The dying trees are from an endangered species of pine that is found only on Yakushima and a neighboring island. Mr. 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.

Image Credit... The New York Times

Mr. Nagafuchi, then a public employee for a city in Kyushu, had already found blackened snow while hiking to Yakushima’s mountaintops in 1992. He started collecting and analyzing the snow as a sort of weekend hobby. To his surprise, he found it contained silicon, aluminum and other byproducts from the burning of coal, which is used to heat homes in China. Using maps of winds, he theorized that the pollutants were carried here from China, across the East China Sea.

The discovery drove Mr. 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, Mr. Nagafuchi climbed to the highest of those stations, atop Mt. Kuromi, a windswept peak that rises 6,000 feet above the sea below. 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 northwest, said Mr. Nagafuchi, 62, who visits Yakushima once a month to collect the data readings. “This is proof that when such a big country industrializes, its effect will spread everywhere.”

When they first started publicizing the findings in the mid-1990s, Mr. 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 sensationalizing the die-off to get public attention. Some scientists questioned why the tree deaths slowed even as China’s pollution problems have grown. Mr. Nagafuchi says he believes the pollution quickly killed off the weak trees, leaving the hardier ones.