However, some researchers in the 20th century argued that this policy should be changed, on the grounds that wildfires in national parks 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.