After a nine-month, 7,000-mile search for a new national Christmas tree to go on the Ellipse, a "lovely tree" has been found right here, less than 15 miles from the White House.

A Colorado blue spruce "of perfect dimensions . . . with flawless branches, 30 feet high, 18 feet wide, and an 18-inch trunk" has been donated anonymously by a Rockville family, according to Bill Ruback, National Park Service ranger in charge of the White House grounds.

Though pledged to secrecy on the tree's donors and its present whereabouts, because the owners do not want to be swamped with visitors, Ruback still lovingly describes its particulars and biographical data as though it were a movie or TV star - which it will be when it is decorated and the President lights it on television Dec. 15 at the opening of the annual Pageant of Peace ceremony on the Ellipse.

The tree was one of 90 offered to the Park Service, from as far away as New Orleans, Milwaukee and Metuchen, N.J., after a Washington Post story five weeks ago detailed the frustrating search for a new tree - to replace the live national Christmas tree cut down last winter after only three years on the Ellipse.

The previous tree, a 40-foot-high blue spruce from the Pocono Mountains of Pennsylvania, was damaged in transplanting and lost many of its lower branches. It was cut up for this year's Yule log fire, which burns throughout the two-week Pageant of Peace.

Ruback and James Lindsay, chief horticulturalist for the Park Service here, had been looking for a new Christmas tree since last October, driving over almost every back road in Maryland and Virginia within 100 miles of Washington and even visiting trees in Pennsylvania and New Jersey. Although they dropped their sights from a 40-feet to a 30-feet tree, still they could find nothing they thought suitable for the Ellipse.

They wanted something from roughly the same elevation and climate as the White House, to improve the tree's chances of survival, and of course it had to be full and perfect on all sides since it is constantly circled by admiring tourists. The Park Service is committed to a "live" national Christmas tree, similar to those decorated annually from 1925 until 1954 when cut trees first began being used. The custom of decorating a tree on the Ellipse began in 1924 when Middlebury College in Vermont sent President Calvin Coolidge a giant fir tree.

The new Christmas tree will be transplanted in October, considered the ideal moving time for trees, and will be fitted with sprinkling hoses, as was its predecessor, to mist its branches during hot Washington weather.

The tree's history, according to Ruback, began in the A. Gude Sons nursery about 30 years ago. It was planted in the front yard of its present owners' house in 1952 "when it was a five or six foot tree with a trunk about four inches in diameter," says Ruback. Spruces grow about a foot a year.

Its present owners, in the best Christmas spirit, have given the tree, "asking nothing in return, except that we fill the hole and replace the grass," says Ruback. The best Colorado spruce previous searches could find, a 38-footer in a small Virginia town, was rejected after its owner demanded first $1,000 then $5,000 because, she told Ruback, "You U.S. government guys all have plenty of money." But the tree had other problems too, Ruback said, and wasn't as perfect as the Rockville tree.

As insurance, so another 7,000-mile, nine-month odyssey in pursuit of the perfect Christmas tree will not have to be launched again, Ruback is proposing the Park Service plant a 15-18 feet tall blue spruce near the Ellipse, as a back-up or stand-in. "Just in case," says Ruback.