IIntroduction: Plant dispersal directly depends on reproduction success, and hence,on sexual systems. In bryophytes, wherein fertilization involves a continuous film ofwater between male and female sexual organs, reproduction in unisexual speciesinvolves the sympatric distribution of male and female sex-expressing individuals.Here, we determine whether these conditions are controlled by the environment. In particular, we test the hypotheses that (i) sex-expressing males and females exhibitdifferent ecological niches and (ii) environmental variation drives sex expression,sporophyte formation, and hence, dispersal capacities.

Methods: We scored 1,080 specimens of the unisexual moss Abietinella abietinaacross Sweden as non-sex expressing, expressing female or male, orsporophytic. We tested whether reproductive stages were related to latitude.Topography and climatic conditions at 1-km resolution were employed tomeasure niche overlap between (i) sex-expressing and non-expressing and (ii)male and female specimens. We finally modelled sex expression and sporophyteproduction depending on these topo-climatic predictors.

Results: Among the 63% of reproductive samples across the entire latitudinalgradient, females outnumbered males by a factor 5.6, and 8% of the femalesamples bore sporophytes. Although the distribution of the sexes was notexplained by topo-climatic variables, the probability of sex-expressing samplesbeing male increased with latitude. It resulted in a higher regional sex ratio in theNorth than in southern regions. Successful sexual reproduction, in terms ofsporophyte occurrence, was confined to central Sweden. It was predicted byintermediate to increasing precipitation seasonality and intermediatetemperature values.

Discussion: Despite a high level of sex-expression, and no significant differencesof niche preference between males and females, sporophyte occurrences wererare. Our results suggest that sporophyte formation was determined by mateavailability and macro-climatic conditions, the latter possibly affectingfertilization success. We further infer that environmental conditions at the prezygoticstage have lower than expected effects on the overall distribution of thismoss. Modelling environmental data at higher resolution, smaller scale and expanding geographic coverage to include more sporophyte occurrences, andcomparing genetic diversity in sporophytic with non-sporophytic populations,are future lines of this research.