Long-Term Reindeer Exclusion Leads to Higher Carbon Storage and Less Recalcitrant Carbon Compounds in Boreal Forest Soils

Abstract 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 10 cm depth beneath the thicker lichen mats. The organic layer SOC stock was 28% higher in exclosures. Spectroscopic analysis using 13 C 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 in the exclosures. 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 promotes a shift toward more decomposable soil organic matter, with potential implications for carbon stability under changing grazing regimes.