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Herbivory may offset climate change-driven treeline expansion into the tundra, but the strength of this effect is rarely quantified. This study leverages a unique semi-natural experiment involving Malla Strict Nature Reserve in northernmost Finland, where the reindeer herding regime shifted from being nearly ungrazed for several decades to being heavily grazed for the past two decades. This is contrasted by low grazing pressure in the adjacent herding district in Norway, which is separated by the border fence preventing free reindeer movement between the two countries. We aimed to quantify the effects of reindeer browsing and grazing on mountain birch treeline position and structure on both sides. We measured seedling numbers and the allometry of trees, vegetation composition, nutrient concentrations in soils and birch leaves, and radial tree growth. We found higher numbers of seedlings and saplings in the area with lower reindeer density, indicating that the treeline may be responding to climatic forcing by expanding into the tundra. Contrastingly, we observed almost no recruitment and treeline expansion in the area with high reindeer density. Furthermore, while birch leaves showed signs of nitrogen enrichment under high reindeer density, we found no differences in soil chemical composition or birch tree growth rates. Our results suggest that the high density of reindeer in Malla Strict Nature Reserve keeps the treeline in a browsing trap, thereby preventing climate change-driven forest expansion. These results are highly relevant for land management decisions that aim to preserve mountain tundra.

Alpine ecosystems harbor remarkably diverse and distinct plant communities that are characteristically limited to harsh, and cold climatic conditions. As a result of thermal limitation to species occurrence, mountainous ecosystems are considered to be particularly sensitive to climate change. Our understanding of the impact of climate change is mainly based on vascular plants however, whereas cryptogams (i.e., lichens and bryophytes) are generally neglected or simply considered as one functional group. Here we aimed to improve our understanding of the drivers underlying temporal changes in vegetation of alpine ecosystems. To this end, we repeatedly surveyed the vegetation on four mountain summits along an elevational gradient in northern Sweden spanning a 19-year period. Our results show that the vascular plant communities remained relatively stable throughout the study period, despite fluctuations in terms of ground cover and species richness of shrubs and graminoids. In contrast, both lichens and bryophytes substantially decreased in cover and diversity, leading to alterations in community composition that were unrelated to vascular plant cover. Thermophilization of the vascular plant community was found only on the two intermediate summits. Our findings are only partially consistent with (long-term) climate-change impacts, and we argue that local non-climatic drivers such as herbivory might offset vegetation responses to warming. Hence, we underline the importance of considering local non-climatic drivers when evaluating temporal vegetation change in biologically complex systems.