As the graphic shows, people cause most wildfires. But lightning-ignited fires burn by far the most territory.
A look at which states were hit hardest by extreme climate and weather events in 2011.
In 2006, wildfires burned seven million acres in the West, the equivalent area of five of the country's largest national parks.
In a desert state like Nevada, heavy rains promote plant growth but can lead to more wildfires.
In a forested state like Montana, hotter and drier years leave forests more susceptible to large wildfires.
Volatile precipitation patterns, with heavy rains followed by drought, can lead to ideal conditions for wildfires in some states.
Wildfires in Washington State have increased significantly in recent years, a shift that appears linked to climate change.
Number of large wildfires on forested federal lands in the Northern Rockies region, including western Montana.