Barriers to sexual reproduction
In a monoicous species both the eggs and sperm are produced on the same plant, whereas in a dioicous species there are separate male and female plants. In most cases of dioicous species the male and female plants need to be fairly close for successful fertilization to take place. This issue is discussed at greater length in the SEXUAL REPRODUCTION SECTION. Thus, you could expect that, all else being equal, a dioicous species would have a harder time producing sporophytes than would a monoicous species and the evidence supports that idea. In a 1950 study of the British moss flora it was found that there were 252 dioicous and 239 monoicous species. Those numbers are relatively close. However, only 148 of the dioicous species were reported to fruit frequently, compared to 232 of the monoicous species.
Free water is essential for sexual reproduction in bryophytes, since it is water that carries the sperm to the egg. In some habitats free water may be rare. Examples are deserts (or semi-deserts) and the polar and sub-polar regions. In such habitats the opportunities for sexual reproduction are likely to be extremely limited. In species where there are separate male and female plants there's an extra hurdle in that the sperm need to travel from the male to the female plants. As you'll have seen in the SEXUAL REPRODUCTION SECTION, the sperm in many species appears to travel relatively short distances. In epiphytic bryophytes sperm may be able to move several metres, for example by water flowing down a trunk, carrying sperm from upper plants to lower ones, or by the aerosol droplets of a dense mist carrying sperm through the air. However, for soil bryophytes fertilization distances are typically just a few centimetres, at most, except where splash cups are involved – in which case sperm could be dispersed a metre or so. The roles of splash cups are discussed in the SPLASH CUP SECTION.
The moss Syntrichia caninervis has separate male and female plants and is common in Californian deserts, where it has been much studied. These studies suggest that more than 99.9% of female plants are unlikely ever to produce sporophytes. What are the reasons for this? In the harsh desert conditions very few plants develop sex organs, so most plants remain infertile through much of their lives. A fertile male plant has to be within about a centimetre of a fertile female plant for a reasonable chance of fertilization. Females greatly outnumber males so the chances of a given female having a male close enough are very small. Even after successful fertilization, many females abort sporophytes fairly early since the conditions are often such that there are insufficient nutrients to support the growth of the sporophyte to maturity. Given all the obstacles to sexual reproduction, it is clear that vegetative reproduction is the dominant reproductive method in Syntrichia caninervis in the Californian desert. Specialized vegetative propagules such as gemmae are not known in this species and vegetative reproduction is by fragmentation of the gametophytes. For more about these studies see the SYNTRICHIA CANINERVIS CASE STUDY.