From a piece on the French organization trying to create urban forests at high speed:

Developed by the Japanese botanist Akira Miyawaki in the 1970s, the concept is to plant tree species that are native to the area in a very dense and layered manner — three per square meter — in order to recreate the richly fertile conditions of the natural primitive forests that once covered the planet. [...] 

Proponents claim that this method can produce a self-sufficient forest in just three years and that these forests grow faster, are denser and contain greater biodiversity than conventional forests. A Miyawaki forest in Japan, according to his own research, can grow one meter a year and can reach maturity in 15 to 20 years — 10 times faster than the average. What’s more, they can in theory be cultivated in all kinds of unconventional locations: roundabouts, factories, schoolyards, or indeed, ring roads. 

Previously on the blog: Tree aesthetics