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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.