City's trees in the long heatwave

In a heatwave, the city’s trees become visible in a way they are not at other times. The rest of the city can retreat indoors, behind glass and air conditioning, but the trees have to stand where they were planted and take it. After three or four days above thirty-five degrees, you start to notice which ones are coping and which ones are not.

The first thing you notice is the shade. On a street with mature trees, the pavement under the canopy is several degrees cooler than the open pavement a few metres away, and the difference is not theoretical: you can feel it as you walk, the way you feel a doorway. On a street without trees, the whole block is one temperature, radiating heat from every wall and every tile. In a heatwave, the street tree is the only piece of public infrastructure that actually lowers the temperature outside. Everything else only moves you away from it.

The trees themselves, though, are working. The leaves that looked casual in spring are now doing something deliberate: transpiring, pulling water up from the roots and letting it evaporate from the surface, which is the mechanism by which a tree cools its own leaves and the air around them. It is the reason a shaded square is cooler than a shaded car park. The tree is air-conditioning the street, for free, on water that the city pipes in. In a long dry spell, the water runs out, and then you see the cost: leaves curl at the edges, turn brown at the tips, and in the worst cases drop early, a month or more before autumn.

Some trees handle this better than others. The ones planted in a narrow pit cut into the pavement, with no room for roots and no access to groundwater, suffer first. The ones with a good strip of soil, or a mature park around them, shrug it off. The choice of species matters too: a tree that evolved in a wet climate will let its leaves burn while a tougher, more drought-tolerant one sits there green. Walking the same route every day, you start to read the city like a gardener reads a bed, noting which species are asking for water and which are fine.

What the heatwave reveals is how much of the city’s softness is borrowed from these plants. Without them, the summer street would be a corridor of reflected heat, the kind that makes even a short walk feel like an errand. With them, the same street is a place you can linger on, sit on a step on, walk slowly through. The difference is not aesthetic. It is thermal, and it decides who can afford to be outside in the middle of the day: the elderly, the delivery rider, the parent with a small child.

The city, for its part, mostly treats its trees as decoration. They are planted in small pits, pruned when they interfere with wires, and watered, if at all, on an irrigation schedule written in an office. A heatwave is the annual moment when that under-investment becomes visible, when the leaves on the most neglected street go brown while the ones on the better-tended street stay green. The argument for more, and better, street trees is usually made in the language of beauty. It is better made in the language of temperature.

By the end of a long July, the survivors will have dropped a few leaves early and leaned a little closer to the kerb, but they will still be there, holding the shade. The city, which in winter thinks of them as objects to be maintained, will in summer remember that they are doing work. When the heat finally breaks, everyone will move back indoors, and the trees will go quiet again, waiting for next year.