Sex ratios, sexual dimorphism and population dynamics in the dioecious shrub shepherdia canadensis (L.) nutt
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AbstractSex ratios, sexual dimorphism and population dynamics were studied in three populations of Shepherdia canadensis L. Nutt, each at different elevations in southwestern Alberta. The study sites were characterized as to soil, climate and vegetation. Shepherdia canadensis was abundant on each site with similar population densities occurring between sites. Pollination in Shepherdia canadensis is by wind or insects. Male flowers tend to be open earlier and longer than female flowers. Intersexual phenological differences were reduced on the upper elevation site which had a shorter growing season. Females on this site also flowered with greater synchrony. Male plants produced greater numbers of flower buds per inflorescence than females except on the mid - elevation site where fruit set was lowest. The low elevation population had the highest numbers of unopened flower buds. Hand pollination experiments indicated pollination limitation of fruit development may occur in later blooming flowers on the low elevation site where intersexual differences in phenology was most pronounced. Nevertheless, fruit set was highest on the low elevation site. Fruit to ovule ratios are low in Shepherdia canadensis possibly reflecting a "bet hedging" strategy in response to environmental uncertainty. Mortality of juveniles was high. Recruitment occurred only on the low elevation site where juveniles were aggregated with each other and with adults. Adults on the low and upper elevation sites were also aggregated. These patterns were not related to vegetative reproduction. The oldest population which occurred at mid - elevation had a random distribution of adults. Above-ground biomass was related to size and size was related to age in this species although the latter relationship was weak. Both relationships were strongest in the younger low elevation population. Biased sex ratios in Shepherdia canadensis were considered as a result of postzygotic influences. The sexes were not spatially segregated in the sites nor was there evidence of intersexual or intrasexual competition. Males predominated on the lower site. This was attributed to greater recruitment of males in more marginal habitat conditions. There was some evidence that some young females on this site delayed flowering until sufficient biomass was accumulated. Males were not larger than females nor did they have a lower mortality rate. All adult plants bloomed each season on the study sites. The mid - elevation population was at unity while the upper elevation population was female biased. Females in the latter population tended to be older than males and males were recruited into the population after recruitment of females failed. The female biased sex ratio may result from differences in the sexes’ ability for recruitment as the habitat changes with time. These results may indicate physiological differences between males and females, expressed at the seed, or juvenile level. Conclusions about sexual selection and sex ratios must be made within the context of temporal and spatial population dynamics. Deviations from the sex ratios of unity in Shepherdia canadensis have little effect on effective population size. Female biased populations showed less variation in seed proteins than male biased populations.
Bibliography: p. 193-208.