Sex expression, plant size, and spatial segregation of the sexes across a stress gradient in the desert moss Syntrichia caninervis

Spatial variation in population sex ratios is common in bryophytes, but the ecological correlates of this variation are not well documented. The dioecious desert moss Syntrichia caninervis presents an example of this variation occurring at the regional scale (kilometers) as well as the microscale (centimeters). Here, our goal is to correlate variation in plant traits and sex expression levels to relevant ecological variation, and specifically to investigate whether spatial segregation of the sexes (SSS) exists at the level of the patch. Five sporophytic patches of S. caninervis were sampled at 10 cm intervals along a transect extending from shrub understory into the exposed intershrub region, thus representing a stress gradient of light intensity and moisture availability. Along a gradient from understory to intershrub microsites, individual plant length and biomass declined significantly, with plants under the canopy twice as large as those in the more exposed microsites. Sex expression declined from 76% under the shrub canopy to < 5% in the intershrub region, with the number of inflorescences per individual declining from 1.5 to near zero along this transect. Of the 778 individuals sampled, only 11 males were recovered, and all of these males occurred within 15 cm of the shrub canopy line. Nevertheless, the high number of female and nonexpressing individuals coupled with a rarity of male individuals resulted in no association between gender and microsite. However, using the distribution of fertilized female inflorescences as a surrogate measure of historical male presence indicated that males were significantly clustered near the shrub canopy line, indicative of spatial SSS. We hypothesize that males are less stress tolerant than females and that this condition may be an outgrowth of the sexes allocating differential resources to sexual reproduction.