Excerpted from Mini-Forest Revolution.

THE HILLTOP VIEW from the Sacré-Coeur Basilica is iconic. From there, Paris is a wide, ivory-gray mosaic where you might spot a juggler on a unicycle in the foreground if you’re lucky. The street view is one of dazzling relics: a Roman arena, medieval houses, a gilded opera house famed for its phantom, the burned and partially renovated Gothic cathedral home to a fabled hunchback, labyrinths of street cafés and kebab stands, 37 bridges over the curving Seine River, and a few trees older than the United States. At the ripe old age of 8,000 years, Paris is not only the seat of the world’s seventh largest economy, but a living archaeological site.

When IT specialist Enrico Fusto moved to Paris in 2015 from his native Italy, only one thing bothered him about the elegant city he now called home. “What surprised me,” he told me in an email, “was the rarity of trees both along the grand boulevards and smaller streets, and the lack of conservation areas within the city where nature is left to develop on its own.” He missed the mere presence of nature and its soothing effects amid the hubbub of human activity.

Others agree, including municipal arborist Arthur Massart of Paris’s Department of Green Space and Environment, whose love for nature and lifelong dedication to trees was the subject of a 2008 article he forwarded to me, along with other documents on the importance of urban vegetation. “Paris is one of Europe’s densest cities, and it’s very paved,” Massart explained to me by phone. “What’s needed are more areas that create a natural cooling effect and boost resilience to increasingly regular summer heat waves. If we cannot create a habitable environment in this city, then we have a big problem.”

Culturally and materially rich, Paris attracts people: In addition to its 2.2 million residents, millions of tourists visit the city every year. Yet the City of Lights is not immune to the discomforts and dangers of the breakdown of the global climate system, and it is already suffering increasingly frequent and intense heat waves. In August of 2003, the temperature in Paris hovered between 30°C (86°F) and 40°C (104°F) for more than two weeks straight, killing 1,100 people, and in July 2019, the city hit a new heat record of 42.6°C (108.7°F).

“In Paris, historically a very paved city, the heat can be suffocating during a heat wave in the areas without greenspace, which are therefore poorly equipped to defend against the heat island effect,” remarked Christophe Najdovski, Paris’s director of public green space, biodiversity, and animal welfare. Najdovski responded by email to several questions I asked him about the city’s tree-planting, biodiversity, and climate change adaptation initiatives.

Over the course of the twenty-first century, Paris’s average summer temperature is expected to rise as much as 5.3°C (9.5°F) (or as little as 1°C [1.8°F]) and the number of days per year with temperatures higher than 30°C (86°F) could increase to 45 days from the current average of ten days. Rising temperatures will come with more frequent and extreme storms, flooding, and drought. A 30 percent reduction in the flow of the Seine River is expected by 2080; along with the Marne River southeast of Paris, the Seine provides nearly half of the city’s drinking water. Ironically, the river also poses increasing flood risks as climate change increases the likelihood of extreme precipitation events.

“The cooling power of every 100 liters of water transpired by a tree equates to the daily output of two central air-conditioning units for an average home.”

Paris is not alone. In 2020, for example, Montreal hit an all-time high for the month of May at 37°C (98°F), and Los Angeles County broke its all-time high temperature record, reaching 49°C (121°F) in September. On top of the 1.1°C (1.98°F) rise in the average global temperature since the 1800s, which has intensified heatwaves, cities are further burdened by the urban heat island effect that Najdovski mentioned. This effect is generated by the concentration of impervious surfaces, as well as heat-generating machines such as air conditioners and cars, and dearth of vegetation. When sunlight strikes concrete, asphalt, metal, and stone, it becomes “sensible heat,” the heat you can feel when thermal energy is transferred to a material. On the hottest summer days, asphalt can easily burn skin.

In contrast to pavement, living plants are never hot. That’s because a leaf converts sunlight into “latent heat,” which we do not experience as heat. Latent heat is the energy absorbed by liquid water that turns it into vapor. Through the process of transpiration, water taken in by roots and transported up a plant’s stem or trunk makes it to the stomatal openings on the leaves, vaporizing just below the surface and cooling the plant in a release of latent heat. Humans do something similar when we sweat — water evaporates from our skin and cools us.

