Effects of grazing on tiller size and population dynamics in a clonal sedge (Carex bigelowii)

Carex bigelowii is common in the subarctic range lands of the Icelandic Highlands, which are grazed during summer, mainly by sheep. C. bigelowii responded to grazing by decreased tiller size and reduced probability of flowering while on a unit area basin, both density and biomass increased. The moss Rhacomitrium lanuginosum is not grazed but is 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 moss and sedge. Analyses of transition matrices showed that all four populations were stable with population growth rates (λ) 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 λ was most sensitive to changes in vegetative fecundity of age class two and to the survival into that age class. 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. -from Author