During a hot day, what is the first thing one looks for to escape the sun? The shade of trees. However, in many communities in southwestern Puerto Rico, that shade is scarce.
A study conducted by the U-TREE PR project revealed temperature differences of up to 12.3 degrees Fahrenheit between nearby communities in the area, which shows how the lack of green spaces and the predominance of cement intensify heat in urban areas and, at the same time, affect vulnerable populations.
Through a monitoring carried out on July 23, 32 community data collectors traveled 11 routes between Mayagüez, San Germán, Hormigueros, Cabo Rojo, Lajas and Sabana Grande. Using sensors installed in vehicles, urban areas and green spaces were compared at three times of day – 6:00 a.m., 3:00 p.m., and 7:00 p.m. – to capture temperature variations.
The analysis revealed how, through evapotranspiration – the process by which water is transferred from the land to the atmosphere – areas with trees, forests, and higher elevation tend to stay cooler.
In urban areas, in contrast, the ground covered by cement, concrete, tar, and asphalt absorbs more radiation and releases it for longer. The report identified developed areas and urban centers such as Mayagüez and San Germán among the warmest places during the day.
“The southwest, according to recent census data, is one of Puerto Rico's regions with the highest proportion of older adults, which makes it a highly vulnerable area, along with children and people with health conditions. Added to this is the low-income population, which has fewer resources to cope with the heat,” explained Yasiel Figueroa Sánchez, manager of U-TREE PR, a project of the organization Caribbean Regenerative Community Development (CRCD, in English).
"Highest priority for mitigation"
The research findings showed that heat does not behave the same in the morning, afternoon, and night. In the morning, patterns of heat accumulated from the previous day were reflected; in the afternoon, the sun more intensely heats surfaces such as asphalt and concrete; and at night, some urban areas take longer to cool down.
In the afternoon, significant territorial contrasts were observed. While Puerto Real, for example, appeared as a relatively cool spot, Boquerón was identified as a "hot spot." Additionally, forested and higher elevation areas around Hormigueros, El Rosario, and Miradero were identified as cooler zones.
In Mayagüez, particularly in the area near the coast, where there is a high concentration of public housing projects, some of the highest temperatures were recorded. Likewise, in urban areas of the municipality, with a high concentration of low-income population, a significant increase in temperatures was also observed.
“That means these are the highest priority places to mitigate heat using green infrastructure or trees, because these are the areas where there is a combination of high temperatures and low tree cover. That's the key there: how we define our 'hotspots' and priority areas,” said Figueroa Sánchez, in a phone interview with El Nuevo Día.
Reforestation as a solution
As part of the solutions to mitigate extreme heat, U-TREE PR will be providing personalized advice to municipalities to develop urban vegetation management plans. The initiative also seeks to support local governments in becoming “Tree Cities,” an international recognition that distinguishes cities committed to the responsible management of their urban trees.
In addition, CRCD will continue its efforts to plant 20,000 native trees by 2029.
The data collected from the study were analyzed and used to create a high-resolution thermal map of the southwestern region. The map allows visualizing how temperatures vary between neighborhoods and municipalities, and which areas present the most critical conditions for designing mitigation strategies, including targeted tree planting. The location of these trees will be determined, in part, by the results of the thermal map.
In the future, Figueroa Sánchez mentioned that they will propose developing "cooling centers" – establishments that will have air conditioning, cool zones, where people can seek refuge during periods of extreme heat.
“Extreme heat is a very broad issue, it needs a multidisciplinary approach, but, from our corner, we are betting on trees and green infrastructure as the first action to be taken because it is the one that takes the longest to yield benefits. So, for that very reason, it should become the most urgent,” he concluded.