Pollen secreted by many plants can cause allergies. But in Japan, this health problem is associated with two types of trees: red cedar and cypress.

Since these trees are native to the country and have been part of their natural environment for hundreds of years, why did they become a problem?

“During World War II, forests in Japan were cut down and destroyed,” Professor Iwao Uehara of Tokyo Agricultural University’s Department of Forestry told the BBC. “Because of the shortage of timber after the war, large numbers of red cedar and cypress trees were planted because they grow relatively quickly.”

According to University of California, Irving historian David Weidman, who specializes in Japanese environmental history, this feature was ideal for meeting the country’s demand for wood during the Reconstruction process, which led to an increase in its cultivation.

“One of Japan’s most important needs immediately after the war was to obtain materials for the reconstruction of the cities,” says Weidman.

“It is important to mention here the aerial bombing campaign that destroyed traditional wooden buildings in Japan in the last months of World War II.”

The Allied occupation of Japan ended in 1952, but red rice and cypress cultivation continued to expand in the following years.

In some cases, natural forests have been cleared and replaced by red cedar and cypress, Professor Uehara said. “Today, Japanese red cedar accounts for 45 percent of Japan’s planted forests, and cypress for 25 percent.”

Both of these tree species depend on wind pollination for reproduction, which means releasing large amounts of pollen into the air.

Due to the decline in the price of imported wood since the seventies of the last century, the economic importance of these trees decreased in the following decades.

Due to the lack of consumption of wood produced from these two species in the country, the red cedar forests became denser and the trees became larger, increasing the problem of pollen clouds.

The Japanese Ministry of Agriculture estimates that red cedar forests alone cover 12% of the country.

But Professor Uehara explains that red cedar and cypress trees are not solely “responsible” for Japan’s springtime allergy crisis. There are other factors that play a role in this crisis, such as air pollution in cities, where certain pollutant particles combine with pollen, making them more allergic.

And there’s also the climate change factor, as warmer temperatures are causing trees around the world to produce more pollen, for longer periods of time, at earlier dates than in previous years.

In a study published in March 2022, University of Michigan researchers found that the length of the pollen production season in the United States and Canada increased by at least 20 percent between 1990 and 2018, and the concentration of pollen in the air increased. 21 percent. So far, according to the study authors, global warming.

This was reported by the Japan Meteorological Society in April / Last April, the pollen outbreak in some parts of the country started two weeks earlier than usual in 2023 compared to last year.

Japanese media have speculated on measures the government could take to mitigate the crisis, from cutting down red cedar forests to using artificial intelligence to monitor pollen emissions.

But Professor Uehara says the problem is deeper than that and has an important lesson not only for Japan, but for the entire world: biodiversity loss can lead to unintended consequences decades later. “The key move was to expand the forest area to include not just one or two trees, but many species of trees,” he said.