In the United States, most forests have historically been affected by humans to some degree, though in recent years improved forestry practices have helped regulate or moderate large scale or severe impacts. However, the United States Forest Service estimates a net loss of about 2 million hectares (4,942,000 acres) between 1997 and 2020; this estimate includes conversion of forest land to other uses, including urban and suburban development, as well as afforestation and natural reversion of abandoned crop and pasture land to forest. However, in many areas of the United States, the area of forest is stable or increasing, particularly in many northern states. The opposite problem from flooding has plagued national forests, with loggers complaining that a lack of thinning and proper forest management has resulted in large forest fires.

National forests are managed by the U.S. Forest Service, an agency of the U.S. Department of Agriculture. The first national forest was established as the Yellowstone Park Timber and Land Reserve on March 30, 1891, then in the Department of the Interior. In 1897, the Organic Act provided purposes for which forest reserves could be established, including to reserve a supply of timber, protect the forest from development, and secure water supplies. With the Forest Reserve Act of 1891, the president of the United States is given the power to set aside forest reserves in the public domain. With the Transfer Act of 1905, forest reserves became part of the U.S. Department of Agriculture in the newly created U.S. Forest Service.

By 1907, President Theodore Roosevelt had more than doubled the forest-reserve acreage, and Congress responded by limiting the president's ability to proclaim new reserves. The National Forest System underwent a major reorganization in 1908, and in 1911 Congress authorized new additions to the system under the authority of the Weeks Act. The management goals provided by the Organic Act were expanded upon by the Multiple-Use Sustained-Yield Act of 1960 to include "outdoor recreation, range, timber, watershed, and wildlife and fish purposes" as well as for the establishment of wilderness areas.

As of September 30, 2014, the Forest Service manages a total of 192,922,127 acres (780,728.15 km²), 188,336,179 acres (762,169.48 km) of which are national forests. The additional land areas include 20 national grasslands, 59 purchase units, 19 research and experimental areas, five land utilization projects and 37 other areas. The National Forest System has an extensive and complicated history of reorganization, so while there are currently 154 named national forests, many of these are managed together as either a single forest or separate forests.

There is at least one national forest in all but ten states: Connecticut, Delaware, Hawaii, Iowa, Kansas, Maryland, Massachusetts, New Jersey, North Dakota, and Rhode Island (although Kansas and North Dakota have national grasslands). In addition, Puerto Rico contains El Yunque National Forest, the only tropical U.S. rainforest. Alaska has the most national forest land, with 21.9 million acres (8.9 million ha), followed by California (20.8 million acres, 8.4 million ha) and Idaho (20.4 million acres, 8.3 million ha). Idaho also has the greatest percent of its land in national forests, with 38.2 percent, followed by Oregon (24.7 percent) and Colorado (20.9 percent). On maps, national forests in the west generally show the true extent of their area, but those in the east often only show purchase districts, within which usually only a minority of the land is owned by the Forest Service.

Falls Creek Falls, Gifford Pinchot National Forest, WA.

Gifford Pinchot National Forest includes over 1.3 million acres of forest lands, wildlife habitat, watersheds & mountains, including Mt. Adams and the Mount St. Helens National Volcanic Monument.

Inyo National Forest

Water: In drought-prone California, the quantity, quality, and timely provision of our water is dependent on the health of our national forests. The forests supply, filter, and regulate water from upper watersheds and meadows, providing clean water throughout the year to communities, homes, and wildland habitats. Water also helps support jobs and industries that are water-dependent.

• About 1.4 million acre-feet of water per year come from the Inyo National Forest

• Or over 464 billion gallons per year

• Over 700,000 Olympic-size swimming pools

• Enough drinking water for California’s population for more than 39 years2, or

• Enough water for over 3.5 million households for a year3 How much is 464 billion gallons worth?

