Self-compatibility and Variation in Seed Production Among Ohio Populations of Federally Endangered Trifolium Stoloniferum (Fabaceae)
Abstract: Sexual reproductive success may affect the long-term persistence of rare, clonal species. To determine if attributes of sexual reproduction in Trifolium stoloniferum contribute to the rarity of this federally endangered plant, I characterized its breeding system and natural levels of sexual reproduction in small and large populations. Inflorescences bagged at the bud stage failed to produce seeds, while hand-tripped (selfed) inflorescences set viable seeds, indicating that Trifolium stoloniferum is self-compatible but does not autonomously self-pollinate. Selfed seeds germinated well and developed into vigorous plants that did not appear to exhibit inbreeding depression. Senescing inflorescences were collected from two small ( 270 ramets) and two large ( 1000 ramets) southern Ohio populations in 1996 and 1997. I determined percent fruit set, the number of initiated seeds, and overall seed quality for each sampled head to measure sexual reproduction and test for differences among small and large populations. When data were averaged across years, percent fruit set and seed initiation per inflorescence were significantly greatest in the smallest population (mean = 73.3% and 61.6 seeds, respectively) and lowest in the largest population (11.0% and 6.3 seeds). Rainfall was abnormally high during the spring of 1996, and many of the initiated seeds failed to mature normally. Seed quality was associated with population in both years. The smallest population occurred at the most open site and consistently produced the highest quality seeds, while the largest population, located in a wooded environment, had the lowest seed viability. Since neither seed production nor seed quality was positively correlated with population size, even small populations may be worthy of preservation. My results do not suggest that overall seed production is the primary determinant of rarity, although dependence on pollinator service, low seed quality in unfavorable years, and low seed initiation and viability in the largest Ohio population may contribute to the endangered status of T. stoloniferum. These factors that potentially limit reproductive success may be more closely linked to habitat characteristics than to population size.