Before the middle of the 20th century, most forest managers believed that fires should be suppressed at all times. By 1935, the U.S. Forest Service's fire management policy stipulated that all wildfires were to be suppressed by 10 a.m. the morning after they were first spotted. Firefighting crews were established throughout public lands, and generally staffed by young men during fire seasons. By 1940, firefighters known as smokejumpers would parachute out of airplanes to extinguish flames in remote locations. By the beginning of World War II, more than 8,000 fire lookout towers had been constructed in the United States. Though many have been torn down due to increased use of airplanes for fire spotting, three are still used each year in Yellowstone. Firefighting efforts were highly successful, with the area burned by wildfires reduced from an annual average of 30,000,000 acres (120,000 km2) during the 1930s, to between 2,000,000 acres (8,100 km2) and 5,000,000 acres (20,000 km2) by the 1960s. The need for lumber during World War II was high and fires that destroyed timberland were deemed unacceptable. In 1944, the U.S. Forest Service developed an ad campaign to help educate the public that all fires were detrimental, using a cartoon black bear named Smokey Bear. This iconic firefighting bear can still be seen on posters with the catchphrase "Only you can prevent forest fires". Early posters of Smokey Bear misled the public into believing that western wildfires were predominantly human-caused. In Yellowstone, human-caused fires average between 6 and 10 annually, while 35 wildfires are ignited by lightning.Some researchers, as well as some timber companies and private citizens, understood that fire was a natural state of affairs in many ecosystems. Fire would help clean out the understory and dead plant matter, allowing economically important tree species to grow with less competition for nutrients. Native Americans would often burn woodlands to reduce overgrowth and increase grasslands for large prey animals such as bison and elk. As early as 1924, environmentalist Aldo Leopold argued that wildfires were beneficial to ecosystems, and were necessary for the natural propagation of numerous tree and plant species. Over the next 40 years, increasing numbers of foresters and ecologists concurred about the benefits of wildfire to ecosystems. In 1963, a group of ecologists consulted by the National Park Service released a report recommending that wildfires should be allowed to periodically burn to restore the environmental balance in parks. The Wilderness Act of 1964 helped to address the role of fire as a natural part of ecosystems. By 1968, the National Park Service had adjusted its fire management policies to reflect changing attitudes. The service determined that fires that started naturally (by lightning) would be permitted to burn if they posed little risk to human life and property. The service also decreed that under prescribed conditions, controlled burns would be deliberately set to restore balance to ecosystems. Fire ecology became better understood after many forests had aged to maturity and were overdue for a large scale burn.

Question: What factors increase flammability in groups of trees?
Answer: aged to maturity

Question: What did forest managers want suppressed?
Answer: fires

Question: Who was the mascot for the campaign by U.S Forest Service to educate the public that al fires were dangerous?
Answer: Smokey Bear

Question: Which natural process can help to make more room for important species of trees to grow?
Answer: Fire

Question: What is the purpose of controlled burns?
Answer: restore balance to ecosystems

Question: Which age group mostly made up the firefighters?
Answer: young men