Seed Germination Ecology of Two Mesic Woodland Winter Annuals, Nemophila aphylla and Phacelia ranunculacea (Hydrophyllaceae)

Abstract

Seeds of Nemophila aphylla and Phacelia ranunculacea (Hydrophyllaceae) were dormant at maturity in May and required high summer temperatures to afterripen enough to germinate at autumn temperatures in autumn. Seeds buried in soil in a nontemperature-controlled greenhouse in May 1989 germinated to 14-64% in light and to 7-51% in darkness at simulated autumn (15/6, 20/10-degrees-C) temperatures in October 1989. During autumn 1989, about 2 5 and 50% of the P. ranunculacea and N. aphylla seeds, respectively, germinated while buried in soil in the greenhouse. Seeds of P. ranunculacea exhibited an annual dormancy/nondormancy cycle during the next 24 months of burial, with only a few buried seeds germinating in the autumn of 1990, while those of N. aphylla that afterripened during the summer of 1990 germinated in the soil that autumn and did not re-enter dormancy. Thus, seed banks in the latter species are attributed to lack of afterripening and not to the ability of nondormant seeds to re-enter dormancy. Seeds sown on soil in the greenhouse in May 1989 germinated in the autumns of 1989-1991. Since seeds only germinated in autumn, the species are obligate winter annuals. Both species form persistent seed banks. Some seeds that germinated in soil samples from N. aphylla population sites were at least 2 years old, while some in two sets of samples from a P. ranunculacea site were 3 years old.