A 27-year-old woodsman and naturalist who has roamed and studied New Jersey's vast and desolate Pine Barrens since childhood was cited today by the Bergen County police as having played a key role in the capture of a man suspected of being the robber-rapist who has caused deep fear in dozens of suburban communities after three nighttime assaults since March.

By tracking 2½-day-old footprints, palm prints and other clues left as the gunman eluded the police in a swampy woodland last Friday night, the woodsman, Tom Brown of Wanamassa in Monmouth County, developed his evidence. It convinced the police that a stocky, 39-year-old bulldozer operator already under surveillance was their prime suspect.

Photographs that Mr. Brown made of the suspect with a long-range lens as the man graded a new road here yesterday were identified by robbery victims who were briefly held hostage here by a gunman Friday night, authorities said. Shortly after yesterday's identifications, the man was arrested.

Details of the Charges

"Brown played an important role, he did a very excellent job for us," said Frank A. Parenti, police chief of this Bergen County suburb of 6,500 people.

"He was just like an old Indian scout who helped the cowboys," said another police chief in the area who has worked on the case since March.

The suspect, Bruce A. Ader, was taken into custody at the two-storyhouse where he lives with his wife and their 11-year-old son in Bernardsville, 35 miles southwest of here.

Mr. Ader was charged initially with armed robbery, assault with a deadly weapon, and breaking and entering, for allegedly forcing his way into two homes Friday night at the end of Arcadia Drive, a cul de sac bordering 400 acres of woods here, and taking the five residents hostage.

In addition, Mr. Ader was charged with raping a 19-year-old baby sitter in neighboring Ramsey on July 22. Bail of $250,000 set last night was continued today. Also, the authorities said, a 17-year-old-girl attacked while baby-sitting in Upper Saddle River on March 26, and a 15-year-old baby sitter raped in Ramsey on Oct. 1 are to view Mr. Ader in a lineup.

Mr. Brown spends 60 to 80 hours a week in the woods strolling, tracking animals and photographing nature. A biology student at Brookdale Community College in Lincroft, Mr. Brown was born and brought up in Beachwood, on the outskirts of the vast stretch of wilderness in New Jersey known as the Pine Barrens. From the age of 7, he walked and camped in the barrens, fascinated by tales of woodlore told by old men in the region, and developed the perceptions and powers of observation vital to tracking, he said.

Writing About the Subject

"It's more than a hobby, it's my life -- if you took me out of the woods, I'd go crazy," Mr. Brown said.

At the moment he is preparing to move into an apartment in Wanamassa, in Monmouth County, with his wife of four months, Judy, and is polishing an unpublished manuscript entitled, "Tracker: The Dying Breed." He said it tries to explain "lots of things people don't see in the woods -- people leave their houses in the morning and don't know if it's cloudy or sunny, they don't smell the roses, look at the trees, or even hear the birds singing."

Most of the time he stalks, he said, adding: "I'll get you right up on any deer you want."

Last April he helped the police and National Guardsmen find a 31-year-old retarded man lost for four days in thick woods around Howell Township in Monmouth County.

Mr. Brown came to the Allendale Police Headquarters Monday morning, unannounced and unknown. He offered his services as a tracker because, he said, he had read that the rapist had fled Friday night in thick, marshy woodlands.

The man had vanished without a trace, eluding about 200 policemen and firemen who combed the woods futilely for seven hours until 5 A.M. Search dogs and spotlights beaming from fire trucks and a helicopter circling overhead were no help to them.

On Monday morning Mr. Brown was taken into the woods off Arcadia Drive by the police, about the time that the five who had been hostages were helping a state police artist prepare a sketch of their attacker.

About 40 yards from the house which the gunman had fled into the woods after robbing two women, Mr. Brown, according to his account, began picking up footprints in the loamy soil. He said he began following them, with the police right alongside. Other items he detected along the trail he was developing included pieces of hair and strands of blue nylon clothing fabric snagged on twigs and branches, he said. He also said he detected palmprints in the soil, apparently left as the fugitive hid on his hands and knees to avoid the police. Some coins were also found.