National Park Service horticulturalists have logged more than 5,000 miles this spring on back roads from Lynchburg, Va., to Princeton, N.J., searching for a new National Christmas Tree to replace the ailing 40-foot "live" tree that was cut down this past winter after only three years on the Ellipse.

Although they've driven over virtually every back road in Northern Virginia and suburban Maryland, avoiding new subdivisions where there are few large old spruces and firs, they could find nothing they thought suitable for the President's Park, no tree perfect enough to appear on television year after year as the national symbol of Christmastide.

Becoming desperate this month, with only 120 tree-shopping days left before the ideal October transplanting date, the Park Service has now lowered its sights and is seeking only a 30-feet, not a 40-feet tree.

"We're looking for a Colorado blue spruce or a White or Nordman Fir, perfectly shaped, with branches to the ground, no open or bare spots. And it's got to look like a Christmas tree," says Bill Ruback, Park Service ranger in charge of the White House grounds, which includes the Ellipse and Lafayette Square.

"And it's got to be from roughly the same elevation as the White House, which is almost sea level, and the same climate, which means pretty much within 100 miles of Washington," says James Lindsay, chief horticulturalist for the Park Service's National Capital Region, and Ruback's partner in the search for a new First Tree.

Its predecessor, donated by the National Arborist Association, came from beside a mountain road near Wilkes Barre, Pa. But the huge, 15-ton tree suffered root damage in transplanting and so many of its lower branches died that last Christmas an entire 25-foot blue spruce had to be cut up and tied on to cover the bare spots. The tree was taken down in January and will become one of this year's Yule logs on the Ellipse.

Though Washington's air pollution and hot, humid climate were not considered responsible for the tree's increasingly scrawny appearance, the Park Service believes choosing a tree already acclimated to Washington-like weather will increase its chances of survival. And to help it keep cool the tree will have, like the last one, its own private shower, a built in network of tubes to mist the branches on hot, muggy days.

Despite its notable lack of success with the previous "live" Christmas tree, the Park Service is determined, for environmental reasons, to have a living National Christmas Tree. The annual raising of a national Christmas tree began in 1924 when Middlebury College in Vermont sent President Calvin Coolidge a giant fir tree, which was erected on the Ellipse. A live tree was planted on the south side of the White House the next year and used until 1941 when two Oriental spruces replaced it. All have since died and cut trees were used from 1954 until 1973.

In addition to the large national Christmas tree, 57 smaller trees, usually 7-feet Norway spruces, are planted in a circle around the tree just before Christmas and then removed in January to ready the Ellipse for the spring tourist season. They are replanted in parks or a Park Service nursery, but Ranger Ruback admits their rate of survival is low.

"We had to dig them out of the frozen ground this year with air compressors. And even last year (when the ground wasn't frozen here) only about 5 per cent survived," Ruback said.

The search for the National Christmas Tree has taken Ruback and Lindsay out into the Virginia and Maryland countryside for weeks at a time. "We crisscrossed every back road on the Eastern Shore, down to Petersburg and Norfolk and all the way down to Lynchburg, often leaving on a Monday and not getting back till Friday," said Ruback.

"Some people thought it sounded like a great assignment, getting to drive through the countryside," Ruback said."But I tell you our eyes and necks were sore from looking.And it's dangerous doing it by yourself, you could drive right off the road looking at trees."

Ruback said they looked mostly in areas "where houses were at least 40 years old, when we were looking for a 40-year-old tree, because these trees are all ornamentals, planted by homeowners, and they grow at the most one foot per year." Neither the blue spruce nor the firs are native to the mid-Atlantic states. They rarely reproduce naturally here and their life spans in this area usually are no more than 90 or 100 years instead of 200 or 300 years, said Lindsay. "We would be happy to get one to live 60 or 70 years - or at least another 11 years until we retire," he said.

They considered other species of evergreens, the deodora cedar and Douglas fir, but rejected them and many other varieties of spruce and fir trees because "they don't really look like a Christmas tree. They're droopy, have a funny shape or are too scrawny," Lindsay said.

"One of the best ones we saw was a 38-foot blue spruce on the front lawn of an elderly widow in a small Virginia town. But she wanted $1,000 for it because she said, 'You U.S. government guys all have plenty of money,' then asked $5,000 and wanted us to plant birches on her front lawn as well," said Ruback. "And the tree had some die-back anyway," said Lindsay.

But their reception has been friendly in most places, "as soon as people realize what we're looking for," Ruback said. "One place a young couple had me in to dinner before I knew what was going on and the next thing I knew its on the front page of the local newspaper about the man from Washington, D.C., coming to town for a Christmas tree," said Ruback.

Unfortunately, while they were happy to donate their tree, and a dinner to boot, the tree wasn't perfect on all sides. "So many things have to be just right," Lindsay said. "It can't be near a fence or house, cannot be in sandy soil where the root ball falls apart, must have only a single trunk, and must be disease and insect free."

And this best of all possible trees must be found in time for Christmas. "We haven't found it yet but we keep thinking it may be just around the corner," said Ruback optimistically, "or maybe someone will call and tell us of one." Ruback's number at the White House is 755-7798.