The story: Cities around the world are in a bad shape, and outdoor pollution kills 4.2m people a year (World Health Organisation data). Concrete and t
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Mini forests via the Miyawaki method
- The story: Cities around the world are in a bad shape, and outdoor pollution kills 4.2m people a year (World Health Organisation data). Concrete and tarmac absorb the sun’s rays rather than reflecting them back into space, and displace plants which would otherwise cool things down by evaporative transpiration. The relentless spread of buildings and roads thus turns urban areas into heat islands, discomforting residents and exacerbating dangerous heatwaves, which are in any case likely to become more frequent as the planet warms.
- Tress are the answer: 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 to the 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.
- Scale matter: But to actually cool an area effectively, trees must be planted in quantity. At the moment, 55% of people live in cities. By 2050 that share is expected to reach 68%.
- Some botanists believe they have a solution to this lack of urban vegetation. It is to plant miniature simulacra of natural forests, ecologically engineered for rapid growth.
- Miyawaki Akira, a plant ecologist at Yokohama National University, in Japan, developed a way to do this starting with even the most unpromising derelict areas. His "Miyawaki method" is finding increasing favour around the world.
- Miyawaki method: Dr Miyawaki’s insight was to deconstruct and rebuild the process of ecological succession, by which bare land develops naturally into mature forest. Usually, the first arrival is grass. Shrubs sprout later, followed by small trees and, finally, larger ones. Incipient and mature woodlands therefore contain different species. The Miyawaki method skips some of the early phases and jumps directly to planting the kinds of species found in a mature wood. When starting a Miyawaki forest, those involved, who often refer to themselves as gardeners, first analyse the soil in which it will grow. If necessary, they improve it by mixing in suitable fertilisers. These need not be expensive. Chicken manure and press mud (the solid residue left behind when sugar-cane juice is filtered) are effective and essentially free. They then select 100 or so local plant species to deploy. These are chosen by surveying the nearby area on foot instead of relying on published guidebooks, which have a habit of being out of date or even simply wrong.
- Mix important: Using a wide mix of species, not all of them trees, is important. Most plantations, having been created for commercial purposes, are monocultures. 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 does that 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 soil fungi, in ways that enable a nutrient exchange which is only now beginning to be understood.
- Process: After selecting their species, the gardeners gather seeds and plant them at random, rather than in rows. And they plant at high density. The seedlings therefore have to fight for sunlight, so only the fastest-growing survive. Trees planted in this way can shoot up as much as 14% more rapidly than normal. For three years, the gardeners water and nurture their handiwork. Then it is left to fend for itself. A couple of decades later the whole thing reaches maturity.
- Success track record: Dr Miyawaki has supervised the planting of more than 1,500 of these miniature forests, first in Japan, then in other parts of the world. India is also keen. In Mumbai, more than 2,00,000 trees are found in Miyawaki forests throughout the city and its suburbs. The authorities in Tirunelveli, in the country’s south, use the Miyawaki method to create green cover in the city’s schools. Hyderabad started growing the largest individual forest of the lot, across four hectares, in 2020. The method is becoming popular outside Asia, too. In Europe, Belgium, France and the Netherlands are all home to Miyawaki forests. There are also a handful in Latin America. Wherever they are planting, though, gardeners are not constrained to follow nature’s recipe book to the letter. Miyawaki forests can be customised to local requirements. A popular choice, for example, is to include more fruit trees than a natural forest might support, thus creating an orchard that requires no upkeep.
- Summary: The Miyawaki method will never work for large-scale reafforestation. It is too labour intensive. Relying on nature and the passage of time is probably the best bet for replanting extensive areas of damaged woodlands, though technophiles dream of speeding things up by distributing seeds by drone. But if the goal is to improve immediate locales, rather than to save the planet from global warming, go for the Miyawaki method!
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