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Life History Variation of Common Mullein (Verbascum Thapsus): I. Latitudinal Differences in Population Dynamics and Timing of Reproduction
James A. Reinartz
Journal of Ecology
Vol. 72, No. 3 (Nov., 1984), pp. 897-912
Published by: British Ecological Society
Stable URL: http://www.jstor.org/stable/2259539
Page Count: 16
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(1) Twenty-four natural populations of the monocarpic perennial Verbascum thapsus were mapped and studied over 3 years. Populations were chosen at the northern (southern Canada) and southern (southern Texas and Georgia) 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. (2) Vegetative individuals colonize a disturbed site for only a few growing seasons after disturbance. However, the populations are long persistent as a dormant pool of seeds buried in the soil. (3) Many genetically maintained differences, related to latitude of origin were noted. (4) Delay of flowering until the third year was most common among northern genotypes and was a response to unfavourable growth conditions. On average, triennial plants produced only one-fifth as much seed as biennial plants. (5) Annual genotypes occurred only in the southern part of the range, at both wet and dry sites. (6) 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. (7) In the common garden, plants from southern populations began to bolt and flower earlier than those from the other populations. Texas plants also flowered for a longer period and their capsules dehisced later. (8) 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 favoured only in the long southern growing season and where survival is uncertain in the second year because of drought or competition. In more northerly climates, biennial and triennial plants are favoured because they can produce more seed in the short but predictable growing season.
Journal of Ecology © 1984 British Ecological Society