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Sex expression, skewed sex ratios, and microhabitat distribution in the dioecious desert moss Syntrichia caninervis (Pottiaceae)¹

² Department of Biological Sciences, University of Nevada, 4505 Maryland Parkway, Box 454004, Las Vegas, Nevada 89154-4004 USA; ³ Center for Evolution, Ecology and Behavior, T. H. Morgan School of Biological Sciences, 101 Morgan Building, University of Kentucky, Lexington, Kentucky 40506-0225 USA; and ⁴ University Herbarium, Jepson Herbarium, and Department of Integrative Biology, 1001 Valley Life Sciences Building, #2465, University of California, Berkeley, California 94720-2465 USA

ABSTRACT

The moss Syntrichia caninervis is the dominant soil bryophyte in a blackbrush (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 expressed ramet sex ratio of 14 : 1 (N = 890), with 85% of ramets not expressing sex over their life span, and an expressed population sex ratio of 40 : 2 : 1 (female : male : mixed-sex, N = 89), with 52% of populations not expressing sex. A greater incidence of sex expression was associated with shaded microsites, higher soil moisture content, and taller ramets. Shaded microsites had higher surface soil moisture levels than exposed microsites. In the exposed microhabitat, surface soil moisture was positively correlated with ramet height but not with sex expression. Male ramets and populations were restricted to shaded microhabitats, whereas female ramets and populations were found in both shaded and exposed microhabitats, suggesting gender specialization. The rarity of mature sporophytes, 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 mixed-sex populations are rare because of factors relating to male rarity and that the differential cost of sex expression reduces the clonal growth capacity of male individuals.

Key Words: bryophyte • cryptogamic crust • desert • haploid dioecy • ramet • sex expression • sex ratio • sporophyte

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