Plants use water mainly for this temperature regulation function, while less than 5 percent of their water use is for cell production and growth. An average mature tree can transpire hundreds of liters of water per day. The cooling power of every 100 liters of water transpired equates to the daily output of two central air-conditioning units for an average home. Imagine what a forest can do.

Case in point: A forested park at Paris’s perimeter recorded temperatures 10°C (18°F) lower than the city center on the same day. Similarly, Washington, DC’s wooded Rock Creek Park was 9°C (17°F) degrees cooler on a hot August day in 2018 than a treeless part of the city a few short miles away. A 2006 study of the heat island effect in New York City revealed that the city’s daily minimum temperature, which occurs at night, is on average 4°C (7.2°F) warmer than surrounding areas.

Trees bolster a city against heatwaves. And with less than 10 percent tree coverage on its city streets, according to a 2016 study, Paris had some catching up to do. Street tree coverage is more than 20 percent in Frankfurt, Amsterdam, and Geneva, for example, and nearly 29 percent in Oslo.

Having pondered Paris’s many shades of white, which seemed to be at the expense of green, Fusto was all ears when he discovered an online talk by Afforest Founder and Director Subhendu Sharma about the Miyawaki Method of forest planting, which involves densely and randomly planting tree, shrub, and ground-cover seedlings to mimic natural conditions, and then monitoring the site for two to three years as the forest establishes itself. The Miyawaki Method, named after Akira Miyawaki, the botanist who developed it, appealed directly to his nagging question about how to invite nature into the dense city. “What I liked was the accessibility of the method,” Fusto recalled. “It seemed simple enough to be implemented by regular people, and effective for restoring biodiversity in urban lots of any size — even very small lots.”

Fusto plunged into research on the method and even contacted Sharma and Miyawaki themselves. He also experimented with native plants in his own garden and started talking to the people around him about the methodology. Fusto found kindred spirits, and together they organized themselves into an informal collective to develop the idea of planting a Miyawaki Method forest in Paris.

By the time a call for proposals had been issued for Paris’s Participatory Budget, which allocates 5 percent of city funds to resident-led projects, the fledgling group had a plan. In 2016, Fusto submitted a project proposal titled “A Sustainable, Open-Source Forest,” and fellow Parisians voted for it, along with several other resident-led environmental, cultural, and social projects.

Not only were residents ready for a project like this, city leaders were, too. Paris had established its first Climate Plan in 2007 and Biodiversity Plan in 2011, both of which were updated in 2018. The 2018 Paris Biodiversity Plan outlined the context for its adoption: “Biodiversity is not just a list of species or genera, but rather, the living, interacting tissue of Earth, whose continuity, functioning and structure are as important as its composition.” The text goes on to assert that emissions reductions cannot be effective for mitigating climate change without a simultaneous global commitment to protect forests, wetlands, coral reefs, and every other ecosystem, thus keeping the carbon stocked therein from being released as CO2.

In a bid to protect wildlife and shelter people from extreme heat, the city proposed to plant 170,000 additional trees, expand wetland area, remove 100 ha (247 ac.) of concrete and asphalt (about 1 percent of city surface), and open 30 additional ha (74 ac.) of vegetation to the public. A guiding objective was that nobody should ever be more than a seven-minute walk to the nearest “cooling island,” which is a location open to the public such as a park, woods, cemetery, church, library, or swimming pool that is cooler than surrounding areas.

The city mapped out tree-deprived neighborhoods and streets for priority planting. And alongside the maps of cooling routes and islands for people were maps of wildlife corridors and reservoirs (both aquatic and terrestrial) within the city.

The vision was to create a web of ecologically functional habitat across the urban matrix that supported the well-being of all citizens. And, importantly, this included “Biodiversity,” which was granted honorary citizenship by the city of Paris in 2016. Najdovski explained that this was a symbolic act: “This recognition contributes to consciousness-raising among as many people as possible about the risks associated with the silent disappearance of our immediate and greater ecosystems.”

