sentence2_structure: Fires<>mainly caused<>by lightning$$$Fires, mainly caused by lightning<>are<>a natural disturbance in temperate grasslands
sentence1: Fires, mainly caused by lightning, are a natural disturbance in temperate grasslands.
sentence2: Fires, mainly caused by lightning, are a natural disturbance in temperate grasslands.
question: Fires, mainly caused by what, are a natural disturbance in temperate grasslands?