Reindeer induced changes in vegetation composition and plant traits on a tundra-forest border

anglais. Reindeer (Rangifer tarandus) are the most common large herbivores in the arctic. With a habitat spreading over the entire circumpolar region, they can have a substantial effect on plant species composition and ecosystem processes and functions. They affect plant species composition by trampling, removal of foliage and by fertilization through dung and urine. These alterations in species composition affect the plant traits in the plant community. Further, plants by themselves can change their traits as a defence mechanism to grazing or as a reaction to alterations in nutrient input. These alterations in plant traits can change the carbon and nutrient cycle and energy balance in an ecosystem. In this thesis, I analyse how reindeer grazing affects the plant species composition of the ground vegetation, and subsequently, the overall community traits connected to the plant composition shift. Further, I test whether grazing regime affects the traits of five plant key-species – Empetrum hermaphroditum, Phyllodoce caerulea, Vaccinium myrtillus, Vaccinium vitis-idaea, and Deschampsia flexuosa – of the ground vegetation. These questions were tested in northern Fennoscandia, on an area where a reindeer-fence separates only winter-grazed areas from also summer grazed regions. After 50 years of summer grazing, the growth of B. pubescens and B. nana was hampered to a level where almost no tall trees were found on the summer grazed side of the fence. The graminoid and moss cover was significantly higher under summer grazing. Summer grazing significantly lowered the leaf P content, whereas the leaf N content was unaffected which led to a lower N:P ratio. None of the other investigated traits – leaf area, specific leaf area, leaf:stem ratio, stem specific density, root specific density and fine:coarse root ratio – were affected by grazing regime, however, the root specific density declined with increased in B. pubescens cover and is thus indirectly affected by grazing. The obtained results show that some plant traits could be directly or indirectly affected by grazing, and so potentially alter the energy balance, carbon and nutrient cycling of an ecosystem. Yet, further studies are needed to define how and to which extent the alterations in plant traits affect the carbon and nutrient cycling of an ecosystem.

anglais. Reindeer are the most common large herbivores in the arctic. By browsing around the entire circumpolar region, feeding on the vegetation and compacting the soil they affect plant growth and vegetation composition. Changes in plant communities would induce alterations in plant properties like leaf size, nutrient and carbon allocation on a community level. This can affect the photosynthetic capacity, growth rates and litter quality and quantity of the plants. As a response to grazing, plants could also directly change their traits, for example, by investing more in root growth as a defence mechanism to defoliation and thus saving carbon and energy. Alterations in plant traits could influence the energy balance, carbon and nutrient cycle of an ecosystem. Since plant traits are important indicators of alterations of the ecosystem’s energy balance, carbon and nutrient cycle; I test how reindeer grazing affects the plant species composition of the understory vegetation, and subsequently, the overall community traits connected to the plant composition shift. Moreover, I evaluate whether grazing regime affects the traits of five plant key-species – crowberry, blue heather, bilberry, lingonberry, and wavy hair grass – of the understory vegetation. These questions were tested in northern Fennoscandia, on an area where a reindeer-fence separates only winter-grazed regimes from year-round and thus also summer-grazed regimes. After 50 years of summer grazing the growth of mountain and dwarf birches were hampered to a level where almost no tall trees or shrubs were found on the summer grazed side of the fence. Alterations in tree and deciduous shrub cover increases nutrient input to the soil due to its high litter decomposability and by enhancing winter soil temperatures by trapping snow and thus increasing the decomposition in winter. These changes affect the plant growth of the understory vegetation. Summer grazing significantly lowered the leaf phosphorus (P) content, whereas the leaf nitrogen (N) content was unaffected, this led to a lower N:P ratio. Meaning that grazing can lead to a P limitation, depressing plant growth. None of the other investigated traits were affected by difference in grazing regime, however, some traits were affected by the mountain birch cover and are thus indirectly affected by grazing. The obtained results show that some plant traits could be directly or indirectly affected by grazing, and so potentially alter the energy balance, carbon and nutrient cycling of an ecosystem. These alterations would lead to changes in feedbacks to the atmosphere and so influence the climate. Further research is needed in order to define the direction and magnitude of alterations in the energy and C balance to make global prediction on how grazing affect the climate system.