urban environment, we need to understand how these impermeable surfaces (roads, pavements, buildings) increase the stresses on street trees as their roots have to spread to areas that are more favourable for them which in turn can cause infrastructure damage and pavement lift. These surfaces also inhibit ground water recharge reducing local water storage creating stress for trees.
Care of our street trees is important. In nature the leaves fallen from a tree are left where they fell as they provide the future nutrients for the soil life and tree and trees do not stand alone, they are surrounded by an understory of supportive and companion plants.
Increased construction with resulting, cement dust and pollutants makes it hard for our trees to breathe as their leaves get coated with this fine dust, increasing their stress and receptivity to diseases.
Food security is a growing issue and maybe it’s time we consider using more productive trees as street trees, move away from worrying about fruit fall in the inner city.
With the increasing demands on our street trees, we need to select and protect the ones that will deliver on these challenges and value and care for our old established street trees so they can continue their significant role that young trees cannot fulfil. •