Written by Oscar Holland, CNN

Architect Stefano Boeri has always been obsessed with trees. The Italian traces his fascination back to a novel he read as a child, "Il Barone Rampante" ("The Baron in the Trees"), in which a young boy climbs up into a world of trees and vows to never to return.

"I think trees are individuals," Boeri said in a phone interview. "Each has its own evolution, its own biography, its own shape."

Unsurprisingly, there is child-like wonder to the architect's best-known building, Il Bosco Verticale, or the Vertical Forest. Built in his home city of Milan, the celebrated complex teems with greenery, its facades transformed into living, breathing organisms.

The project's two residential towers -- measuring 80 meters (262 feet) and 112 meters (367 feet) respectively -- play host to around 20,000 trees, shrubs and plants. They spill out from irregularly placed balconies and crawl up the structures' sides. By Boeri's estimates, there are two trees, eight shrubs, and 40 plants for each human inhabitant.

Greenery is placed on balconies across the two buildings' facades. Credit: Michael Hierner

The purported benefits of this garden architecture transcend aesthetics. Greenery, supposedly, provides shade to apartments, psychological benefits to residents and a home to wildlife. (There are, Boeri said, "hundreds of birds, more than 15 different species" nesting on the towers' various floors.)

But the architect's proudest claim is that the buildings absorb 30 tons of carbon dioxide and produce 19 tons of oxygen a year, according to his research , with a volume of trees equivalent to more than 215,000 square feet of forestland.

"The ability to enlarge green surfaces inside and around our city is one of the most efficient ways to try to reverse climate change," he said. "So, a vertical forest is one of the possible ways to ... enlarge biological surfaces, in the horizontal and the vertical. (The solution is) not only gardens. Why not also the side of the building?"

Other energy-efficient features, including geothermal heating systems and wastewater facilities, have attracted less attention. Nonetheless, they help the towers to not only resemble trees, but function like them too, the architect said.

"The buildings really work as trees, from the roots up," he explained. "They have a trunk, and they distribute (water and energy) through the different branches."

Scalable urban model

In September 2018, Vertical Forest was named among four finalists for the RIBA International Prize, a biennial award honoring the world's best new buildings. Despite the plaudits, Boeri claims the project's real success is that it serves as a prototype.

The architect has far more ambitious designs. His firm has already unveiled plans for new Vertical Forest buildings in European cities including Treviso in Italy, Lausanne in Switzerland and Utrecht in the Netherlands.

In the Chinese city of Liuzhou, Guangxi province, he has masterminded an entire " Forest City ," scheduled for completion in 2020, which comprises tree-covered houses, hospitals, schools and office blocks over a sprawling 15-million-square-foot site. (Boeri said that he's also been approached about producing similar "cities" in Egypt and Mexico.)

The Vertical Forest in Milan was completed in 2014. Credit: Giovanni Nardi for Stefano Boeri Architetti

Each location requires detailed research to decipher which species of plants and trees will flourish. (Levels of humidity, wind and sunlight are the most challenging variables.) Before opening his Milan towers, Boeri spent two years observing the growth of different plants, and only used those that proved most resilient.

Having already carried out numerous studies (and proven the project's real estate worth), future ventures will invariably become more attractive to developers -- and cheaper to deliver. On this front, Boeri highlights an upcoming project in the Dutch city of Eindhoven. The 19-story, 125-unit tower will mark the first time the Vertical Forest concept is applied to social housing.

Vertical Forest is located in Boeri's home city of Milan. Credit: Giovanni Nardi for Stefano Boeri Architetti

"The challenge is to realize Vertical Forest for low-income people, not only rich people," he said. "The apartments will be rented to young couples and families (with less) money. And this is, for me, a very, very important step. We have reduced the cost of construction and used prefabrication for the facade without reducing the number of trees or plants.

"I want to demonstrate that it's possible to put together the two main issues for the future of our cities: climate change and poverty."

A green 'manifesto'

Boeri is, undoubtedly, an architect on a mission. A manifesto published on his firm's website commits to launching "a global campaign on urban forestry" that spans city farms, roof gardens, green facades and other forms of public greenery. He recently helped establish the first World Forum on Urban Forests, which is set to take place in Mantua, Italy, later this month.

Yet Boeri is neither dogmatic nor protective of his vision. "I'm proud to say we didn't put any kind of copyright," he said, "because we know well the social value of this kind of building."

It could easily be argued that designers and architects like Thomas Heatherwick and Ole Scheeren -- whose upcoming projects in Shanghai and Ho Chi Minh City , respectively, feature prominent urban forestry -- owe a debt of gratitude to Boeri's vision. But the Italian remains modest about his role in what is a burgeoning architectural movement.

Boeri claims that the volume of trees on the two towers is equivalent to more than 215,000 square feet of forestland. Credit: Stefano Boeri

"There will be architects and urban planners who realize buildings, with the same philosophy, that will be better than what we've done," he said. "It's already happening. I'm very happy to see that.

"As architects, we are used to dealing with natural materials," he added. "But living nature is something very different. We normally think that living nature is only an ornamental part of our buildings. What we have done with the Vertical Forest ... is to demonstrate that it is possible to transform living nature into a basic component of architecture."