Human actions such as tossing lit cigarettes, leaving campfires unattended, arson and burning debris often cause wildfires, according to the National Park Service. Natural phenomena such as lightning and lava flow also contribute, though less frequently in America. Equipment usage sometimes causes wildfires, states the Arbor Day Foundation.
Sparks from rock falls also cause wildfires. Human activity is not the leading cause of wildfires in some other countries, and it is not always the primary cause of wildfires every year. Human activities are estimated to be responsible for up to 90 percent of wildfires by the National Park Service. However, the Arbor Day Foundation gives a slightly lower estimate, stating that human actions caused more than 83 percent of wildfires during the year 2006.
Plants generally maintain a balance between water loss and water absorption; however, in relatively dry conditions or droughts, plants become dried out and more flammable. This creates conditions that increase the likelihood of a forest fire.
The amount of land burned by forest fires varies based on different factors, such as landscape, humidity, precipitation, wind, temperature and the type of flammable material in the forest fire's path. While humans caused more wildfires in America overall, fires that were caused by lightning burned more land in 2006, according to the Arbor Day Foundation.