When Shelton Johnson was 5, his family took him to Berchtesgaden National Park in the Bavarian Alps. Now 52, he still remembers his sense of awe. "The mountains, the sky being so close—it affected me profoundly," said Mr. Johnson, who now works as a ranger at Yosemite National Park in California. In 23 years on the job, Mr. Johnson has been equally struck by how few of his fellow African-Americans visit the national parks. So a few years ago, he decided to write Oprah Winfrey, the American entertainment icon. "Every year, America is becoming increasingly diverse, but that diversity is not reflected in the national parks, even though African-Americans and other groups played a vital role in the founding of national parks," he wrote. "If the national parks are America’s playground, then why are we not playing in the most beautiful places in America" "The Oprah Winfrey Show" recently aired two episodes from Yosemite in response to Mr. Johnson’s appeal. The National Park Service is expanding its efforts to diversify both its guests and its work force as the agency prepares to celebrate its centennial in 2016. Surveys show that visitors to America’s 393 national parks—there were 285. 5 million of them in 2009—are mainly non-Hispanic whites, with blacks the least likely to visit. The Park Service now says the problem is linked to the parks’ very survival. "If the American public doesn’t know that we exist or doesn’t care, our mission is potentially in jeopardy," said Jonathan B. Jarvis, who took over as director of the Park Service last year. In a Park Service survey it commissioned in 2000, only 13 percent of black respondents reported visiting a national park in the previous two years. Jim Gramann, a visiting social scientist with the Park Service who is overseeing a review of a follow-up survey in 2008 and 2009 that is to be released early next year, said the gap persisted. "The demographic face of America is not reflected in national park visitation, with a few exceptions," Mr. Gramann said. But some officials acknowledge that the parks may not seem welcoming to specific ethnic groups. They cited rules that limit the number of people in picnic areas or the number of tents that can be pitched at specific sites, which can clash with the vacation style of extended Latino families. But no group avoids national parks as much as African-Americans. The 2000 survey found that blacks were three times as likely as whites to believe that park employees gave them poor service and that parks were "uncomfortable places. " Attendance tends to be more homogenously white at wilderness parks like Yosemite, where a 2009 survey found that 77 percent of the visitors were white, 11 percent Latino, 11 percent Asian and 1 percent black. Mr. Johnson said he was more likely to meet someone from Finland or Israel in the park than from an African-American neighborhood in the United States. "It’s something that’s pervasive in the culture—it doesn’t matter whether you’re Oprah or a postal worker," Mr. Johnson said.