Sex expression and growth rates in natural populations of the desert soil crustal moss Syntrichia caninervis

The low elevation Mojave Desert cryptobiotic crust is dominated by the moss Syntrichia caninervis. In 16 populations of this moss, stem height and pop ulation were significantly associated with sex expression, with longer stem s expressing sex more frequently and producing more perichaetia. The above- ground age of stems ranged up to 10 years, with a mean growth rate across p opulations of 0.36 mm year(-1). The overall stem sex ratio, as a proportion of the total number of stems, was 0.30 female : 0 male : 0.70 non-expressi ng. The probability of expressing sex in a given season was 0.09. The absen ce of sexual reproduction (no sporophytes) and likely absence of the male s ex from the area indicate that the non-expressing plants are female individ uals. The slow growth rates, low rates of sex expression, absence of male p lants, and absence of sexual reproduction in this crustal species may help explain why re-establishment of mosses on desert soils can take decades. (C ) 1998 Academic Press.