Premise of research: Natural populations of many mosses appear highly female-biased based on the presence of reproductive structures. This bias could be caused by increased male mortality, lower male growth rate, or a higher threshold for achieving sexual maturity in males. Here we test these hypotheses using samples from two populations of the Mojave Desert moss Syntrichia caninervis. Methods: We used double digest restriction-site associated DNA (RAD) sequencing to identify candidate sex-associated loci in panel of sex-expressing plants. Next, we used putative sex-associated markers to identify the sex of individuals without sex structures. Key results: We found an 18:1 phenotypic female:male sex ratio in the higher elevation, moister site, and no sex expression at the drier, low elevation site. In contrast, based on genetic data we found a 2:1 female bias in the low stress site and only females in the high stress site. The area occupied by male and female genets was indistinguishable. Conclusions: These data suggest that both differential mortality and sexual dimorphism in thresholds for sex expression contribute to population sex ratio biases, and that sex-specific life history and survival interact with environmental stress to determine the frequency of sexual reproduction in S. caninervis.