From Alaska to the Everglades, from Maine to Maui, the United States comprises more than 2 billion acres of real estate. And one out of every three of those acres belongs to you.

As a nation that never had a monarchy, the U.S.A. has no royal preserves set aside for the king and his cronies. Since the birth of the republic, rather, any land not privately claimed has been held in trust for the American people. Today, the nation's public lands range from the tiny rings of green around traffic circles in Washington, D.C., to the Wrangell-St. Elias Wilderness in Alaska, a 9 million-acre swath of tundra that is the biggest single public parcel.

The public lands include the highest and lowest points in the nation, the country's tallest waterfall, the largest ski resorts, the headwaters of most major American rivers, and such national treasures as Yellowstone, the Grand Canyon, and the Blue Ridge Parkway. They are key sources of mineral and energy wealth, and teeming centers of biological diversity. A bio-survey of a single city park in Danbury, Conn., in 2001 found 2,519 distinct living species (including 300 types of beetles).

For residents of Nevada, where the federal government owns 83 percent of all land -- not to mention Alaska (68 percent), Utah (64 percent), Idaho (61 percent) -- public land ownership is a basic fact of daily life and a key source of political dispute. But most Americans have considerably less involvement with the public lands they own.

That's why the last Saturday in September each year has been designated National Public Lands Day, when tens of thousands of people volunteer a day of their time for cleanup, paint-up and fix-up on city, state and national lands.

The 12th annual Public Lands Day will take place tomorrow. Anybody who wants to help can volunteer at 1-800-865-8337, or www.npld.com.

"It's a wonderful way to give something back to these areas that add so much to American life," says Robb Hampton, who serves as program director of the national day for its sponsor, the private National Environmental Education & Training Foundation. The foundation says the volunteer effort is based on the Depression-era Civilian Conservation Corps.

Hampton says that about 750 public sites will have jobs for volunteers this Saturday, ranging from upgrading hiking trails to repairing bridges and removing graffiti. As land managers pay increasing attention to the preservation of native species, a key task for the volunteers will be clearing out invasive weeds and other alien plants and planting native flora in their place.

Volunteers will work in national parks, forests and wildlife preserves, at state parks across the country, and in many city parks, large and small.

Federal land managers say the need for volunteer help is particularly strong this fall, because thousands of park and forest employees have been dispatched to the Gulf Coast for emergency response in the wake of Hurricane Katrina.

The United States has held large areas of public land as long as there has been a United States. The 13 original colonies ceded about 240 million acres to the new national government at its birth (Washington, in turn agreed to assume the colonies' Revolutionary War debts). The Louisiana and Alaska purchases and other 19th-century acquisitions added more than a billion acres in public holdings.

And then Washington started giving the land away. As rail barons pushed to link the Atlantic and Pacific after the Civil War, Washington turned over 128 million acres to railroad companies -- until populist pressure stopped the free transfers in 1871.

Under the Homestead Act of 1862 -- championed by Abe Lincoln "so that every poor man may have a home" -- the government turned over 80 million acres, for a small fee, to war veterans and western settlers. The last of the homestead acts was not repealed until 1986.

Today federal land comes in the form of parks, forests, lakeshores, battlefields, monuments, military installations and the National Mall along Constitution Avenue. There is a National Historic and Scenic Trails System, a National Landscape Conservation System, and a National Wild and Scenic Rivers System. All 50 states maintain state parks and preserves, with a national total greater than 3,300.

The most stringent controls on public land are found in the National Wilderness Preservation System -- plots of virgin land where roads, structures, machines, weapons, pets and vehicles (even bicycles) are banned. When the Wilderness Act was passed in 1964, the system began with 9 million acres set aside in 13 states. Today there are 106 million acres of wilderness areas, in 44 states; Rhode Island, Connecticut, Delaware, Maryland, Iowa and Kansas have no federally protected wilderness.

The public lands produce constant political friction. The most heated current disputes surround the Bush administration's determination to open more public land to mineral and energy development.

But the burning 19th-century debate, as to whether the federal government should own land at all, seems to be settled. Under the Federal Land Policy and Management Act of 1976, Congress expressly promised "to retain federal lands in public ownership." There is no discernible movement in contemporary politics to reverse that policy.

Online sources of information about the United States' public lands:

* www.publiclands.org Maps and guidance from the Public Lands Information Center.

* www.npld.com Information on National Public Lands Day and places to volunteer.

* www.volunteer.gov To volunteer to work on public lands, on the national day or anytime.

* www.nationalgeographic.com/geographyaction/backyard Information and maps from the National Geographic Society.

* www.wilderness.net Information and maps on federally designated wilderness areas.

* www.recreation.gov Listings and guide for public recreation areas -- federal, state, and local.

Karen Geronimo, with husband Harlyn Geronimo, great-grandson of the Apache warrior Geronimo, says a blessing at the future Geronimo memorial in Gila Wilderness, N.M.