During the war, oil and gas shortages led Japan to turn to the nation's most abundant natural resource – forests – as a source of fuel for home and industry. The result was widespread deforestation of natural forests, with the mountains around major cities like Tokyo, Osaka and Kobe completely stripped bare of trees.

"After World War Two, many of Japan's mountains became barren, causing disasters in various regions," says Noriko Sato, a professor and forestry researcher at Kyushu University in Fukuoka, Japan. (Bare mountains can increase the incidence of landslides and flooding). "Large-scale afforestation was carried out by public works, funded by tax revenues, to prevent soil erosion."

Aiming for rapid reforestation, the government chose to plant reams of only two different native, fast-growing evergreen species that could quickly reforest landscapes and provide wood for future use in construction: the Japanese cedar, sugi, and the Japanese cypress, hinoki.

Today, these hinoki and sugi plantation forests still cover around 10 million hectares (25 million acres) – a fifth of Japan's entire land area.

The problem is, sugi and hinoki trees also produce large amounts of lightweight pollen which can easily drift into cities. It's this pollen, often released all at once from the monoculture plantations, that is responsible for most seasonal allergies in Japan. The issue has become all the worse since these trees release ever more pollen after maturing at 30 years of age – now the case for nearly all of them.

"Pollen allergies have become a national health issue in Japan," says Sato. "Addressing this problem is urgent."

In 2023, Japan declared allergies a national social problem and the central government set out an ambitious plan – reduce pollen by 50% in 30 years. As a first step, it aims to reduce the forest areas planted with sugi trees by 20%.

But swapping out forests covering over 2% of Japan in 10 years is a massive endeavour. Plus, simply cutting these trees down won't be enough – they also need to be replaced with new forests to avoid soil erosion or accidentally undercutting Japan's own climate targets.

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Still, even before the 2023 government declaration, some local actors and non-profits had begun efforts to turn these forests into biodiverse ecosystems, and some are already seeing the benefits. The small town of Nishiawakura, Okayama, for example, has created an entire economy around reducing the 84% of its forests made up only of hinoki and sugi, turning wood into heat for eel farms as well as chopsticks and timber.

In 2020, Kobe, a larger port city in central Japan with a dense urban core and vast forests within its city limits, began an effort to turn more than 180 hectares (445 acres) of plantation back into natural broadleaf forests in a 15-year cycle.

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Similar projects are beginning in other parts of Japan. One project in Hotani, Osaka, is now restoring wetlands and grasslands. And the largest effort aims to turn 10,000 hectares (25,000 acres) of plantation forests in Gumna prefecture to meadows and mixed deciduous woodland.