Previous research suggests that while sexual reproduction generally increases with environmental stress it may decrease with extreme stress, at the edge of eukaryotic life. In this study, we explored the limits to sexual reproduction in an extremophile, bryophyte system to ultimately understand the processes that limit sexual reproduction. We used field data from geothermal sites at Lassen Volcanic National Park, California, to demonstrate that sexual reproduction, as measured by the number of sporophytes per shoot, decreases with increasing environmental stress. We found that the number of sporophytes per shoot is positively correlated with distance from geothermal features. When Pohlia nutans plants were transplanted to mesic conditions, high numbers of gametoecia and sporophytes were produced, regardless of where along the environmental stress gradient plants originated, suggesting that physiological stress rather than local adaptation is constraining sexual reproduction in this extremophile system. We discuss our results with respect to previous work on sex in extreme environments.