Frederick Olmsted, creator of New York's Central Park, called it 'the lungs of the city'. He was right. Green spaces aren't just pleasant retreats in the concrete jungle, but living organs vital to urban health.

A city's 'green infrastructure' filters pollutants from water before it gets into rivers, reduces the amount of water washed out by storms, oxygenates the air, cuts energy use by shading buildings, and absorbs carbon dioxide and other gases. Urban forests lower the temperature of the city, which is normally higher than in the surrounding countryside. And, of course, they provide recreational space and wildlife habitat.

Beijing, China
As it prepares to host the 2008 Olympics, Beijing has planted over 800,000 trees in its 680-hectare Olympic Forest Park, in the north of the city. Designed to evoke traditional Chinese 'mountain-water' art, it will include an enormous artificial mountain and a 122-hectare Olympic lake, as well as carefully chosen native plants and animals. After serving as a venue for tennis and other sports, the Park will provide a quiet refuge and clean air for the congested city.

Barcelona, Spain
The best view of Barcelona is from the Parc de Collserola, an 8,000-hectare oasis of pine, oak and bubbling springs in the mountains bordering the city. The city established it in 1987, since when Barcelona's parks and gardens have doubled, while its streets now also boast 150,000 flourishing trees.

Rio de Janeiro, Brazil
The 3,300-hectare Floresta da Tijuca is the world's largest urban forest, providing habitat for many rare and endemic species. But by 1844 it had been so degraded by coffee and sugarcane cultivation that the city's water supplies were threatened. Within 12 years, Manuel Gomes Archer, the forest administrator, had replanted 72,000 native Atlantic rainforest trees, restoring it almost single-handedly.

Nairobi, Kenya
Only 2 per cent of Kenya is still forested, so the 600-hectare Ngong Road Forest Sanctuary in Nairobi is a particularly precious resource. While providing a carbon sink and stabilizing the water table, it hosts 190 species of birds and more than 300 species of plants. Its many insects and arachnids are still being catalogued.

Louisville, Kentucky
The Jefferson Memorial Forest, 25 kilometres from the city centre, is - at 2,400 hectares - the largest city forest in the United States of America, and was originally established in 1946 as a tribute to Kentuckians killed in the Second World War. Hikers, campers and picnickers enjoy its oaks, ferns and wildflowers, as well as its many birds, including great blue herons and horned owls.