Reindeer grazing changes the vegetation of northern pine forests below ground as well as above it. In a recent study in Finland, trees and understorey vegetation in grazed forests had nearly 30% less shallow-root biomass than those in ungrazed areas. Root structure also changed, which may affect the ability of plants to take up water and nutrients.

Changes in pine-forest root systems associated with reindeer grazing may affect forest carbon and nutrient cycling. The impacts could be significant across reindeer-grazed boreal forests worldwide, which store around one third of the Earth’s carbon, with more than half of that carbon located in soils.
The researchers collected and compared soil samples from dry pine forests in Angeli, Raja-Jooseppi and Kätkäsuvanto in Finland. The sites were either grazed by reindeer or had been fenced to exclude reindeer grazing for 50–100 years.

The study showed that reindeer grazing reduces the quantity of tree and understorey roots particularly in the uppermost 0–5 cm of soil, where most root biomass is found.

Root abundance is influenced by the quantity and composition of vegetation as well as soil temperature. In the pine forests studied, reindeer grazing had thinned the insulating lichen cover, exposing the soil to greater temperature fluctuations. In reindeer-grazed areas, winter soil temperatures were well below freezing, reaching as low as -10°C, compared with ungrazed areas, where the minimum temperature only rarely reached -5°C.

“Winter cold and frost damage in particular may help explain the reduced amount of roots in reindeer-grazed areas. Mechanical disturbance caused by reindeer, such as trampling, may also contribute to the reduction in roots,” says postdoctoral researcher Maria Väisänen of the Water, Energy and Environmental Engineering Research Unit at the University of Oulu, Finland.

The study also found that reindeer grazing changes roots, making them shorter, thicker and less branched. “The changes indicate reduced exploration for nutrients and water, as the roots became structurally less exploratory,” Väisänen says. The change may result either from root plasticity or from shifts in plant species composition. Reindeer grazing reduced both crowberry and lingonberry, which have thin and highly branched roots.

Roots take up nutrients and water and contribute carbon-rich root litter and root exudates to the soil. The amount, structure and location of roots in the soil play a central role in soil nutrient and carbon cycling.

In northern coniferous forests, roots are an essential pathway for transferring carbon from plants into the soil. Roots are estimated to account for 50–70% of the carbon entering the soil. Living roots release carbon compounds into the soil, which can stimulate microbial decomposition and the release of carbon into the atmosphere. Dead roots, in turn, form root litter in which carbon can be stored in the soil for long periods.

How was the study conducted?
The soil samples were divided into five-centimetre sections down to a depth of 25 cm. Fine and coarse roots were separated from the samples by washing, and their total amount was weighed. Fine roots were also classified according to root length, diameter and branching. The researchers also measured lichen-layer thickness, dwarf-shrub abundance, and soil temperature and moisture at the study sites.

The project was led by a research group at Umeå University, Sweden, and forms part of Johannes Cunow’s doctoral research. The University of Oulu,Finland participated in the project as a research partner.

The study Reindeer grazing induces spatial and functional shifts in root systems of boreal pine forests was published in August 2026.