| Summary: | Reindeer have been shown to alter boreal ecosystems by changing the vegetation cover and influencing belowground processes. By providing nutrients for plants and by exerting trampling stress on the field layer vegetation, reindeer induce shifts in the vegetation where ground-lichen are being replaced by evergreen shrubs. Nutrients also stimulate microbial activity in the soil which has been shown to increase decomposition rates, thus potentially reducing the carbon (C) stock in boreal soils. Microbial activity is potentially further increased by higher soil temperatures caused by a removal of the lichen layer which could further increase C losses from the soil. However, findings of the effects of reindeer on the C stock diverge and potential implications of changes in C quality have rarely been considered. Here I show that reindeer reduce the C stock in lichen-dominated boreal forests in northern Fennoscandia and shift the C quality towards higher recalcitrance and reduce concentrations of labile C. I found that a reduction of tree and lichen biomass by reindeer reduces soil organic carbon (SOC) pools directly through lower litter inputs. Further, the reduction of lichen biomass and increase in evergreen shrubs directly reduces the quality of SOC. My results suggest that reindeer-induced vegetation changes have a direct control over SOC quantity and quality in the study sites and that microbial adaptations to resource availability may only play a minor role in shaping SOC, since year-round low soil temperatures strongly inhibit microbial decomposition. Parts of the methods section has been written in the active form (e.g., I did .) to highlight which analyzes I conducted myself since some of the data that I used was not collected by me.
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