While Professor Nagafuchi and a small number of collaborators say their research is not politically motivated, they admit they may be finding more receptivity among a public that already resents China for supplanting Japan as Asia's largest economy, and for what is seen as its haughty attitude in a territorial dispute over islands both countries claim. Japanese officials still dispute whether airborne pollutants are responsible for killing the pine trees, but they and other scientists have at least begun to view Yakushima, which is far from Japan's own industrial centres, as a pristine laboratory for understanding how China's growing environmental problems could be affecting its neighbours. Many islanders are already believers, and they worry that the pollutants may be threatening their health. "We are starting to feel like the canary in a coal mine," said the island's mayor, Koji Araki. "Our island is right downwind from China, so we get the brunt of it." Whatever the cause, the tree die-off is a worrisome turn for this small, mountainous island off Kyushu, the southernmost of Japan's main islands, whose moss-carpeted forests provide a rare patch of primitive nature in an otherwise densely populated nation.

There are fears here that a growing smog problem could scare off the hikers and other ecotourists upon whom many of the island's 14,000 residents depend for their livelihoods. Most visitors come to see Yakushima's majestic cedar trees, which have so far been unaffected by the mysterious ailment killing the pines. The cedars won the island the distinction of a UNESCO World Heritage site in 1993. The cedars were logged for centuries to build some of the great Buddhist temples in the ancient capital, Kyoto. The biggest remaining tree, the gnarled Jomon cedar, measures five metres around at the base and is estimated to be at least 2600 years old. The dying trees are from an endangered species of pine that is found only on Yakushima and a neighbouring island. Professor Nagafuchi, a professor of ecosystem studies at the University of Shiga prefecture in central Japan, said he noticed the problem when satellite photographs showed a large increase in the number of dead trees between 1992 and 1996.

Professor Nagafuchi, then a public employee for a city in Kyushu, had already found blackened snow while hiking to Yakushima's mountain tops in 1992. He started collecting and analysing the snow as a sort of weekend hobby. To his surprise, he found it contained silicon, aluminium and other byproducts from the burning of coal, which is used to heat homes in China. Using maps of winds, he theorised that the pollutants were carried here from China, across the East China Sea. The discovery drove Professor Nagafuchi to quit his city job and eventually become a university professor, doing much of his research on Yakushima. He has set up small monitoring stations around the island to measure levels in the air of ozone and sulfur emissions, which are typically the byproducts of burned coal or automobile exhaust. On a recent afternoon, Professor Nagafuchi climbed to the highest of those stations, atop Mount Kuromi, a windswept peak that rises 1800 metres above the sea. After hooking up his laptop to download data from the station's small digital recorder, he pointed out the thin, gauzy haze that clouded what he said should have been pristine air.

"The worst is when winds blow from Beijing and Tianjin", two Chinese cities about 900 miles to the north-west, said Professor Nagafuchi, 62, who visits Yakushima once a month to collect the data readings. "This is proof that when such a big country industrialises, its effect will spread everywhere." When they first started publicising the findings in the mid-1990s, Professor Nagafuchi and his main partner, Kenshi Tetsuka, an islander who started a small environmental group to protect the pines, were at first derided by forestry officials and established scientists who said they were sensationalising the die-off to get public attention. Some scientists questioned why the tree deaths slowed even as China's pollution problems have grown. Professor Nagafuchi says he believes the pollution quickly killed off the weak trees, leaving the hardier ones. His ideas began to win limited acceptance in the early 2000s, amid evidence of a growing influx of Chinese pollutants across Japan. The national government's Forestry Agency began to allow Professor Nagafuchi to set up his monitoring stations, and is doing joint research with him and Mr Tetsuka, though it still believes the deaths are caused by an infestation of bugs, and a runaway population of deer, which can strip small trees of pine needles.

They point out that there had been die-offs of pine trees on Yakushima even before China's economic take-off. "We don't agree with him, but we respect his research," said Hiroharu Ijima, a Forestry Agency official on Yakushima. Public anxieties about environmental effects from China have soared this year, after Beijing recorded alarming increases in pollution levels. That was followed by officials in western Japan issuing warnings in their own cities of high levels of particulate matter measuring 2.5 micrometers or less, known as PM 2.5, that are small enough to become embedded in human lungs. Several Japanese cities have issued warnings this year for residents to stay indoors when the pollutant levels spike. When the air grew particularly hazy on Yakushima one day last month, local officials asked if they could use one of Professor Nagafuchi's monitoring stations to measure PM 2.5. The level was above government-recommended safe levels, prompting officials to order a local elementary school to cancel a field trip to a nearby forest.

Residents who believe the pollution is caused by China described feeling helpless, saying they doubt there is action their government could take even if it becomes convinced Professor Nagafuchi is right. "There is not much we can do about this, except ask the Chinese to spend more money on environmental clean-up," said Mr Tetsuka.. "I'm afraid it will only get worse and worse." AP