F. Herbert Bormann, a plant ecologist whose research with colleagues on a swath of New Hampshire forest in 1971 documented a new environmental horror in the United States — acid rain — died on June 7 at his home in North Branford, Conn. He was 90.

The cause was complications of a lung infection, his daughter Rebecca Bormann said.

Dr. Bormann and his team of scientists discovered the threat of acid rain in a small watershed in the White Mountains, where they had gone to analyze chemical interactions in the area’s ecosystem. They found rain and other precipitation to be much more acidic than expected. Over the next few years, they tested rain throughout the Eastern United States and found that acidity had increased 100 percent to 1,000 percent since the early 1950s.

The scientists traced the acidity to sulfur dioxide emissions and various nitrogen oxides from faraway smokestacks. The gases are converted to sulfuric acid and nitric acid in the air.

Dr. Bormann and his team detailed some of the pernicious effects of acid rain, including reduced forest growth and fish kills, in Science magazine in 1974. Their laboratory experiments showed that tomato plants, birch leaves and pine needles were damaged when misted with acid water, confirming similar conclusions reached in Sweden.