Suburban tree in a car park. Trees in urban areas help to reduce the urban heat island effect, where urban areas heat up as concrete structures absorb heat. Trees and other vegetation cool the air as water evaporates from plants as part of a process called transpiration. Trees also provide shade to cool the ground in their shadow, and also bind together the soil they grow in, preventing it from eroding. The soil will absorb water, helping to reduce the flooding caused by water run-off from paved urban areas. Trees also help trap pollutants such as dust and smoke particles. Photographed in Milwaukee, Wisconsin, USA.