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
Other kinds of rare lightning also occur with unusually high frequency in the vicinity of forest fires.