Lightning was far more damaging back then. The Carboniferous period is associated with the formation of large deposits of charcoal, which have been linked to massive wildfires. Part of the reason is that this period had ideal conditions for fires to start. Not only was the oxygen concentration at a record high, but there were also huge deposits of wood that could serve as fuel. Lightning then often acted to close the fire triangle by providing the spark needed to ignite the fuel. Quoting the introduction to this paper:
Fusain occurs widely in Carboniferous coals and sediments. It is now recognized to represent charcoal and be the product of wildfire. The occurrence of fire is partly constrained by atmospheric oxygen levels, availability and nature of fuel and by aspects of climate (rainfall and seasonability in particular). The majority of fires in the Carboniferous were probably started by lightning strikes or by volcanic activity.