EFFECTS OF GRAZING ON TILLER SIZE AND POPULATION-DYNAMICS IN A CLONAL SEDGE (CAREX-BIGELOWII)

The clonal sedge, Carex bigelowii Torr. ex Schwein., is common in the subarctic range lands of the Icelandic Highlands, which are grazed during summer, mainly by sheep. The effects of grazing on tiller size and the population dynamics were studied by comparing grazed and ungrazed sites at two altitudes. On a tiller basis, C. bigelowii responded to grazing by decreased tiller size and reduced probability of flowering while on a unit area basis, both density and biomass increased. The moss Racomitrium lanuginosum (Hedw.) Brid., also abundant at the study sites, is not grazed but sensitive to trampling. The moss dry weight was four times greater at an ungrazed site than at a comparable grazed site. There was a negative relationship between the moss biomass and the density of C. bigelowii tillers on the grazed site. suggesting intense competition between the moss and the sedge. The past population dynamics of tillers was studied by using historical records on tillers with dead above-ground shoots. Analyses of transition matrices showed that all four populations were stable with population growth rates (lambda) at or close to unity. All populations had similar stable age distributions and reproductive values. Perturbation analysis of the matrices using elasticity indices showed that lambda was most sensitive to changes in vegetative fecundity of age class two and to the survival into that age class. The similarities in the dynamics and growth rates of the ungrazed and grazed populations predicted by the matrix model suggests that the difference in the density of C. bigelowii tillers and the moss cover must have been established quickly after that grazing intensities were changed. It is concluded that individual tillers of C. bigelowii not only tolerate relatively intense grazing within a broad scale of climatic severity, but also benefit from it through a competitive advantage.