Biased Sex Ratios

Polar and warm deserts are marginal areas that are notable for a marked presence of dioecious species. In the Arctic, although the number of dioecious species is not significantly greater than the global average of 2-3%, the extent to which they provide a large degree of ground cover is very noticeable due to the ecological success of the various species of dwarf willow that grow at high latitudes. Observations of sex ratios in dioecious species often record a bias. A preponderance of males is commoner than a bias towards females (Lloyd, 1974). Nevertheless, female-biased sex ratios although less common are very frequently observed in arctic willows (Crawford & Balfour, 1990; Fig. 4.33). The advantages of dioecy are most frequently discussed in terms of the advantages of avoiding inbreeding. When closely related pairs of species are compared in which one member of each pair is dioecious then a greater allelic diversity and heterozygosity can be demonstrated in the dioecious species (Costich & Meagher, 1992). However, dioecy also allows the possibility of morphological and physiological specialization between pollen and seed bearing parents. These differences need not be confined to sexual organs, as separation of the sexes automatically allows for the differentiation of secondary sexual features in plants as well as in animals.

Studies of the form and function of non-floral parts of dioecious species have drawn attention to the role of physiological properties such as growth rate, source-sink relationships (Laporte & Soule, 1996), leaf demography, and gas exchange in influencing the relative extent of sex ratio bias (Dawson & Bliss, 1993; Marshall et al, 1993). In many cases these physiological differences can be interpreted as possible cause for differential mortality between the sexes and thus account for the bias in their

Fig. 4.33 Distribution of some female-biased sex ratios as observed in a number of arctic and subarctic species of willow Salix polaris, S. arctica, S. glauca, S. myrsisinifolia, S. repens (Crawford & Balfour, 1990).
Fig. 4.34 Sex distribution in stands of balsam poplar (Populus balsamifera) at Richmond Gulf in subarctic northern Quebec. (Reproduced with permission from Comtois et al., 1986.)

Fig. 4.35 Distribution of sex ratios in young and old colonies of creeping willow (Salix repens) in an actively accreting dune and slack system at Tentsmuir National Nature Reserve (Fife, Scotland) as observed in 1989. The dates refer to coastline changes. Note that the younger colonies of creeping willow did not begin to establish until the late 1960s. Re-examination of the younger colony in 1999 found a sex ratio similar to that in the old colonies. (Reproduced from Crawford & Balfour, 1990.)

Fig. 4.35 Distribution of sex ratios in young and old colonies of creeping willow (Salix repens) in an actively accreting dune and slack system at Tentsmuir National Nature Reserve (Fife, Scotland) as observed in 1989. The dates refer to coastline changes. Note that the younger colonies of creeping willow did not begin to establish until the late 1960s. Re-examination of the younger colony in 1999 found a sex ratio similar to that in the old colonies. (Reproduced from Crawford & Balfour, 1990.)

sex ratios. In marginal areas such as semi-deserts and in the Arctic and northern limits of the boreal forest examples can be found of distinct microhabitat preferences between the sexes of dioecious plants. In northern Quebec, the balsam poplar (Populus balsamifera) grows as small stands which are normally unisexual and either poly or monoclonal (Comtois et al., 1986). Although there was an overall equal male-female sex ratio there was a non-random distribution of the sexes in relation to habitat conditions (Fig. 4.34) with more female stands predominating in maritime regions. Conversely, elevation gradients in adult sex ratios with a predominance of male clones at high elevations have been observed in Populus tremuloides (Grant & Mitton, 1979). In arctic willows there is frequently a bias towards females (Fig. 4.33) irrespective of species or geographic location (Crawford & Balfour, 1990). Contrary to intuitive expectations (Harper, 1977), the burden of bearing seed does not impair the growth of the female sex in arctic willows. On the contrary, in those cases where growth has been measured in terms of annual growth rings, the female plants have shown themselves to be superior to male plants A differentiation between the sexes in either form or physiology raises the question as to whether the different sexes are opposed in their use of resources. In subsequent studies of arctic willows it was shown that in these female-biased populations the female plants were especially common in wet, higher nutrient, cold habitats, while the male plants predominated in the more xeric, low nutrient habitats with higher soil temperatures and greater drought exposure (Dawson & Bliss, 1989a,b). Similarly, in a study of five diverse dioecious wildpollinated species in Utah it was found that male plants were more predominant in the drier microsites (Freeman et al., 1976). The species included salt grass (Distichlis spicata), meadow rue (Thalictrum fendleri), box elder (Acer negundo), the gymnosperm desert shrub Mormon tea (Ephedera viridis), and saltbush (Atriplex confertifolia).

The causal mechanisms for these common trends with other examples of changes in sex ratio with habitat are still not clear. It cannot be substantiated that the

Male Biased Sex Ratios

Fig. 4.36 Sea buckthorn (Hippophae rhamnoides) in fruit in autumn on a dune system in east Scotland. This is one of the few species outside the family Fabaceae which has nitrogen-fixing root nodules. The species is dioecious and as colonies become mature the female plants shade out the males and fruit production ceases. The species also spreads vegetatively with underground stolons (see Fig. 7.46). Inset shows details of fruits and foliage.

Fig. 4.36 Sea buckthorn (Hippophae rhamnoides) in fruit in autumn on a dune system in east Scotland. This is one of the few species outside the family Fabaceae which has nitrogen-fixing root nodules. The species is dioecious and as colonies become mature the female plants shade out the males and fruit production ceases. The species also spreads vegetatively with underground stolons (see Fig. 7.46). Inset shows details of fruits and foliage.

individuals in the separate sexes are themselves more fit in their respectively preferred microsites. In some cases the sex of the plants may be labile causing one sex to be more common in one habitat type than the other. Labile sex appears to be more common among dioe cious and monoecious flowering plants than among hermaphrodites. The majority of plants with labile sex expression are perennials, which indicates that flexibility in sex is more important for species with long life cycles. Environmental stress, caused by less than optimal light, nutrition deficiencies, adverse weather or water conditions, often favours the development of male organs (Korpelainen, 1998). Alternatively, there can be a differential use of resources resulting in differing survival probabilities for the two sexes in the different microsites. Irrespective of the proximate cause of the differences in sex ratio it has been argued that for the marginal conditions of the sites in Utah, the male plants would be in a better position to disseminate their pollen from dry windy ridges while the reproductive effort of female plants would be favoured by the longer favourable moisture regime of depression sites. For creeping willows (Salix repens) it has been noted on a sand dune site in eastern Scotland (Tentsmuir National Nature Reserve) that on newly accreted land which had been colonized within the last 20 years (Fig. 4.35) the willows had a 50:50 sex ratio whereas the older colonies had a preponderance of females (Crawford & Balfour, 1990). It would appear that in this case at least, the females had a superior competitive power than males. A similar phenomenon is seen in sea buckthorn (Hippophae rhamnoides; Fig. 4.36) where the female plants are able to shade out the males with the result that mature colonies consist entirely of female plants isolated from a pollen source and where the lack of regeneration results in the ultimate demise of the colony.

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