"In 1980, when Ronald Reagan famously said that most air pollution was caused by plants, most people guffawed. Yet he may have been onto something. Scientists have since learned that some flora do produce volatile organic compounds (VOCs) that contribute to smog, and now a study has found that widespread land-management and forest-growing practices are making the problem worse by allowing certain VOC-emitting species to thrive. "
Smoggy Forests - Popular Science: