Choosing trees to cool urban areas in hot weather

The discomfort associated with hot, dry summers in some urban areas can be reduced by planting the most appropriate trees in open city spaces, according to a recent study. The researchers identified the Indian laurel fig as the most effective tree for this purpose in the Greek city of Chania. As populations continue to grow and urban areas expand, open spaces in cities can help improve the health and well-being of citizens. In addition to other benefits, such as reducing pollution levels, green spaces help cool and reduce discomfort in urban environments in hot, dry weather by improving the micro-climate around them. Trees planted in open spaces reduce urban temperatures in two ways: through shading and via 'evapotranspiration'. Evapotranspiration is the loss of water from soil and plants to the atmosphere, which cools the surrounding air by using heat from the air to evaporate water. This study determined the most appropriate choice of trees to plant in open spaces in the city of Chania, Crete, by focusing on the role that evapotranspiration plays in cooling the surrounding microclimate. Chania has a temperate Mediterranean climate with a yearly mean temperature of 18.9�C and about 5-6 months of hot, dry weather. The researchers measured the temperature and relative humidity for shaded and sunlit pavements and under the following five species of trees already growing in Chania: the Indian laurel fig (Ficus retusa ssp. nitida), stone pine (Pinus pinea), Canary Island date palm (Phoenix canariensis), bitter orange (Citrus aurantium), and the olive tree (Olea europaea ssp.europaea). These measurements were used to calculate evapotranspiration rates in millimetres per day and the Discomfort Index, an indicator of the feeling of discomfort caused by heat. Areas planted with trees were, on average, 3.1�C cooler than sunlit pavements. Plants with a high level of evapotranspiration were the most effective in making the micro-environment under the trees more pleasant and reducing the discomfort caused by hot, dry summers. Results from the study suggest that Indian laurel fig trees were the most effective in reducing discomfort, followed by stone pine, Canary Island date palm, bitter orange and olive trees. This was due to Indian laurel fig trees having the highest relative humidity, the highest rate of evapotranspiration and the lowest temperature under the tree canopies compared with the other trees. The high relative humidity under the fig trees contributed to a more pleasant micro-climate via higher evapotranspiration rates (6.9mm per day), which lowered temperatures, in addition to increasing the humidity of the dry summer atmosphere. Trees are a cost-effective means of lowering temperatures in hot urban spaces compared with the installation of sun awnings and outdoor cooling fans. The researchers suggest this study could be used help plan outdoor environments in other cities with similar characteristics.