Alongside narrow Haycock Road in Fairfax County, near Metro's proposed West Falls Church station, stands a large tulip poplar tree where - local tradition has it - the fabled Confederate raider, John Singleton Mosby, often encamped.

The owners of the so-called Mosby Tree, fearing that it is doomed to be cut down by Metro's plans for an access road to the station, have launched a campaign to save it.

There is no official agreement, however, on the authenticity of the purported Mosby connection with the tree, which is 150 feet tall and measures 181 inches around its lower trunk.

Oscar Kiessling, a 76-year-old retired federal government economist, and his wife, Alice, a physician, have asked the Fairfax County Historical Commission to join the preservation effort, with no decision reached. The Kiesslings also are contesting condemnation papers on a silver of their property containing the tree, filed for Metro by the Justice Department in U.S. District Court at Alexandria.

Planning and engineering officials at the Metro transit authority's headquarters in Washington said they sympathize with the owners' desire to save the tree, and are studying ways to do so.

"I think there is a good possibility we can get by without disturbing the tree," Earl Long, a Metro planner, said. He added that Metro officials were informed only recently of the claimed Mosby connection.

Horace Jones, a Metro engineer, said the tree is in no imminent danger, in any event. Design work on the West Falls Church station was suspended in 1975 pending the ultimate decision on construction of Interstate Rte. 66 through the area. Metro would occupy the I-66 median strip.

Although U.S. Secretary of Transportation William T. Coleman Jr. recently approved the I-66 construction, many questions remain unresolved before work could begin. The question of whether to build the Metro route itself is being studied, and no decision is expected until late this year.

Until after that decision is reached, Jones said there would be no construction activity affecting the fate of the tree.

Kiessling, who moved to Haycock Road in 1930 when the nearby State Rte. 7 - now a busy commuter and commercial artery - was a rutted dirt road, cited the recollections of oldtimers, now dead, and an account by a 90-year-old woman who grew up in the area.

Mosby, the central figure in the historic tale, was a Confederate officer during the Civil War whose mounted troops raided and harrassed Union soldiers operating in the Northern Virginia area. Nicknamed "the Gray Ghost," Mosby was romanticized in a television series during the Civil War centennial of the early 1960s.

According to Kiessling, the late Will Ryder recalled in the early 1930s that he as a child had seen Mosby ride into Falls Church.

"We all knew what was going on," the Ryder account says, "and that Mosby had a meeting place near West Falls Church where his troop assembled on call. It was on a knoll overlooking the Leesburg Pike (now Rte. 7), under a giant oak tree and a very large tulip poplar, about 300 yards apart."

Kiessling said the oak tree, located on the present Earnest Layman property on Birch Street, was destroyed by a windstorm 10 years ago. The tulip poplar, Kiessling contends, is the one on his own property.

Margaret Ballard Smith, now 90 and living in New Hampshire, told Kiessling that she played as a child under the tulip poplar, already large.

Mrs. Smith said she was told the Mosby tale by her father, Lyman Ballard, who himself had camped in the area as a Union soldier and who returned there to live - to the age of 96 - after the war.

Nan Netherton, historian for the Fairfax County office of comprehensive planning, said she has no doubt of the sincerity of the Kiesslings in advancing the Mosby story, but said it has not been positively authenticated.

"We would like to have authentication as to the age and condition of the tree," she said, "and specific evidence of it having been used (by Mosby) as the local traditions have it."

The Mosby connection aside, the Kiesslings said they can see no reason to destroy the tulip poplar or a nearby grove of Canadian hemlocks planted by Ballard.

Kiessling said he has been informed by Charles Finley, head of the Virginia Forestry Association, that the purported Mosby Tree is the sixth largest tulip poplar in the state.