June 5 (Bloomberg) -- Global plant growth increased 6 percent over the last two decades, mainly because of climate changes that reduced cloud cover in some areas and boosted rainfall in others, according to a study that will published in the journal Science.

The greatest gains appeared in tropical rainforests such as the Amazon in Brazil, where plant growth surged by 42 percent from 1982 through 1999, said Ramakrishna Nemani, earth scientist at the University of Montana and author of the study in the June 6 issue.

The 1980s and 1990s, two of the warmest decades since records began, saw significant climactic changes including three ``El Nino'' events that skewed global rainfall patterns, Nemani said. The result was that more sunlight spurred growth in South America while increased rainfall helped flora in South Asia, he said.

``In areas like India, where there are more than a billion people, it may have helped with food reserves,'' Nemani said. In the U.S., ``it probably didn't have that much of an impact.''

U.S. corn yields increased by almost a third in roughly the same period, to an average 130 bushels an acre in 1996 through 2001 from 100 bushels in 1980 through 1984, according to the Washington-based American Corn Growers Association. Still, the gains are more likely the result of improved hybrids and crop- management practices than warmer, wetter weather, said association President Keith Dittrich.

``Global warming is affecting our environment, but to say the weather has been part of (increased harvests) would be hard,'' Dittrich said in an interview.