What if a city had no trees?

Look down a city street. You might see trees along the sidewalk and in the park.

What if all of those trees were gone? Would the city feel different?

Most cities have trees: along streets, in parks, in schoolyards and backyards. It is easy to think of them as decoration. But trees quietly do several jobs for a city every day.

This lab is a thought experiment. Imagine a city where every tree has been taken away. What would change on a hot summer day, during a rainstorm, and for the birds and people who live there? Choose a path to find out.

Urban trees provide what ecologists call ecosystem services: cooling, stormwater interception, air filtering, energy savings and wildlife habitat. These are measured in cities today, so we can reason about a treeless city from real data.

This thought experiment removes the whole urban tree canopy and asks which services disappear, how big each one is according to the EPA's summaries of research, and what cities do to keep or grow their canopy.

On a hot day, trees make cool shade. You can rest under them.

When it rains, tree leaves catch some of the raindrops. Birds and squirrels live in the branches.

Trees cool a city in two ways. Their leaves shade the ground and buildings. They also drink water through their roots and let it evaporate from their leaves, which takes heat from the air. This is called evapotranspiration.

Trees catch rain on their leaves and soak it up, so less water rushes off roofs and streets. They trap some pollution on their leaves, and they give homes and food to insects, birds, bats and squirrels.

The EPA describes two cooling mechanisms: shade, and evapotranspiration, in which trees draw water through their roots and evaporate it from their leaves, using heat from the air. One review of 308 studies found urban forests were on average 3.0 °F (1.6 °C) cooler than non-green urban areas.

Urban trees can absorb 15 to 27 percent of annual rainfall, reducing runoff; they filter fertilizers, pesticides and other pollutants; pollutants such as particulates, nitrogen oxides and ozone deposit on their leaves; and shading buildings cuts air-conditioning demand.

Now imagine Olly's city with no trees. There is no shade on the sidewalk. The streets feel hotter.

When it rains, more water rushes along the street to the drains. The birds have fewer places to rest.

Without trees, the Sun would beat down on roads, roofs and parking lots. These hard surfaces soak up heat and give it off slowly, even after sunset. A city already tends to be warmer than the countryside around it, and losing its trees would remove one of its best ways to cool down.

Rain would land on hard surfaces instead of leaves and soil, so more of it would run into storm drains and streams, carrying pollution with it. Birds, bats and squirrels would lose their homes.

Buildings, roads and other infrastructure absorb and re-emit more of the Sun's heat than forests or water. In the United States, heat islands raise urban daytime temperatures about 1–7 °F and nighttime temperatures about 2–5 °F above outlying areas, and conventional roofing can reach 66 °F above the surrounding air on warm days.

Removing the canopy would remove shade and evapotranspiration, and with them the cooler pockets that urban forests create; the rainfall trees intercept would become runoff, and the habitat they provide would vanish. Exact numbers for a whole city without trees are not something the sources give.

Trees work like a big umbrella and a sponge.

They block the Sun, cool the air and soak up rain. Without them, the city gets hotter and wetter streets.

Dark, hard surfaces like asphalt and many roofs reflect less sunlight and store more heat than plants and soil. Trees are different: they shade those surfaces and use the Sun's energy to evaporate water, which cools the air around them.

Trees also slow down rain. Leaves catch drops, and trees soak up some of the water, so less rushes off the pavement. Each of these jobs is small for one tree, but a whole city's trees add up.

Conventional paving and roofing reflect less solar energy and absorb and emit more heat than vegetation, and dense urban geometry traps heat and blocks cooling winds. Heat islands often become more pronounced after sunset as materials slowly release stored heat.

Trees counter this with shade, latent cooling and interception. The EPA's figures are averages from research and vary with climate, species and placement: in a desert city, for example, trees must be species suited to hot, dry conditions, and placement matters for how much a building is cooled.

Many cities plant new trees every year. Neighbors and kids can help!

Young trees need water and care, so people look after them as they grow.

Cities and neighbors plant trees to cool streets and catch rain. The EPA advises choosing trees that suit the local climate, planting them in the right place (trees on the west side of a building help cool it most), and caring for them while they are young.

Native species usually need less care. More tree cover is also linked to fewer heat illnesses such as heat stroke, so planting trees helps people as well as birds.

EPA guidance for community tree planting stresses species suited to the local climate, careful placement (deciduous trees or vines to the west are most effective for cooling a building), early maintenance that declines over time, native species to lower costs, and avoiding utilities and sidewalks.

The EPA links greater tree cover to fewer heat illnesses and fewer heat-related deaths in cities, and notes that green infrastructure can cut nearby buildings' energy demand by about 10 percent. These are reasons many cities run canopy programs today.