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In recent years, small patches of dense woodland, some no bigger than a tennis court, have begun to appear in parks, school grounds and even beside motorways in cities across Europe and Asia. Known as tiny forests or micro-forests, they are usually planted using a method developed in the 1970s by the Japanese botanist Akira Miyawaki.

The method differs sharply from conventional tree planting. Instead of spacing young trees several metres apart, volunteers plant three to five saplings in every square metre, using a wide mix of native species: trees and shrubs that grow naturally in the area. The soil is first improved with compost and other organic material, and the ground is then covered with a thick layer of straw to keep in moisture. Because the young trees compete for light, they grow upwards quickly, and after two or three years the forest is expected to look after itself without further watering or weeding.

Supporters make impressive claims for the approach. They say that the forests grow far faster than ordinary plantations, attract a wide variety of insects and birds, and help to cool the streets around them in hot weather. For many communities, the social benefits matter just as much: planting days bring together residents who might otherwise never meet, and schools use the sites as outdoor classrooms.

Not all scientists are convinced. Some point out that many of the claims about growth rates come from the organisations that promote the method, rather than from independent research. Others note that planting so many trees so close together means that a large proportion of them die within the first few years, which raises the cost of each tree that survives. There is also concern that a single small forest, however dense, cannot replace the benefits of large areas of mature woodland.

Researchers who have studied the sites generally agree on one point: the success of a tiny forest depends heavily on the people who look after it in its early years. Where volunteers continue to water and weed during dry periods, most sites do well; where interest fades after the planting day, the results are far less impressive. For this reason, some programmes now offer training and small grants to local groups who agree to care for a site for at least three years.