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Kautokeino, Norway

Ravolainen V.T.,University of Tromso | Yoccoz N.G.,University of Tromso | Brathen K.A.,University of Tromso | Ims R.A.,University of Tromso | And 2 more authors.

Large herbivores can change ecosystem functioning by impacting plant diversity. However, although such impacts are expected to be scale-dependent in ecosystems with wide-roaming ungulates, scaling issues rarely enter empirical assessments. We here test the hypothesis that the impact of increased reindeer abundance on plant diversity in alpine tundra is scale-dependent. Based on potentially high productivity of the focal habitat units and hence the possibility of positive grazer impacts on plant diversity we predicted higher α and β diversity at the habitat scale where reindeer densities are high. We also explored whether there were differences in diversity patterns at larger scales, including the scale of reindeer management districts. We estimated grazing disturbance as high versus low reindeer density in selected districts (a total extent of 7421 km2) of Northern Norway where reindeer-induced vegetation shifts are debated. We focus on dominance patterns because they can quantify the vegetation state and thus performed additive partitioning of Simpson diversity on multiple scales assessing also species' contributions to diversity. Contrary to our predictions, we found only weak scale-dependent effects of reindeer grazing on plant diversity. Under high reindeer densities there was evidence for a landscape-scale homogenization of the vegetation, but the predicted α and β diversity increases at the habitat scale were not found. Consistently through all scales considered, four shrub species contributed the most to plant diversity. These results contradict the idea that reindeer at high stocking densities induce shifts in plant species dominance in productive habitats. We conclude that context-dependencies such as spatial scales of management units and habitat types need to be explicitly considered in evaluations of the impacts of large ungulates on plant diversity. © 2010 Springer Science+Business Media, LLC. Source

Gonzalez V.T.,University of Tromso | Brathen K.A.,University of Tromso | Ravolainen V.T.,University of Tromso | Iversen M.,Sami College | Hagen S.B.,University of Tromso
Plant Ecology

Grazing constitutes a selective pressure on vegetation recruitment through modification of the seed banks. Here we address changes in seed bank density and its life history trait composition in century-old pastures, where contrasting reindeer densities have developed during the last decades. We cover the actual scales used by these wide-roaming herbivores by sampling 70 productive tundra habitats over 7,421 km2 in the reindeer summer pastures of Northern Norway. Results showed that the seed bank density was significantly lowered where reindeer densities had increased in recent decades, whereas the century-long history of grazing probably explains the main seed bank traits typical of grazing tolerant plants. The dominant trait characteristics were small seeds (<0.5 mg), seeds lacking dispersal mechanism and individuals with a graminoid growth form. Finally, differences between regions introduced trait variation independent of grazing history, reinforcing the importance of using scales covering several contexts when designing ecological studies. © Springer Science+Business Media B.V. 2009. Source

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