Population biology of the rare copper moss, Scopelophila cataractae

The so‐called copper mosses include a number of rare species that exhibit very broad intercontinental geographic distributions comprised of highly disjunct occurrences. In one species, Scopelophila cataractae, only the haploid gametophyte generation exists in the United States, although sporophytes occur in tropical America and in Asia. Gametophytic plants were sampled from all U.S. populations to determine what factors limit sexual reproduction. More than 50% of the plants in every population were devoid of gametangia, and no population contained plants with both male and female gametangia. Morphological differences between plants from putative male and female populations (when each was interpreted to be unisexual) suggested gametophytic sexual dimorphism, but generalized sexual differences were not maintained under common garden conditions. Experimental growth of plants on soils with low, moderate, or high concentrations of metals demonstrated extensive morphological variability, and thus genetic polymorphism, among five asexual populations. Morphological traits were also significantly plastic in response to differing soil types, but there was no evidence of differences in patterns of plasticity between sexes or populations. All populations produced higher cover area and individual plants formed larger leaves on the most highly metal‐contaminated soil. Populations varied significantly in growth on less contaminated soil, again suggesting genetic variability.