• Estimated wholesale market value: over $134 million4

• To Los Angeles households: over $15.3 billion5

The Ocala National Forest ls the second largest nationally protected forest in the U.S. State of Florida. It covers 607 square miles (1,570 km²) of Central Florida. It is located three miles (5 km) east of Ocala and 16 miles (26 km) southeast of Gainesville. The Ocala National Forest, established in 1908, is the oldest national forest east of the Mississippi River and the southernmost national forest in the continental U.S. The word Ocala is thought to be a derivative of a Timucuan term meaning "fair land" or "big hammock". The Ocala National Forest is in the southeastern conifer forests and the Florida sand pine scrub ecoregions. Dry, sandy areas support Florida longleaf pine sandhills and Florida peninsula inland scrub. Longleaf pine sandhills are woodlands dominated by longleaf pine (Pinus palustris). Inland scrub consists of sand pines (Pinus clausa) growing amid shrublands of evergreen oaks.

Salmon–Challis National Forest is located in east central sections of the U.S. state of Idaho. At 4,235,940 acres (6,618.66 sq mi, or 17,142.24 km²) it is one of the largest national forests in the lower 48 states and also has most of the land area of the Frank Church–River of No Return Wilderness, which is the largest wilderness area south of Alaska.

Borah Peak, the tallest mountain in Idaho, is also found here. The Wild and Scenic Salmon River weaves through the rugged terrain of the Sawtooth Mountains and Sawtooth National Recreation Area, while it flows for over 75 miles (121 km) through the forest. The Big Lost River has its headwaters within the national forest.

Challis National Forest, the more southerly of its two parts, lies primarily in Custer County, but also has major areas in Lemhi and Butte counties, as well as smaller areas in Clark and Blaine counties. It has a total area of 2,463,471 acres (3,849.17 sq mi, or 9,969.31 km). There are local ranger district offices located in Challis, Clayton, and Mackay.

Salmon National Forest, which lies to the north, is primarily located in Lemhi County, with spillover into Valley and Idaho counties. It has a total area of 1,772,469 acres (2,769.48 sq mi, or 7,172.93 km).

Sequoia National Forest is located in the southern Sierra Nevada mountains of California. The U.S. National Forest is named for the majestic Giant Sequoia (Sequoiadendron giganteum) trees which populate 38 distinct groves within the boundaries of the forest.

The Giant Sequoia National Monument is located in the national forest. Other notable features include glacier-carved landscapes and impressive granite monoliths. The Needles are a series of granite spires atop a narrow ridge above the Kern River.

The Sequoia National Forest covers 1,193,315 acres (1,864.555 sq mi; 4,829.17 km²), and ranges in elevation from 1,000 feet (300 m) in the foothills of the Sierra Nevada to over 12,000 feet (3,700 m). Its giant sequoia (Sequoiadendron giganteum) groves are part of its 196,000 acres (790 km²) of old growth forests. Other tree species include:

• Jeffrey pine (Pinus jeffreyi)

• Red fir (Abies magnifica)

• Coast Douglas-fir (Pseudotsuga menziesii var. menziesii)

• Ponderosa pine (Pinus ponderosa)

• White fir (Abies concolor)

• Lodgepole pine (Pinus contorta)

The Tongass National Forest in Southeast Alaska is the largest U.S. National Forest at 16.7 million acres (26,100 sq mi; 6,800,000 ha; 68,000 km). Most of its area is temperate rain forest and is remote enough to be home to many species of endangered and rare flora and fauna. The Tongass, which is managed by the United States Forest Service, encompasses islands of the Alexander Archipelago, fjords and glaciers, and peaks of the Coast Mountains. An international border with Canada (British Columbia) runs along the crest of the Boundary Ranges of the Coast Mountains.At 16.5 million acres, Tongass National Forest is more than half as big as the entire state of North Carolina. And while the forest is best known for its wide expanses of Sitka spruce, western hemlock and cedar, it is also geologically and climatically diverse enough to accommodate majestic glaciers and icefields. The trees themselves are remarkable for their size and longevity –some up to 800 years old.

USDA Forest Service

Tongass National Forest map

Over one-third of Tongass National Forest is designated as federal wilderness, containing habitat for a variety of wildlife including brown and black bears, mountain goats, black-tailed deer, wolverines, river otters, harbor seals and bald eagles.

Sadly, Tongass National Forest is one of America's most exploited forests, with decades of forestry clearcut scars to prove it. Currently, some of its wildest stretches of forest--called "inventoried roadless areas"--are under threat, which could lead to similar dangers for wild forests across the county.