Sexual Differences in Biomass and Nutrient Allocation in the Dioecious Rubus Chamaemorus

Abstract

I compared the seasonal pattern of biomass distribution and amounts of N and P in male and female ramets of the dioecious perennial herb Rubus chamaemorus L. Two populations, one in an open and one in a shaded habitat, were studied for 2 yr. The cost of fruit production in terms of reduction in vegetative growth was estimated in a field experiment. Ramets were larger, but the proportions of aboveground biomass, N, and P that were allocated to reproduction were lower at the shaded site than at the open site. The annual aboveground shoot of R. chamaemorus is either single—flowered or nonfloral. Female flowers had a lower dry mass and nutrient content than had male flowers. However, the ripe fruit had a dry mass, and a content of N and P, several times as great as those of male flowers. Fruit production interfered with vegetative growth; fruit—producing female ramets produced smaller leaves, and began producing rhizome branchlets later, than male ramets and than female ramets whose flowers had been excised to prevent fruit development. Fruit—producing female ramets had a higher mortality, and a lower probability of flowering in the subsequent year, than had male ramets. The sexual differences in vegetative growth and ramet mortality were smaller in the shaded than in the open habitat. In the shaded habitat, $\approx$50% of the ramets on male clones, and $\approx$30% of the ramets on female clones, were floral. The results suggest that male reproductive effort is greater than female up to the time of flowering, but that the total female allocation to reproduction is greater than the total male even at relatively low levels of fruit set. The predominance of R. chamaemorus populations with male—biased floral sex ratios is proposed to be the result of males having both a greater competitive vigor and a higher flowering propensity than females.