Demography of Two Natural Populations of Musk Thistle (Carduus Nutans)

Abstract

(1) Mortality and fecundity were determined for two populations of musk thistle (Carduus nutans) for the period June 1975 to May 1977. The populations occurred in old-fields but differed in density and competing vegetation. (2) Three life-cycle patterns were observed: winter annual; spring biennial; and autumn biennial. Greatest mortality occurred in summer and late spring. There were significant differences between the populations in the time of germination, mortality, rosette growth rates, time to flowering, and seed production. (3) Generally those cohorts that germinated after autumn 1975 were slower growing, had higher rates of mortality, took longer to flower and produced fewer seeds per individual. (4) The autumn 1975 cohort was divided into September, October and November germination cohorts. The October and November cohorts had lower rosette growth rates, lower survivorship, flowered later and produced fewer seeds per individual. (5) Survivorship, flowering and seed production were all closely associated with rosette size. Larger plants were more likely to survive and to flower, and they produced more seed. (6) Early autumn germination with competition-free growth favours high survivorship and early flowering. Variation in the demography of musk thistle can best be explained by phenotypic plasticity rather than genetic causes.