Sex expression, skewed sex ratios, and microhabitat distribution in the dioecious desert moss Syntrichia caninervis (Pottiaceae)

The moss Syntrichia caninervis is the dominant soil bryophyte in a blackbru sh (Coleogyne ramosissima) community in the southern Nevada Mojave Desert, with a mean cover of 6.3%. A survey of the 10-ha study site revealed an exp ressed ramet sex ratio of 14 female:1 male (N = 890), with 85% of ramets no t expressing sex over their life span, and an expressed population sex rati o of 40 female :2 male : 1 female male (female:male: mixed-sex, N = 89), wi th 52% of populations not expressing sex. A greater incidence of sex expres sion was associated with shaded microsites, higher soil moisture content, a nd taller ramets. Shaded microsites had higher surface soil moisture levels than exposed microsites. In the exposed microhabitat. surface soil moistur e was positively correlated with ramet height but not with sex expression. Male ramets and populations were restricted to shaded microhabitats, wherea s female ramets and populations were found in both shaded and exposed micro habitats, suggesting gender specialization. The rarity of mature sporophyte s, found in 0% of the ramets sampled and in only 3% of the populations, is probably due to the rarity of mixed-sex populations. We hypothesize that mi xed-sex populations are rare because of factors relating to male rarity and that the differential cost of sex expression reduces the clonal growth cap acity of male individuals.