Asphalt and concrete are poor materials for both scenarios: they retain heat but not rain so lead to higher temperatures and flooded streets. Conversely trees and greenery help with both these challenges; they make our streets more liveable through providing shade and releasing water vapour (evapotranspiration), which cools the air, roads and buildings. On top of this tree canopies can reduce surface water runoff by as much as 80% compared to asphalt, which lowers the risk of surface flooding.
Trees also fight climate change by absorbing carbon dioxide (CO2) from the air and converting it into wood and oxygen in a process known as carbon sequestration. Here are a few of the many other additional benefits of greenery for our physical and mental health:
- Studies have found that having plants, hedges and trees in streets can reduce levels of nitrogen dioxide by 40% and particulate matter by 60%.
- Researchers found asthma rates among children aged four and five fell by a quarter for every additional 343 trees per square kilometre.
- Parks and green spaces assist in improving child development, including better dexterity and coordination, and the opportunity to build social connections and relationships.
- Emerging evidence indicates that engaging with nature benefits those living with ADHD, depression and dementia by improving cognitive functioning and reducing stress.
- Property prices are elevated by proximity to green infrastructure: for example homes adjacent to parks are worth 20% more than similar homes elsewhere.
- Active travel is improved by creating safe green corridors that encourage people to walk or cycle to work.
- Evidence shows that people who live close to and use green spaces tend to exercise more, and have a reduced risk of type II diabetes, cardiovascular disease, obesity, premature death, and preterm birth.
- Wildlife such as birds, bugs and bats need greenery to survive and these creatures provide incalculable benefits to human health and wellbeing e.g. pollinating our crops, providing birdsong and beauty.
The Netherlands: plant a tiny forest
In the Netherlands, a conservation group IVN Nature Education has helped cities and households to plant 100 miniature forests since 2015.
A mini-forest is made by planting saplings close together, three per square metre, using native varieties adapted to local conditions. A wide variety of species – ideally 30 or more – are planted to recreate the layers of a natural forest. These forests can be as small as a tennis court so are ideal for congested cities with little space, and are often sited in schoolyards or next to roads.
They are based on the work of the Japanese botanist Akira Miyawaki, who has planted more than 1,000 such forests in Japan, Malaysia and elsewhere. Advocates for the method say these forests grow 10 times faster and become 30 times denser and 100 times more biodiverse than those planted by conventional methods.
Scientists say such ecosystems are key to meeting climate goals, estimating that natural forests can store 40 times more carbon than single-species plantations. The Miyawaki forests are designed to regenerate land in far less time than the 70-plus years it takes a forest to recover on its own.
In 2017, researchers at Wageningen University in the Netherlands monitored newly planted mini-forests and concluded that they “increase the biodiversity compared to the nearby forest. Both the number of species groups and the number of individuals is generally higher than in the reference forests.”
The higher biodiversity is due partly to the forests’ young age and openness, explained Fabrice Ottburg, an animal ecologist who led the Wageningen study. This allows more sunlight to reach flowering plants that attract pollinators. Diversity is also boosted by planting multiple species, which “provide more variety in food and shelter for a higher diversity of animals like insects, snails, butterflies, amphibians, bugs, grasshoppers."