BAKER, NEV. -- In a big white tent set up among the salt sage and prickly pear of the continent's largest desert, a group of politicians will gather this morning for a once-common official ceremony that the Reagan administration has steadfastly resisted until now: the dedication of a new national park.

Today's formal opening of Great Basin National Park, a ruggedly scenic desert-to-alpine expanse of eastern Nevada, marks the first major addition to the national park system since President Reagan and his first secretary of the Interior, James G. Watt, imposed a moratorium on park acquisitions when they took office.

The new park rises from khaki-brown alkali flats through pale green hillside forests on up to dusty white fields of summer snow on jagged mountain peaks where golden eagles search for prey far above the timberline. Its 77,000 acres (about twice the area of the District of Columbia) represent an intact slice of the entire ecosystem of the Great Basin, the huge western desert shaped like an upside-down arrowhead covering parts of six western states.

This principle -- preserving a complete ecosystem -- is the hot concept among advocates of expanding the park system.

Accordingly, Congress' decision to create a national park for the Great Basin ecosystem has sparked hope among conservationists for a series of new parks representing untainted ecosystems. Among the leading candidates to become national parks are stretches of tall-grass prairie in Kansas and Oklahoma and the full length of a wild Oregon river, the Rogue.

If Congress continues to add to the park system, it will reverse a major policy change brought about when Reagan took office.

During the 1970s -- and particularly during President Jimmy Carter's term -- there was massive expansion of the national park system. The combination of Carter's interest in the environment and the political power of the late Rep. Phillip Burton (D-Calif.), who chaired the House Interior subcommittee responsible for creating parks, resulted in the addition of dozens of new parks, seashores, lakeshores, historic sites, recreation areas and the like to the national park system.

The apogee of this trend came in Burton's Omnibus Parks Act of 1978, known in Congress as the "Park-of-the-Month" bill because it added 13 units to the park system in a single year. Carter signed the last of the "park barrel" bills, creating 43 million acres of national parks and monuments in Alaska, shortly before he lost to Reagan in 1980.

The Reagan administration, with Watt in charge of most public land policies, reversed this direction. As soon as he took over at Interior, the "There's no way you're getting half a million people way out here."

-- Chuck Berger

park service's parent agency, Watt announced a "new beginning:" He would halt the rapid expansion of the park system and concentrate money and personnel instead on maintenance and improvement of existing Park Service lands.

Despite opposition in Congress and the conservation community, Watt and his successors at Interior have managed to maintain this no-growth policy. In the first six years of the Reagan administration, the only additions to the park system were small ones such as a boyhood home of President Harry S Truman.

The contrast between the park policies of Carter and Reagan can be seen clearly in the records of the Land and Water Conservation Fund, a federal trust fund that can be used only to acquire park land. Carter spent about $700 million from the fund each year of his term. This year, with fund revenues running about the same as they were a decade ago, Reagan will spend $188 million.

As a result, the national park system today is almost the same collection of land, buildings and monuments it was when Reagan took office. The National Park Service now has jurisdiction over 339 "units," ranging from single-building sites like Ford's Theater in Washington and the James A. Garfield birthplace in Orange, Ohio, to enormous swatches of mountain and range like the 6 million-acre Noatak wilderness in Alaska.

The biggest and best-known of these "units" are the national parks themselves, mostly in the wide-open West. The park being dedicated here this morning is the 49th national park established in the United States since the idea was born at Yellowstone in 1872.

Public interest in the parks continues to grow. Annual park visits increased almost 30 percent from 1980 to 1986, from 220 million to 281 million, the Park Serice says. It was that kind of public pressure, plus some exigencies of Nevada politics, that prompted Congress to end the Reagan/Watt moratorium and pass the Great Basin National Park Act last fall.

Until today, Nevada was the only western state without a national park. Its neighbors Utah, Arizona and California have 13 among them. This indignity, plus the need to enhance the state's tourist economy, prompted Nevada's congressional delegation to push hard for creation of the park. In addition, some members of Congress said the park bill was seen on Capitol Hill as a kind of farewell gift to Paul Laxalt, the veteran Nevada Republican who retired from the Senate this year.

The newest national park is in an unlikely spot for tourists. It is about as remote from population centers as any place in the continental 48 states can be. The nearest metropolitan area, Salt Lake City, is 4 1/2 hours away along the empty stretches of Route 50, "the loneliest road in America," a highway famous for occasional signs reading "Cattle on Highway -- Next 110 Miles" and "Next Services -- 83 Miles."

Great Basin National Park encompasses a locally famous tourist attraction, Lehman Caves, an intriguing series of underground chambers. About 50,000 people visited the caves last year. During congressional debate on the new park, it was suggested that a national park on the site would attract up to 500,000 tourists annually. The residents of Baker, a village of about 75 people just outside the gate of the park, hoot at that estimate.

"We probably need the park; it could help our town," said Chuck Berger, proprietor of the Outlaw bar on the main street of what Park Service literature refers to somewhat expansively as "downtown Baker." "But, hey -- there's no way you're getting half a million people way out here."

Those who make the trek to Great Basin will find a broad range of vegetation and natural habitat. Reflecting the ecology of the vast natural basin, the park's territory takes in five "life zones," from dusty dry desert where rattlesnakes slither through the cactus to an alpine glacier 2.5 miles above sea level where overheated hikers can reward themselves by plopping into an open field of snow that never melts.

The new park also contains several stands of bristlecone pine, the small tree known to botanists as Pinus longaevus because it has the greatest longevity of any living thing. Near the top of Mt. Wheeler, the park's tallest peak, there are living bristlecones that were already a few centuries old when Jesus was born.

A less exotic feature of the new park is the presence of herds of cattle grazing on the hillside grasses. Although such use would be taboo in other national parks, Congress felt strong pressure to accommodate the ranchers here in Baker who hold longstanding grazing permits. Accordingly, the legislation creating the nation's newest national park decrees that the cow is part of the natural ecosystem of Great Basin National Park.