Reindeer grazing induces spatial and functional shifts in root systems of boreal pine forests

The spatial distribution of roots, laterally and vertically, is critical for nutrient and water uptake and for driving root‐associated carbon inputs and turnover. Large herbivores, such as reindeer, influence ecosystems belowground by altering aboveground vegetation composition and soil microclimate. However, how these herbivore‐driven changes translate into shifts in root distribution and functional traits remains largely unexplored. We compared root systems in northern boreal Scots pine forests with and without > 50 years of reindeer exclusion. Specifically, we assessed how grazing affects the spatial distribution of root biomass and traits (root length density, specific root length, root tissue density, diameter, and branching frequency) across soil depths and distances from trees, in relation to grazing‐induced changes in vegetation and soil microclimate. Fine root biomass declined with soil depth but not distance from trees; grazing reduced biomass predominantly in the top 5 cm by 28% (−39 g m ⁻² ), leading to a total reduction of 24% (−59 g m ⁻² ). Grazing caused a functional shift toward thicker, shorter, and less‐branched roots, especially near the soil surface and farther from trees. For example, in the top 5 cm depth, specific root length increased from 37 to 48 m g ⁻¹ between 1 and 3 m from trees without grazing, but only from 17 to 19 m g ⁻¹ under grazing. These shifts coincided with lower aboveground ericoid shrub biomass and amplified soil temperature extremes. Reindeer grazing decreased fine root biomass and modified the functional composition of root systems and soil microclimatic conditions across the forest floor. Together, these changes suggest that root‐mediated processes in boreal forests could be susceptible to heavy grazing pressure, particularly at the soil surface and open spaces, with potential implications for carbon and nutrient dynamics.