President Obama seems most comfortable outside on an 18-hole golf course, not hunting bear in Colorado, as Theodore Roosevelt did while president in 1905, or deep-sea fishing for tarpon in the Texas Gulf, as Franklin D. Roosevelt, an avid angler, did on a getaway from the White House in 1937.

Yet as president, Mr. Obama has visited more than 30 national parks and emerged as a 21st-century Theodore Roosevelt for his protection of public lands and marine reserves. His use of the Antiquities Act of 1906, which gives a president unilateral authority to protect federal lands as national monuments, has enabled him to establish 23 new monuments, more than any other president, and greatly expand a few others.

On Wednesday, he set aside some 87,000 acres of federal land along the Penobscot River in north-central Maine as the Katahdin Woods and Waters National Monument. The action will safeguard the wild country around the 5,267-foot Mount Katahdin, the state’s highest peak. Then, on Friday, he announced a fourfold expansion of a marine monument designated by President George W. Bush off the coast of Hawaii.