Trees are vital for cooler cities, but they need help too

As the heat of a recent summer lingered, the crunch of dry leaves and the yellow or bare crowns of trees offered an all-too-familiar scene. These large, leafy trees are crucial for making our cities livable in many ways - blocking sunlight, reducing pollution, storing carbon, providing habitats, absorbing stormwater and even improving mental and physical health.

However, as climate change brings hotter conditions, the role of these trees becomes increasingly vital. In cities, trees act as natural air conditioners, with their canopies being up to three degrees cooler than the surroundings. This is due to evapotranspiration, where trees absorb water and release it as vapor. The shade they provide also helps prevent the surrounding area from heating up.

Studies have shown that cities with more tree canopy have lower rates of cardiovascular disease, better school grades and healthier outcomes for newborns. Despite the importance of trees, warmer climates make it harder for them to survive. Mature trees require up to four times the water of younger ones, and it can take decades for trees to reach full maturity and reap the maximum benefits.

Experts emphasize the importance of protecting older trees and selecting smarter tree species for the future. Rather than focusing on planting new trees, efforts should be made to conserve and maintain existing ones. Proper watering techniques, such as deep soaking and using irrigation bags or cushions, can help trees access the water they need.

When choosing new trees, biodiversity and climate adaptation should be prioritized over aesthetics. It is recommended that urban forest populations include no more than 10% of any one species, 20% of a single genus or 30% of any family. The CITREE database, which categorizes nearly 400 species of woody plants, can be a valuable resource for selecting drought-tolerant trees suitable for a city's specific conditions.