FITNESS, POPULATION-GROWTH RATE AND FLOWERING IN CAREX-BIGELOWII, A CLONAL SEDGE

Data on the clonal sedge, Carex bigelowii, from three sites along an a ltitudinal gradient in Swedish Lapland were analysed at two levels: 1) between tiller systems within sites and 2) between tiller populations at different sites. At the tiller system level we studied the compone nts of fitness (measured as the finite rate of increase in the number of tillers), and found that the two main components (mean daughter til ler production and tiller generation time) were positively correlated at two of the sites. They are hence counteracting each other in their effects on fitness. At the same two sites, higher mean daughter tiller production was mainly caused by a decrease in the proportion of tille rs without daughter tillers, together with an increase in the proporti on of tillers with more than one daughter tiller. At the third site th e same phenomena were observed but, in addition, the proportion of til lers with one daughter tiller was slightly reduced as well. Tiller sys tems at this site also differed from the others by showing a positive correlation between fitness and variation in daughter tiller productio n. At no site was mean rhizome length of tiller systems correlated wit h fitness. At the population level, the tiller densities at the lower and intermediate sites appeared to have been stationary (no net trend) during the past 10-20 years. At the upper site, the number of tillers in the sampled quadrats had increased, either due to high mobility of the tiller systems or to an actual increase in overall population den sity. We also measured the flowering rates in the populations and foun d no major differences between the sites. No seedling establishment of C bigelowii has been recorded in this area and the costs of sexual re production, in terms of decreased vegetative reproduction, are therefo re expected to be low. This expectation was in accordance with an earl ier study finding no such costs associated with flowering and seed set .