ON A COOL MARCH morning in 2018, 40 volunteers planted 1,200 saplings into 400 m2 (4,306 ft.2) of soil beside one of Europe’s busiest freeways, at the Porte de Montreuil. A year later, they planted another mini-forest a mile north of the first one, also on the beltway. The Paris Tree Plan released in October 2021 articulates a goal to “transform the beltway into a circular forest” to the extent possible, while also experimenting with urban forest planting approaches elsewhere in the city, using the Miyawaki Method or otherwise. In the meantime, Fusto’s team, which formalized itself into a nonprofit called Boomforest, has planted mini-forests at a Paris community garden, in a clearing within a degraded suburban woods, and in the city of Lyon, all the while consulting groups throughout the country on the Miyawaki Method.

Fusto and I met on a rainy afternoon in late 2020 to visit the first beltway mini-forest. He unlocked a gate behind a sign demarking the site as a Participatory Budget investment, and we walked up to the thicket of tall and mostly leafless deciduous trees growing on a meter-high oblong mound. The floor of the dormant forest was green in the mild Paris winter on account of grass and other small herbaceous plants growing among the trees. A few evergreen shrubs lent shades of deeper green.

Walking around the stand of young trees, we chatted over the roar of traffic. “This coming year in March of 2021, the forest will be three years old,” Fusto announced, as if speaking of a child about to start preschool. “So, we’re entering into the famous period of autonomy. We’ll soon see if it’s true that we no longer need to maintain it.”

During the first three summers, small teams of neighborhood volunteers weeded the incipient ecosystem on a monthly basis; the city watered the trees only once or twice each year. Not all weeds needed pulling out, though. “The bindweed, thistle, and black locust needed to be weeded, but not the dandelion, which is too short to bother the young trees and shrubs and is quickly shaded out anyway.” Actually, he said, pointing toward the middle of the grove, “you can see that even the bindweed is no longer growing in the middle, but only at the more exposed edges.” Over the previous summer, Fusto added, this older plot needed less weeding than the younger mini-forest that had been planted a year later.

A further sign of this little ecosystem’s growing maturity is that insects and other small animals are showing up. Fusto has seen bees, European firebugs and ladybugs, mole burrows, and a pigeon nest. There are also lots of earthworms under the loose and spongy forest floor, especially as compared to the adjacent grassy stretch, where Fusto found little visible evidence of life when he took soil samples.

When Fusto and I spoke again in August 2021, he said the last weeding had been in March — pretty much at the mini-forest’s three-year anniversary. The trees are in very good health, he said, and “most are taller than us.” Furthermore, the vegetation layers — ranging from shrubs to understory and overstory trees — were now noticeable, even though some won’t reach their full height for another 15 to 20 years or more. When they were first planted, the plants were uniformly less than a meter tall. Even one year prior, Fusto said, the layers were not yet quite apparent.

While weeds were no longer present in the increasingly shaded area, a few forest species (beech, oak, and gooseberry) were sprouting new plants around the edge of the forest. It is not that last year’s two-year-old trees had already produced acorns or beechnuts, but rather that these seeds had arrived from nearby trees outside of the mini-forest and the seedlings had been allowed to take hold — not mowed, as they would have been elsewhere. The gooseberries, which take fewer years to mature, may have self-seeded from the mini-forest.

By the winter of 2021, Boomforest hadn’t yet added another beltway mini-forest to the original two, although plans for a third were in the works. The association was busy responding to other requests. In November 2021, they applied the Miyawaki Method in front of a 28-story suburban apartment building in Colombes, a small city tucked into a bend in the Seine River northwest of Paris. The mayor, who planted a few trees himself, explained to the media that Colombes would likely plant several more mini-forests as a way to help combat climate change.

Boomforest’s initiative is gaining traction one local hero at a time, resulting in the sporadic placement of tiny ecosystems rather than the strategic development of an ecosystem corridor — at least for now. As the hyper-local landscape transformations prove themselves over time, though, perhaps the Miyawaki Method will become a centerpiece of Paris’s ostensibly biodiversity-sensitive landscaping strategy.

The above excerpt is from Hannah Lewis’s new book Mini-Forest Revolution (Chelsea Green Publishing, June 2022) and is reprinted with permission from the publisher.