Wildfires in the Sierra Nevada are caused by a combination of factors, including lightning strikes, human activities, and the presence of dry vegetation.

Lightning: Lightning strikes are a natural cause of wildfires, igniting dry vegetation during thunderstorms.
Human activity: Human activities such as campfires, discarded cigarettes, and accidental burning can also start wildfires.
Dry vegetation: The presence of dry vegetation, especially during periods of drought, creates ideal conditions for wildfires to spread rapidly.

The combination of these factors makes the Sierra Nevada susceptible to wildfires, particularly during the dry summer months. Wildfires can have significant impacts on the environment, including habitat loss, air quality degradation, and soil erosion.