Leaf Demography and Clonal Growth in Female and Male Rumex Acetosella.

Abstract

The cost of reproduction by female plants generally exceeds that of male plants because females produce seeds and fruits in addition to flowers (Darwin 1877, Lloyd and Webb 1977, Willson 1983). If the vegetative growth of female and male plants is equivalent, then the greater female reproductive effort should result in reduced growth and/or survival. In a dioecious clonal plant that reduction might be manifest as lower ramet production, and the sex ratio of ramets should become increasingly male-biased. In contrast to this expectation the ramet sex ratio of Rumex acetosella tends to be female-biased (Harris 1968, Putwain and Harper 1972, Lovett Doust and Lovett Doust 1985), even though females allocate a greater proportion of their resources to reproduction than do males (Putwain and Harper 1972). In eight Massachusetts populations of Rumex acetosella, we found one male-biased population and a female bias at the other sites. In these latter seven sites the estimated sex ratio of genets (the genetic individuals) did not differ significantly from 1: 1 (Lovett Doust and Lovett Doust 1985). The greater reproductive effort by female plants, plus the female bias in sex ratio of ramets suggested differential growth by females. Biased sex ratios in clonal plants can be caused by a variety of factors, including, for example, differential grazing by herbivores (see Lovett Doust and Lovett Doust 1985, Agren 1987), sex-associated effects of intraspecific competition (e.g., Onyekwelu and Harper 1979, Lovett Doust et al. 1987), and differences in the numbers of ramets produced and sustained by female and male plants (Sakai and Burris 1985). We have examined and rejected the possibility that differential herbivory on male and female Rumex plants drives the ramet sex ratio to being female-biased. (In fact, although males and females suffered comparable herbivory in absolute terms, the proportion of the available foliage that was removed from females was greater [Lovett Doust and Lovett Doust 1985].) The present study was designed to explore the hypothesis, suggested by David Lloyd (personal communication), that differences in the vegetative vigor and seasonal phonology of males and females might explain the female-biased sex ratios generally found in populations of Rumex acetosella.