For nearly 40 years, a low-key government agency based in Los Altos has preserved some of the most scenic landscapes in the Bay Area for hikers, bicyclists and horse riders, from redwood forests to ridge-top meadows with sweeping ocean views.

Six years from now, however, the open-space purchases may come to an end.

After acquiring 59,401 acres — an area about twice the size of San Francisco — since 1972, the Midpeninsula Regional Open Space District is facing an unprecedented financial challenge. Because of increases in its staff size and growing debt, the agency projects it will run out of money to buy any more land by 2017.

“We’re not in a crisis, but we see one coming,” said Steve Abbors, general manager of the district. “We’re going to ask the public what they want us to do.”

The growing concern means outdoor lovers and taxpayers could be asked to open their wallets to help — an uncertain proposal that has critics questioning whether the agency, commonly known as “Midpen,” has bought enough parkland.

Midpen officials are considering charging admission for the first time to some of their 26 preserves in Santa Clara and San Mateo counties, like state and county parks do. Most likely they also will go to the ballot, in 2014 or 2016, to ask voters who live in the district boundaries of northern Santa Clara and southern San Mateo counties for the two-thirds majority needed to pass a parcel tax, probably $12 to $18 a year.

Environmentalists say the open space district has been a critical bulwark against urban sprawl from Crystal Springs Reservoir near San Carlos to Mount Umunhum in South San Jose, and that its job isn’t finished. Without the district, motorists driving along Interstate 280 from San Jose to San Francisco would see an unbroken series of subdivisions and trophy homes in the hills above Los Gatos, Palo Alto and other communities, rather than the broad expanses of publicly owned forests and meadows there now.

“Our population is growing and changing. As Mark Twain said, they aren’t making any more land,” said Darla Guenzler, executive director of the California Council of Land Trusts. “There’s never been a single poll that shows the public ever regrets the public land that has been put aside.”

Some of the open space agency’s critics, however, say it has bought enough real estate.

“There has to be some culmination of the mission,” said Terry Gossett, executive director of Californians for Property Rights, in Moss Beach. “At some point they should stop buying land, and put the emphasis on maintaining what they have.”

At the current rate of spending, with moderate property tax growth, the district estimates it will have only $1 million a year by 2017 to buy land, down from $13 million this year. There are three primary reasons:

First, after the Silicon Valley housing bubble burst, property tax revenues went flat. Residents of northern Santa Clara and southern San Mateo counties pay $17 per $100,000 of assessed value in property taxes to Midpen. That works out to be $85 a year for a house worth $500,000. From 2001 to 2008, those property taxes soared 64 percent, from $16 million to $26.3 million. But over the past three years, they have stagnated at $27 million a year.

Second, the district has increased debt, selling revenue bonds every few years to raise money to make big land deals, like the former Alma College property it purchased across from Lexington Reservoir or Driscoll Ranch near La Honda. This year it will pay $9.7 million in interest and other debt service, up more than 50 percent from a decade ago.