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Dry and unproductive Scots pine forests in the northern boreal forest zone provide conditions where ground-dwelling lichens can thrive. These lichens are a crucial winter forage for reindeer. Reindeer reduce lichen biomass both by consumption and trampling, leading to cascading effects on microclimate, litter inputs, and soil carbon dynamics. To investigate long-term impacts of reindeer exclusion, we assessed the plant biomass, soil nutrients, and soil organic carbon (SOC) quantity and quality in three dry pine forest sites in northern Finland, where reindeer have been excluded for over 50 years. Within exclosures, lichen biomass was 4.5 times higher and dwarf shrub biomass nearly doubled compared to grazed controls. These vegetation changes were associated with a 33% higher soil moisture and 0.8 °C lower summer soil temperatures at 8 cm depth beneath the thicker lichen mats. The organic layer SOC stock was 28% higher in exclosures. Spectroscopic analysis using ¹³C NMR spectroscopy revealed 9.5% higher O-alkyl carbon (carbohydrates) content and lower methoxy (–5.1%), alkyl (–6.9%), and carbonyl (–8.7%) content, reflecting a younger, more labile carbon pool. Despite these changes, soil nutrient concentrations and C∶N ratios remained unchanged between treatments, suggesting that altered SOC storage and composition result primarily from increased litter inputs and microclimatic changes.  We conclude that in lichen-rich boreal forests, long-term reindeer exclusion enhances SOC stocks and promote a shift toward more decomposable soil organic matter, with potential implications for carbon stability following disturbance. 

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.

Boreal forests provide important habitat for biodiversity, store vast amounts of carbon, and support a range of ecosystem services, including forestry and reindeer herding. Across northern Fennoscandia, reindeer (Rangifer tarandus) grazing overlaps extensively with forestry, creating both ecological and cultural tensions. Yet, the impacts of reindeer on the structure, function, and carbon dynamics of boreal pine forests remain poorly understood.

My thesis investigates how reindeer influence forest structure and dynamics in boreal pine (Pinus sylvestris) forests. Using detailed field measurements from long-term reindeer exclosures in lichen-rich pine forests across broad climatic and productivity gradients, I quantified reindeer impacts on soils, vegetation, saplings, and mature trees, linking these to variation in microclimate, nutrient availability, and forest composition.

Reindeer reduced lichen cover and biomass by more than 50%, which in very dry, lichen-dominated forests led to warmer and drier summer soils and an earlier spring thaw. These microclimatic shifts, together with reduced litter input, were associated with 22% lower soil organic carbon stocks, although the remaining carbon was chemically more stable. Reindeer had only minor direct effects on forest regeneration, which was primarily shaped by changes in field- and ground-layer vegetation. Effects on tree growth and stand biomass depended strongly on forest structure: winter grazing in older forests increased stand biomass, whereas grazing in younger forests mainly caused declines.

Overall, these findings demonstrate that reindeer effects on carbon storage and tree dynamics are highly site-specific. Sustainable forest and herding management will therefore benefit from close collaboration between herders and forest owners to identify grazing strategies that align forest carbon goals with viable herding practices, directing grazing toward areas where outcomes are most beneficial for both ecosystems and livelihoods.