Typically during thunderstorms most lightning strikes carry a negative electric charge  only a few carry a positive charge  Thunderstorms with unusually high proportions of positive-charge strikes tend to occur in smoky areas near forest fires
The fact that smoke carries positively charged smoke particles into the air above a fire suggests the hypothesis that the extra positive strikes occur because of the presence of such particles in the storm clouds
A positive-charge strike is as likely to start a forest fire as a negative-charge strike is.