LIFE-HISTORY VARIATION OF COMMON MULLEIN (VERBASCUM-THAPSUS) .1. LATITUDINAL DIFFERENCES IN POPULATION-DYNAMICS AND TIMING OF REPRODUCTION

Natural populations (24) of the monocarpic perennial V. thapsus were mapped and studied over 3 yr. Populations were chosen at the northern (southern Canada) and southern (southern Texas and Georgia [USA]) limits of its range in North America, and in North Carolina and came from a range of the habitats occupied by the species. Genetic components of life history variation were studied by growing seed from several different populations in a common garden in Durham, North Carolina. Vegetative individuals colonize a disturbed site for only a few growing seasons after disturbance. The populations are long persistent as a dormant pool of seeds buried in the soil. Many genetically maintained differences, related to latitude of origin were noted. Delay of flowering until the 3rd yr was most common among northern genotypes and was a response to unfavorable growth conditions. On average, triennial plants produced only 1/5 as much seed as biennial plants. Annual genotypes occurred only in the southern part of the range, at both wet and dry sites. Longer average periods before reproduction were negatively correlated with the percentage of the ground surface covered by vegetation. Most population differences in length of these periods were maintained in the common garden. In the common garden, plants from southern populations began to bolt and flower earlier than those from the outer populations. Texas plants also flowered for a longer period and their capsules dehisced later. The pattern of a short period of population growth, followed by a long period of slow decline of the buried seed pool, causes population growth rates to be dependent solely upon the number of seeds produced and independent of the time of their production. Annual genotypes are favored only in the long southern growing season and where survival is uncertain in the 2nd yr because of drought or competition. In more northerly climates, biennial and triennial plants are favored because they can produce more seed in the short but predictable growing season.