Sex expression and sex dimorphism in sporophytic populations of the desert moss Syntrichia caninervis

Ten sporophytic populations of Syntrichia caninervis contained an average of 22 individuals cm^-2, with a sex ratio of 7.9 Female:1 Male: 3.1 Nonexpressing (N = 300). In each of the populations, female individuals outnumbered male individuals. A representative population size of 50 × 25 cm was estimated to contain 27,250 organically distinct individual plants. Given the wide disparity in reproductive investment in this dioecious species, sex-specific traits were investigated at the level of the individual in biomass, total stem length, number of ramets, number of branches, length of annual growth interval, length of longest ramet, age, number of inflorescences, number of ramets expressing sex, and consecutive seasons of sex expression. The only significant sex dimorphism recorded was in consecutive seasons of sex expression, with nonsporophytic female individuals exhibiting a higher frequency than males. However, nonexpressing individuals had lower biomass, shorter total stem length, fewer branches, and shorter ramets than males and females, and fewer ramets than female individuals. When the biomass of female and male individuals was controlled for inflorescence number, no significant differences between the sexes in biomass were found. There appears to be a threshold size for sex expression, with all individuals above 2.0 mg dry weight expressing sex (N = 108). Biomass and total stem length are strong positive correlates (r = 0.88), and individual biomass is a better predictor of the number of inflorescences produced per individual than is stem length (r = 0.85 vs. r = 0.69, respectively). Independent of stem length, individual biomass had a positive and significant relationship with inflorescence number; however, independent of individual biomass, stem length was not associated with inflorescence number. Overall sex expression was 0.74 (individual level) and 0.65 (ramet level). The functional sex ratio was assessed at the inflorescence level, and ranged from 9.7 ♀ : 1 ♂, to 14.9 ♀ : 1 ♂ over the most recent four years. Fertilization frequency was 0.69 (individual level) and 0.34 (perichaetial level). Over the last three years, 63% of all fertilized perichaetia resulted in an abortive sporophyte.