Antietam received about half of the $360,000 park officials requested from the federal government to beef up security at the battlefield by hiring three new rangers, buying more security-related equipment and establishing a security training base, Howard said.
The nation's elevated threat level costs national parks nationwide about $65,000 per day to provide extra security, said Mel Poole, superintendent at Catoctin Mountain Park in Thurmont, Md.
Security concerns are especially high at Catoctin, which surrounds the Camp David presidential retreat. Catoctin rangers help provide security for the camp's high-profile visitors.
The U.S. Secret Service requested that portions of Catoctin be closed to the public on 153 occasions since the terrorist attacks on Sept. 11, 2001, Poole said. Many of those closures were on weekends, the park's busiest time.
At the C&O Canal National Historical Park, which runs about 185 miles along the Potomac River from Cumberland, Md., to Georgetown in Washington, D.C., all park employees are paying close attention to the bridges, water plants and other structures on or near park property, Assistant Superintendent Kevin Brandt said.
To ensure adequate security despite budget shortfalls, vacant law enforcement positions have been filled more quickly than open jobs in areas such as park maintenance, Brandt said.
As a result, he said, tasks such as removing the tree roots that threaten the structural integrity of the dozens of stone culverts along the canal have been shelved. Towpath hikers might encounter detours in areas where the culverts' stone masonry is unstable, Brandt said.
Harpers Ferry National Historical Park in Jefferson County, W.Va., has also strengthened security in response to the heightened homeland security threat alert - reduced last week from orange, the second-highest level, to yellow - but park visitors might not notice the change, said Marsha Starkey, the park's public information officer.
"Our main concern is that visitors to the park feel safe, but we don't want them to feel like they're in an armed camp," she said.
Park visitors are likely to note the absence of the park's annual Freedom's Birth fireworks display. The annual late June event is among the programs and services that will be cut at Harpers Ferry this year due to increased security costs and other factors affecting the park's nearly $6 million operating budget, Starkey said.
Such sacrifices must be made to keep the park open, she said.
"So far, it looks like we're going to make it through the year - by the skin of our teeth," Starkey said.
The National Park Service gets only two-thirds of the funding needed to maintain national parks properly - an annual shortfall of more than $600 million, said Thomas Kiernan, president of the National Parks Conservation Association. The association is a watchdog group based in Washington.
National parks are "ill-equipped to confront threats such as habitat destruction and dwindling wildlife populations, and unable to serve satisfactorily the countless visitors and thousands of schoolchildren who request educational opportunities," Kiernan said on March 13.
Harpers Ferry faces a $3 million annual funding shortfall - money that's needed to protect the park's natural resources and historic structures and provide visitors with educational tours and other services, said Joy Oakes, the National Parks Conservation Association's mid-Atlantic regional director.
Gettysburg (Pa.) National Military Park faces a budget shortage of more than $3.5 million, said Katie Lawhon, public affairs specialist at the park.
"The park does have some funding problems, but visitors this summer shouldn't see anything different," Lawhon said.