Ecological life history of Polymnia canadensis, a monocarpic species of the North American Temperate Deciduous Forest: Demography

A demographic investigation was conducted to assess variation in life history of Polymnia canadensis (Asteraceae), a geographically-widespread, herbaceous species of deciduous forests in eastern North America. During 19851994, 23,063 seedlings of P. canadensis were monitored at five central Kentucky study sites. Numbers at the end were: biennials, 554; triennials, 142; winter annuals, 16; monocarpic perennials, 2; tricarpic perennials (three years), 3; and dicarpic perennials (some skipped years) that matured in the first year of life, 23; in the second year, 60; and in the third year, 9. Weekly cohorts of P. canadensis generally exhibited Deevey Type III survivorship with highest seedling mortality in summer associated with low soil moisture. Wide spatial variation in life history was displayed by the fact that fall germination cohorts at dry sites generally had greater germination and survivorship than at mesic sites during seedling establishment, while the reverse was true for spring cohorts. This led to more reproductive individuals in fall germination cohorts than spring cohorts at dry sites and generally the opposite case at mesic sites. Forest shade in mesic sites caused slower growth and a greater frequency of longer-lived reproductive individuals than in open, dry sites, but it also resulted in higher survivorship than at dry sites during moderate drought. Annual population growth rate averaged across four years was not significantly different between a dry site and mesic site, indicating that despite forest shade, P. canadensis persisted in the mesic site as well as it did in the dry site. Population structure varied among years, seasons, and study sites; at two study plots, a 2-year flowering cycle of mass seeding and senescence persisted for 4 years. At a smaller scale, there was little difference in survivorship between study plots within sites or between quadrats within study plots, while in a few instances there were large differences in the number of reproductive individuals. Weekly cohorts that germinated early within seasonal cohorts had greater number of reproductive individuals than later weekly cohorts, but not consistently greater survivorship. Several droughts induced temporal variation that was as important as spatial variation. During these droughts, the population size of all cohorts that germinated prior to the droughts declined to zero at all sites, and biennials were the longest lived type of reproductive individuals. During drought, Deevey Type I survivorship was prevalent, and lack of seed rain led to dependence on persistent seed banks for recruitment in some seasonal cohorts.