- A new study finds that simply increasing tree cover does not always make cities cooler. In humid urban areas, dense tree canopies can even raise the Heat Index by trapping moisture.
- The cooling benefits of urban trees depend on a combination of canopy structure, photosynthetic activity, local climate, urban density and airflow, rather than tree cover alone.
- Researchers argue that urban greening must be climate-smart, prioritising the right tree species, the right locations and thoughtful planting design over simply planting more trees.
Urban greening is widely promoted as a solution to rising temperatures. But can planting more trees actually make people feel hotter in some cities? A new study published in Nature Communications by researchers from IIT Gandhinagar and Northeastern University, USA examines this paradox.
Instead of looking only at air temperature or land surface temperature, the study focused on the Heat Index (HI) which is the “feels like” temperature that combines heat and humidity because humans respond to both temperature and moisture in the air.
The study finds that while urban trees generally help cool cities by providing shade, reducing incoming solar radiation, and lowering surface temperatures, these benefits vary in different cities. In humid cities, dense tree canopies can inadvertently increase heat stress by boosting evapotranspiration, which releases more moisture into the air. Where buildings are closely packed and airflow is restricted, this extra humidity can raise the Heat Index even if the air itself becomes cooler. On the other hand, semi-arid cities experience more consistent cooling because the drier atmosphere can absorb additional moisture without becoming uncomfortably humid.
The study’s first author, Angana Borah, says the same tree can produce very different thermal comfort outcomes depending on where it is planted, the surrounding urban density and the local climate. As cities warm faster than their rural surroundings, climate-responsive greening is important, the study notes.
Canopy structure plays a role
The study analysed three different aspects of vegetation — EVI (Enhanced Vegetation Index), LAI (Leaf Area Index) and fPAR (Fraction of Absorbed Photosynthetically Active Radiation) — in 138 Indian cities with diverse climates — tropical savanna, tropical monsoon, humid subtropical, hot semi-arid, and hot desert — from 2003 to 2020.
Leaf Area Index (LAI) is a measure of how dense a canopy is, based on total leaf area while Enhanced Vegetation Index (EVI) represents a satellite measure of how green an area is and Fraction of photosynthetically active radiation (fPAR) is a measure of how actively the vegetation is absorbing sunlight for photosynthesis, which is linked to evapotranspiration, indicating tree health.
Using explainable artificial intelligence (AI) models, the researchers identified the conditions under which vegetation influences human heat exposure. The study found that the structure of the tree canopy, including its leaf density, spatial arrangement and shading pattern, plays a greater role in determining urban heat than simply increasing the number of trees.
One of the authors of the study, Udit Bhatia an associate professor of civil and computer engineering at IIT Gandhinagar shares that the research examines three key aspects of urban vegetation and identifies the thresholds (EVI ∼ 0.4; LAI ∼ 0.05 – 0.1 and fPAR ∼ 0.25 – 0.35 ) at which they influence heat stress. “The study of photosynthetic activity explains why specific urban cooling succeeds in one city but becomes counterproductive in another city,” he says, adding: “In some regions, dense canopies can restrict ventilation, while higher photosynthetic activity can lead to increased moisture through transpiration. This extra moisture can create a localised greenhouse effect due to trapped humidity, thus reducing the cooling effect.”
Mohammad Rahman, a senior lecturer of urban horticulture at University of Melbourne, who is not part of the study, says the findings are significant but not entirely surprising from an urban climatology perspective. “Trees generally provide substantial cooling benefits. However, the magnitude and even the direction of their effects can vary depending on the local climate and the density of the cities.”
Matching right species to the right climate
In hot, semi-arid cities such as Gandhinagar, Jaipur, Ahmedabad and Coimbatore, where humidity remains relatively low through much of the peak summer, dense tree canopies combined with high photosynthetic activity and greater greenness consistently improved cooling, as per the study. In tropical savanna cities such as Bengaluru and Chennai, on the other hand, dense canopies and greater surface greenness helped lower heat but trees with higher photosynthetic activity released more moisture into the air. This increased humidity and, in turn, raised the Heat Index, making conditions feel hotter despite lower air temperatures. “So, we need localised studies to determine how a tree would provide thermal comfort in one city versus another,” Borah says.
“Gandhinagar has a higher threshold of photosynthetic activity, almost two times more than that of Bengaluru. Native trees such as Arjuna (Terminalia arjuna) and neem (Azadirachta indica), and not ornamental trees, meet our criteria for Gandhinagar,” says Bhatia.
Looking beyond tree cover
The findings suggest that urban greening strategies need to move beyond simply increasing tree cover. Instead, urban planners should consider the choice of tree species, how and where they are planted, the need to maintain airflow around tree canopies, and the city’s climate when designing effective and equitable heat adaptation measures.
Bhatia says the study is a step towards understanding how these insights can be translated into local climate adaptation strategies for urban heat. Rahman adds, “In humid tropical and subtropical cities, urban planners may need to balance the shading benefits of trees with the need to maintain adequate open spaces for ventilation.”
Rahman, however, cautions that while satellite observations are valuable for identifying broad patterns across cities, local planning should also be informed by street-level assessments that capture the thermal conditions people experience.
Experts, however, point to the study’s limited consideration of local wind patterns. Raghu Murtugudde, Emeritus Professor at the University of Maryland and an earth systems scientist, who was not involved in the research, says wind should be an important factor in urban tree planning. “Winds can have a strong effect on water loss through evaporation and, consequently, on cooling. That’s where hyperlocal differences matter,” he says.
According to Murtugudde, urban landscapes with tall buildings, narrow streets and dense clusters of trees create complex airflow patterns that can influence the Heat Index and, ultimately, thermal comfort. “This is a nice big picture, but actual planning will need to consider such street-level parameters,” he adds.
Bhatia agrees that understanding how different elements of the urban environment, including trees, buildings, pavements and wind patterns, interact to shape local thermal conditions is the next logical step. “Using thermodynamic modelling, we can map how specific tree species interact with local airflow and humidity in confined urban spaces,” Bhatia says.
As cities get denser, cooling neighbourhoods will require more than simply adding trees. Effective urban planning will depend on precise, location-specific data to determine which tree species are best suited to a neighbourhood, how dense the vegetation should be, and how the canopy is likely to develop over time.
Trees provide a range of ecological benefits beyond cooling, from improving air quality and supporting biodiversity to storing carbon. The question, therefore, is not whether cities should plant millions of trees, but which trees they should plant and where. “Let us not confuse mere greening with better cooling. The right tree species has to be prioritised for a given climatic region to get maximum cooling benefits. One size does not fit all,” Bhatia says.
Banner image: People rest under the shade of trees in New Delhi as the city is gripped by a heat wave in June 2024. (AP Photo/Manish Swarup)
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