U.S. and Canadian scientists plan to observe a controlled forest fire in northern Ontario today as part of ongoing research programs in both countries to determine whether nuclear war could cause a "nuclear winter" of unsurvivable darkness and cold.

Canadian foresters plan to set the fire this afternoon in woods near the town of Chapleau, if weather conditions permit, to burn off 1,600 acres of dead balsam fir trees killed by an infestation of spruce budworm. There are about 50 such controlled fires in Canada each year to clear away deadwood, officials there said.

This time, they have invited as observers university scientists and researchers for the U.S. Defense Nuclear Agency and the Los Alamos National Laboratory who are testing the theory that nuclear war would blast so much dust and smoke from burning cities into the atmosphere that the sky would be blackened, cutting off sunlight and allowing the land to cool.

According to the "nuclear winter" theory, many of those spared the ravages of the nuclear blast and subsequent radiation fallout would perish from the cold or starve to death because of the destruction of crops and animals in the "quick freeze."

Milton Gillespie, a testing official at the Defense Nuclear Agency, said, "One of the biggest and toughest questions we are facing is: once the smoke gets in the air, how long does it stay there?"

Although critics of the theory have suggested that rain and other natural processes would wash most of the particles out of the atmosphere before they could affect the climate, computer simulations of atmospheric behavior largely have confirmed it and have predicted that a war in July could drop temperatures by 30 to 100 degrees Fahrenheit for a matter of weeks.

Gillespie indicated that U.S. officials hope that next year or in 1987, they will be able to do field testing during controlled forest fires in Canada to determine what happens to the smoke. Wildfires such as those that have raged on the West Coast recently are considered too dangerous to be studied in complicated experiments, although Gillespie noted that there was no detectable effect on the atmosphere outside the immediate region of those fires.

The controlled fires in Canada are set in wind conditions that contain the fire by forcing it into one central inferno. A helicopter with a "drip torch" suspended from it flies in circles to create concentric rings of fire. Smoke plumes sometimes rise more than 18,000 feet into the air.

There is not enough money in the U.S. nuclear agency's current budget for the kind of sophisticated effort required for field testing of the nuclear winter theory, Gillespie said. He said representatives of his agency intend only to observe the process and will seek funding for comprehensive testing in 1986 or 1987. The cost for such testing would be between $200,000 and $2 million, he estimated. The current U.S. budget for nuclear winter research is about $2.1 million. Gillespie said he expects this to be increased to about $5.5 million during the coming fiscal year.

In both the United States and Canada this spring, the discussions of nuclear winter have led to major clashes over the Reagan administration's Strategic Defense Initiative. Opponents have argued that the theory demonstrates that even a limited nuclear exchange could have devastating consequences. Administration spokesmen have responded that the theory makes more urgent the need to build up U.S. nuclear deterrent capability and to proceed with President Reagan's initiatives for arms reductions.

The theory has appeared to deepen Canadian unease over nuclear arms at a time when the Canadian government is deliberating whether to accept a U.S. offer to participate in research on the initiative, which critics call "Star Wars."

A recent study by the Royal Society of Canada concluded that a nuclear exchange between the superpowers probably would destroy Canadian agriculture even if no Canadian targets were bombed.

"There will be no bystanders," Kenneth Hare, the University of Toronto scientist who chaired the study, told the Canadian Press news agency.