FOR 26 YEARS, Washington officials have been hotly debating what kind of memorial the nation should erect to honor Franklin Delano Roosevelt. They've done this without realizing that the most appropriate and useful one possible already exists, although it is slowly being bulldozed and burned.

The chairman of the FDR Memorial Commission has been trying to persuade Congress to build an FDR Memorial on the western edge of the Tidal Basin near the Jefferson Memorial. This revised $26 million plan is a scaled-down version of a $50 million design. But it, too, has been attacked as having "remarkably high" construction and operating costs. h

As former interior secretary Cecil Andrus pointed out, "President Roosevelt himself made it very plain that he did not want a large, costly monument erected to his honor in Washington." Instead, to satisfy admirers who want a memorial in place by the 100th anniversay of FDR's birth -- Jan. 30, 1982 -- Andrus suggested the remaining of a park in Washington. "As a leading conservationist and a celebrated planter of trees on his estate at Hyde Park, N.Y., he surely would believe such recognition appropriate if he were among us today," he said.

But there's no reason to settle for a mere park. An opportunity exists to salvage what remains of one of Roosevelt's dreams.

While crossing the Great Plains "Dust Bowl" in July 1932 as he sought the presidency, Roosevelt conceived the idea of planting large numbers of trees to reduce the wind erosion which was then killing the Plains.

FDR had seen with his own eyes the black blizzards of blowing topsoil and swirling sand which drove farmers off their land, stopped auto traffic which drove farmers off their land, stopped auto traffic at midday and deposited dusty debris over cities as far away as Washington and on the decks of ships in the Atlantic.

"This is my baby," Roosevelt later said of the project to plant 100-foot-wide shelterbelts or windbreaks in long, east-west strips along farmers' fields to reduce erosion and to protect crops from the hot winds out of the south. Unlike many of man's grandiose schemes, this dream had practical result.

More than 222 million of "FDR's trees" were planted in the Prairie States Forestry Project from 1935 to 1942. From the Dakotas, these shelterbelts stretched south a thousand miles into the Texas panhandle in a 200-mile-wide belt in the transitional area between the tallgrass eastern prairies and the shortgrass western plains.

"Never before or since has man planted so many trees in so few years under such trying environmental conditions," wrote John R. McGuire, then chief of the U.S. Forest Service, in a 1976 foreward to an impressive study of Great Plains trees by Wilmon H. Droze, a Texas sholar.

FDR's trees have not only protected crops, livestock and humans from the high winds, the piercing winter cold and the scorching summer heat of the Plains, but they have also provided places for farm children to play, for families to picnic, for game animals and birds to flourish. These half-mile-long rows of conifers and deciduous trees have provided unusual places of natural beauty and human refuge in a largely treeless region so sparse and lonely that early travelers called it the "Great American Desert."

The project's first tree, an Austrian pine planted on the cotton farm of Horace E. Curtis 46 years ago this spring, survives -- as does the satisfied farmer who supervised the planting of that first group of shelterbelt trees 5 miles west of Willow in southwestern Oklahoma. Many of the other shelterbelt trees still stand, but time is taking its toll.

Given little national attention since FDR's death in 1945, the shelterbelt project has become a program of largely forgotten trees.

While the trees were planted with federal finance assistance, they stand on private land, with maintenance left to individual farmers. As the memory of the duststorms of the "Dirty Thirties" has faded, many farmers and agribusiness operators have allowed their shelterbelts to deteriorate. As these windbreaks lose efficency in shielding crops and farm homes, high costs pressure some farmers to bulldoze the shelterbelts and plant crops in the cleared land. There is little educational work to inform farmers of the benefits of keeping these trees.

"It's not that people on the Plains don't appreciate trees," explains David F. Van Haverbeke, a research forester in Lincoln, Neb. "There just isn't any cost-sharing program to give a lot of farmers the economic incentive to keep them up." So, many of FDR's trees have been destroyed.

This is why the rehabilitation of the shelterbelts on the Plains would be such a fitting memorial to FDR. His trees are already in place and bringing benefits. With a little extra effort, the nation could come to regard these shelterbelts as a suitable living memorial to that "celebrated planter of trees." Now there is neither a center where the history and purpose of the shelterbelt project is explained nor even highway markers pointing to the existence of these trees. i

Even if Congress did not want to fund a historical center on the Plains to explain FDR's trees, it might spare something to make education on the shelterbelts available and to give farmers some economic or tax incentive to renovate existing shelterbelts and to plant more trees where needed. Whatever was spent on such a plan would be regained manyfold in sustained agricultural productivity and protection of necessary farmland, especially important now that large parts of the Plains face another potentially devastating period of drought.

Trees meant a lot of FDR; he listed his occupation as "tree grower" when registering to vote in New York, William R. Emerson, director of the Franklin D. Roosevelt Library at Hyde Park, points out. "The planting of these shelterbelt trees was very close to FDR's heart," Emerson says. "Nothing except the rebuilding of the Navy was so close to his life and concerns. Attention to these shelterbelt trees would be the most seemly observation of the 100th anniversay of his birth in 1982."

While it might seem a bit unusual to advocate a memorial to a Democratic president during a frugal, Republican administration, many remember that Ronald Reagan supported FDR throughout Roosevelt's lifetime and that, even in the 1980 campaign, Reagan often wrapped himself in FDR's words and memory.

It somehow seems possible and fitting that President Reagan could lead in designating a living memorial to a president whose wisdom helped sustain farmland in Reagan's native Midwest and whose economic intiatives helped Reagan's own family survive during the Great Depression of the Thirties.