Key takeaways
- Raising tree cover to 30 percent could have prevented about a third of the premature deaths tied to urban heat across 93 European cities in the summer of 2015.
- Of more than 2.5 million buildings studied across eight major cities, most sat in neighborhoods below 30 percent tree canopy.
- Scientists writing in PLOS Climate in 2026 argued that cities should fund trees as critical infrastructure, alongside education, security, and transportation.
Rio de Janeiro's renowned forest has its origins in a water conservation effort. By the mid-1800s, the Tijuca hills had been largely deforested for sugarcane and coffee farming. In 1861, Major Manuel Gomes Archer was tasked by the imperial government with reforesting the area. Over the span of 13 years, more than 100,000 seedlings were planted under Archer's supervision. Today, this reforested area spans 3,953 hectares within Tijuca National Park.
Urban forests like this one provide essential services that benefit public infrastructure. They cool city streets through shade and the release of water vapor, allow stormwater to seep into the ground rather than accumulate on pavement, reduce air pollution, and offer residents parks that enhance mental well-being. In Tampa, for example, the city’s tree canopy has been quantified to prevent 7.48 million cubic feet of stormwater runoff and eliminate 1,004 tons of air pollutants. Researchers highlighted the importance of trees in a July 2026 study published in the journal PLOS Climate, emphasizing that cities should treat trees as vital infrastructure, comparable to education or transportation. "It has to be elevated to that level," Manuel Esperon-Rodriguez, an ecologist from Bangor University and the lead author of the paper, told Grist.
The rising heat in urban environments has introduced another compelling reason for cities to expand their forest coverage. A study in The Lancet from 2023, led by the Barcelona Institute for Global Health (ISGlobal), calculated that increasing tree cover to 30 percent in 93 European cities could have prevented roughly one-third of premature deaths linked to urban heat islands during the hot summer of 2015. At that time, these cities had an average tree cover of about 14.9 percent, and researchers attributed approximately 6,700 summer deaths to the extreme heat. According to Tamara Iungman, the lead researcher at ISGlobal, "Predictions based on current emissions reveal that heat-related illness and death will become a bigger burden to our health services over the next decades."
However, the distribution of these benefits isn’t uniform across cities. Cecil Konijnendijk, a Dutch urban forester and professor at the University of British Columbia, introduced the 3-30-300 rule in 2021. He suggested that every resident should see three trees from home, live in a neighborhood with at least 30 percent tree canopy, and have access to a public green space within 300 meters. Researchers from RMIT University tested this rule against more than 2.5 million buildings in eight cities and found that only Seattle and Singapore met the 30 percent canopy criterion. Thami Croeser, the lead researcher of the RMIT study, noted that "Previous research has shown depression, anxiety, obesity and heatstroke are more prevalent in urban areas that lack access to shady tree canopy and green open spaces."
At MIT's Senseable City Lab, researchers assess greenery from the sidewalk level, employing Google Street View to create a Green View Index for cities. Carlo Ratti, the lab's director, explained to The Associated Press, "Streets are important because that's where we spend most of our time."
Determining which cities boast the best urban forests can vary depending on how city boundaries are defined. Typically, they’re determined by four main factors: the size of continuous forest within or at the boundaries of the city, the percentage of land under canopy or legal protection, street-level greenery (as assessed by MIT's Treepedia), and the accessibility of the forest to residents.
