SPOKANE The trial of three former Missoula, Mont., environmental activists accused of spiking trees in Idaho offers a chance to tell their side of the story, a lawyer said Monday.

The Post Office Creek timber sale in March 1989 was ''the kind of sale that John and the others felt should not occur,'' attorney Robert Chastain said of his client, John P. Blount.

Blount, 32, of Masonville, Colo., and Jeffrey C. Fairchild, 27, Ashland, Wis., each are charged with two counts of tree spiking, two counts of destruction of federal property and two counts of conspiracy.

A third defendant, Daniel A. LaCrosse, 36, Salem, N.H., is charged with conspiracy to spike trees and conspiracy to destroy government property.

U.S. District Judge Justin Quackenbush told the jury the trial would last a week.

Blount, known in the Missoula environmental community as ''Spicer,'' won't deny spiking trees in the Clearwater National Forest near Powell, Idaho, to try to stop the planned helicopter logging operation, Chastain said.

Blount will admit buying spikes and paint and making one of two trips to the logging sale site the government contends he and others made to spike trees, Chastain said. But his mission was to try to save the old growth trees in a pristine part of the national forest, his attorney said.

''There was no intent to hurt anybody,'' Chastain said. ''He wanted those trees to continue growing.''

Attorneys for Fairchild and LaCrosse did not make opening statements.

The government's first witness, Tracy Stone-Manning, testified that she was shocked to read a letter given to her by Blount on the steps of the University of Montana's Rankin Hall environmental studies building in April 1989.

''I hadn't known this had happened,'' said Stone-Manning, who was an environmental studies graduate student and now heads the Five Valleys Land Trust. ''It was news to me.''

She said she retyped the letter correcting spelling errors and deleting some profanity then sent it to the Forest Service, as Blount had requested.

A Forest Service supervisor at Orofino received a letter in April 1989 warning that 500 pounds of bridge spikes had been driven into trees to prevent loggers or sawmills from cutting them.

Blount, Stone-Manning and others lived for varying periods of time at a residence called Sherwood House, where they became involved in protests against environmental and forest policies in an activist group called Earth First, she said.

Stone-Manning was granted immunity from prosecution for her testimony after coming forward last year after Blount was arrested in Colorado in a domestic dispute with a former girlfriend, Guenevere Lilburn, who also will testify, assistant U.S. attorney George Breitsameter said.

Two others who participated in the spiking raids have pleaded guilty to lesser charges and agreed to testify.

Arvid E. Hartley and Neil K. McLain pleaded guilty to misdemeanor charges June 3 of spiking trees with the intent to hinder a timber sale.

They are scheduled to be sentenced Aug. 5 in Missoula, Mont.