RELATIONSHIP BETWEEN SYNAPTOSPERMY AND DIOECY IN THE LIFE HISTORY STRATEGIES OF BUCHLOE DACTYLOIDES (GRAMINEAE)

Buchloe dactyloides (Nutt.) Engelm. is a perennial dioecious grass, exhibiting synaptospermy in that the 1–5 seeds in the dispersal unit (bur) are not released prior to germination. A major positive effect is that at least 50% of the burs contain both a male and female, making sexual reproduction possible and reducing founder effects upon colonization by one or a few burs. The objectives of this research were to examine the relationships between synaptospermy and dioecy in the life history strategies of Buchloe dactyloides, and to investigate whether the potential costs of synaptospermy would exceed its benefits if the species were not dioecious. An experiment that tested the effects of tightly clumping three seedlings vs. a uniform spacing of single seedlings at two densities and two moisture levels indicated only slightly negative effects of this aspect of synaptospermy. Although number of tillers and shoot biomass per plant were lower for clumped seedlings at all density and moisture combinations, only 50% of the possible uniform vs. clumped comparisons were significantly different. In addition, these effects, even at low moisture and/or high density, were not great enough to affect survivorship during the 16-week growth period. Experiments under both optimal and low moisture conditions showed that seeds within burs germinated at a significantly higher percentage than isolated or “hulled” seeds; in contrast to the differential timing of germination from burs, almost all of the seedlings from hulled seeds were produced during the first of three germination periods. Although survivorship for these seedlings was quite high under optimal conditions, it was greatly reduced under low moisture, being only one-fourth of that for seedlings arising from burs. The possible reduction in the wind dispersal of propagules due to their teeth and to their greater weight and size appears offset by more effective ungulate and water dispersal. Considering the evidence that the bur anchors the seedling in the soil, protects the seeds from fire, reduces precocious germination by requiring considerable moisture for saturation, and enhances seed longevity, the benefits of synaptospermy appear to outweigh the costs, even if those benefits related to dioecy are not considered.