When the aging visitor center at the base of Yosemite Falls needed a makeover, the $13.5 million price tag loomed as far out of reach as the giant granite cliffs atop the valley there.

The federal government could not afford the project, but it did not die there.

Private and nonprofit donors gave $12 million, and an army of volunteers gave years of their labor.

The Yosemite Falls restoration project illustrates the extent to which America's parks increasingly rely on private donations.

As tight budgets limit park maintenance and improvements, groups outside government are stepping up for projects such as tearing out barbed-wire fences in California's Mount Diablo State Park, restoring carriage roads in Acadia National Park on a Maine island and building a new visitor center at Ano Nuevo state beach on the San Mateo County coast.

"There has been a philanthropic element from the beginning of our national parks when people like John D. Rockefeller donated land, but we're certainly seeing an increase in recent years," said Bob Hansen, president of the Yosemite Fund.

The nonprofit group spearheaded the upgrading of trails, a meadow and parking lot beneath the famous falls.

Park managers say it is healthy for people to support their outdoor places, especially in trying economic times.

Yet some park advocates say state and federal parks may have begun leaning too hard on donor dollars.

Both the National Park Service and the California State Parks have asked nonprofit groups in recent years to pick up salaries for some seasonal workers.

Donations should not cover such baseline services, they argue.

"The partnership groups should provide a margin of excellence for parks, not a margin of survival," said Blake Selzer, a budget specialist with the National Parks Conservation Association.

Hundreds of nonprofit groups help individual national parks, and more than 80 groups help individual California state recreation areas with money, labor and volunteers.

In theory, the outside groups provide the icing on the cake, while the government performs the bread-and butter basic operations and maintenance.

The line, however, is blurring between what government and outside agencies do, some park watchers say.

In Contra Costa County, the Mount Diablo Interpretive Association last year rejected a state request to consider paying the salary of an interpretive employee to tell visitors about the Mount Diablo environment.

"It opens up Pandora's box if we start paying salaries," said Rich McDrew, president of the 300-member association. "I think volunteers can do more in the parks, but it's not our mission to start paying salaries."

Volunteers in the interpretive association produce brochures, lead hikes and repair signs.

In Maine, a nonprofit group balked at paying Acadia National Park workers' salaries, but the group did raise more than $4 million to create an endowment fund to pay for crews to maintain historic gravel roads.

The endowment is thought to be the first of its kind to pay for road upkeep in a national park.

"We're adding value to the parks," said Ken Olson, the Friends of Acadia executive director.

In California, a nonprofit group that raised $2.6 million last year spends much of its money on sending low-income children to a camp and parks in East Bay Regional Park.

That is 12 times more than money than the group, the Regional Parks Foundation, raised in 1993.

"There is a greater need for nonprofits as the state, local and federal funding has become more challenged," said Rosemary Cameron, the foundation's executive director.

Park advocates say contributions have risen because of a strong public environmental spirit and an increasing sophistication of groups in attracting donations.

Many donors give to a park they visited in their childhood, said Curt Buchholtz, head of an alliance called the Rocky Mountain National Parks Associates.

"Not everyone can be a park ranger," he said, "but anyone can be a park steward and help care for a park."

Recent improvements at Yosemite Falls in California cost $13.5 million, but private and nonprofit donors provided $12 million.