Varieties of Mediterranean thyme (Thymus vulgaris) produce oils with different chemical compositions, and the ones with stronger smelling compounds like phenols are more effective at deterring herbivores. Producing phenols typically comes at a cost, though, as these plants are more sensitive to freezing. But in southern France’s Saint-Martin-de-Londres basin, winters are getting warmer. Since the 1970s, the basin has seen fewer freezing nights during the cold season.
Looking at 24 populations across the basin in 1974 versus 2010, one study found an increase in the proportion of plants that produce phenolic compounds. These plants are even popping up in areas where they didn’t grow in the 1970s. Since the plant’s genes determine the chemical composition of its oils, it’s likely that genetic changes are behind wild thyme’s response to warmer winters.