City air is in a sorry state. It is dirty and hot. Outdoor pollution kills4.2m people a year, according to the World Health Organization. The relentless spread of buildings and roads turns urban areas into heat islands,discomforting residents and _exacerbating_ dangerous heatwaves, which are inany case likely to become more frequent as the planet warms. A possible answer to the twin problems of pollution and heat is trees. Their leaves may destroy at least some chemical pollutants (the question is debated)and they certainly trap airborne particulate matter, which is then washed tothe ground by rain. And trees cool things down. Besides transpiration, they provide shade. Their leaves have, after all, evolved to intercept sunlight,the motor of photosynthesis. To cool an area effectively, though, trees must be planted in quantity. In2019 researchers at the University of Wisconsin found that American citiesneed 40% tree coverage to cut urban heat back meaningfully. Unfortunately, notall cities are blessed with parks, private gardens or even ornamental street trees in sufficient numbers. And the problem is likely to get worse. One group of botanists believe they have at least a partial solution to this lack of urban vegetation. It is to plant miniature forests, ecologically engineered for rapid growth. Over the course of a career that began in the1950s their leader, Miyawaki Akira, a plant ecologist at Yokohama National University, in Japan, has developed a way to do this starting with even themost unpromising abandoned areas. And the Miyawaki method is finding increasing favor around the world. Dr. Miyawaki's insight was to deconstruct and rebuild the process of ecological succession, by which bare land develops naturally into matureforest. Usually, the first arrival is grass. Shrubs sprout later, followed by small trees and, finally, larger ones. The woodlands therefore contain different species. The Miyawaki method skips some of the early phases andjumps directly to planting the kinds of species found in a mature wood. Using a wide mix of species, not all of them trees, is important. But trees,shrubs and ground-covering herbs all coexist in natural forests, and the Miyawaki versions therefore have this variety from the start. Not only doesthat pack more greenery into a given space, it also encourages the plants to grow faster-for there are lots of positive ecological relations in a natural forest. Vines rely on trees for support. Trees give shade to shrubs. And,beneath the surface, plants' roots interact with each other, and with soilfungi, in ways that enable a nutrient exchange which is only now beginning to be understood. 

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