WITHIN-POPULATION VARIATION IN DEMOGRAPHY OF TARAXACUM-OFFICINALE - SEASON-DEPENDENT AND SIZE-DEPENDENT SURVIVAL, GROWTH AND REPRODUCTION

1 Lefkovitch transition matrices were used to determine vital demograp hic rates of a natural population of Taraxacum officinale in Morgantow n, WV, USA. Separate size transition matrices were calculated for each of four seasons, October-January, January-April, April-July, and July -October, to test if demographic rates vary as a function of season an d if size-specific rates vary differentially among seasons. Season-dep endent demography was also compared for four phenotype classes segrega ted by cluster analysis of leaf morphology. 2 The finite rate of incre ase for the entire population was largest in autumn (October-January) and declined throughout the rest of the year. Overall, there was a sma ll reduction in the population size. Size-specific probabilities of su rvival, growth and fertility varied dramatically among seasons. Sensit ivity analyses showed that small individuals were particularly importa nt to population growth from autumn to spring. Larger individuals were more important during summer. 3 Highly season-dependent demographic r ates have large implications for population distribution and persisten ce since increased vulnerability to perturbation during particular sea sons may constrain population growth and stability. Although T. offici nale is a long-lived perennial, annual censuses may mask the importanc e of certain individuals or life history traits for maintenance of gen etic variability and population viability. 4 Seasonal and annual finit e rates of increase also varied as a function of phenotype class. Of t wo phenotype classes which had identical annual growth rates, one grew better in cool seasons while the second performed better in warm seas ons. Direct competition for resources should be reduced by such invers e patterns of demography across seasons. 5 If phenotype classes are to some degree genetically determined, the differential responses observ ed here suggest that temporal variation in the environment could expla in the maintenance of genetic diversity within populations.