2.1 Field sampling.
We collected plants from four populations of S. cataractae (Sc1 to Sc4) in a former copper mine in Silver Hill, North Carolina (USA) in September 2016. We use the term population to refer to physically unconnected and scattered patches of this species (separated by mostly bare soil), even though the distance between these patches was very short (~20 to ~300 m). These populations were exposed to different concentrations of heavy metals in the field: the two growing in the center of the mine (Sc2, Sc3) were exposed to higher concentrations of Cd and Cu than the two growing in microhabitats located at the edges of the mine (Sc1, Sc4) 68. At every site, we sampled gametophytic tissue of S. cataractae from several moss patches.