Article

Plant Ecology

, Volume 212, Issue 8, pp 1299-1312

First online:

Open Access This content is freely available online to anyone, anywhere at any time.

Effects of sheep grazing and temporal variability on population dynamics of the clonal herb Geranium sylvaticum in an alpine habitat

  • Marianne EvjuAffiliated withDepartment of Research and Collections, Natural History Museum, University of OsloNorwegian Institute for Nature Research (NINA) Email author 
  • , Rune HalvorsenAffiliated withDepartment of Research and Collections, Natural History Museum, University of Oslo
  • , Knut RydgrenAffiliated withFaculty of Science, Sogn og Fjordane University College
  • , Gunnar AustrheimAffiliated withMuseum of Natural History and Archaeology, Norwegian University of Science and Technology
  • , Atle MysterudAffiliated withCentre for Evolutionary and Ecological Synthesis (CEES), Department of Biology, University of Oslo

Abstract

An improved understanding of population-level consequences of grazing on plants can be facilitated by an assessment of grazing effects on all stages in the life-cycle. In this study, 6 years of demographic data for three populations of the perennial herb Geranium sylvaticum were analysed. We examined the effects of sheep grazing (high sheep density, low sheep density and no sheep) and interannual climatic variability on vital rates and population growth rates (λ). Grazing did not affect survival or flowering rates, but reduced rates of growth and increased rates of clonal reproduction. At the population level, high contributions from retrogression and clonal reproduction buffered reduced rates of growth and stasis, and no consistent differences in λ between populations exposed to different sheep densities were found. Instead, large between-year variability in λ, independent of sheep density, was detected, related to variation in the local summer climate. The results indicated, however, that grazing effects on λ were more severe in unfavourable than in normal years. Our study highlights that increased clonal reproduction rates functioned as a tolerance mechanism towards grazing in this herb, which forms a mechanism to explain how moderate population responses to grazing in some herbs can arise.

Keywords

Herbivory Interannual variability LTRE analysis Matrix projection models Plant tolerance