Reindeer grazing keeps carbon steady in northern forests

Winter is changing across the North. Snow arrives later, sits differently, and melts sooner. New field experiments show that these shifts can alter how northern coniferous forests trade carbon with the air. 

But there’s a twist: the forests’ response depends on reindeer. Where herds still graze, winter’s whiplash barely budges the carbon balance.

Researchers at the University of Oulu in Finland ran multi-year experiments in two classic boreal sites: Oulanka in the east and Kevo in the far north. They compared places open to reindeer with fenced “exclosures” where grazing has been excluded for decades – 25 years at Oulanka and 55 years at Kevo. 

From 2019 to 2023 the team manipulated snow, creating patches with deeper and shallower winter packs to see how carbon release changed in summer.

Why winter matters for a summer sink

Northern forests hold about a third of Earth’s forest carbon. Trees are the headline, but understory plants and soils do a huge share of the work. Snow controls their winter environment. A deep, fluffy blanket insulates. Thin, crusty snow lets the cold bite deeper. 

Those winter conditions echo into spring and summer, shifting soil temperature and moisture, the pace at which microbes decompose litter, and the tug-of-war between carbon taken in by photosynthesis and carbon breathed out by soil life. 

Grazing adds another lever, trimming lichens and reshaping the understory that blankets the soil.

The team measured carbon exchange during the growing seasons of 2019–2023 in grazed areas and long-term exclosures at both sites. 

They paired those measurements with “snow treatments” that either added or removed snow to create shallow and deep packs. Then they tracked how much carbon the understory and soil released once summer arrived.

Analyzing snow and carbon release

At Kevo’s 55-year exclosure, winter snow depth mattered a lot. Shallow snow increased carbon release from the understory and soil. Deeper snow decreased it. 

In contrast, in the areas with ongoing grazing – and even in the 25-year exclosure – carbon release stayed steady no matter what happened to the snow. 

It is notable that snow depth did not affect carbon exchange in areas with ongoing reindeer grazing. 

“Altogether, this could indicate that northern coniferous forests may be relatively well resistant to short-term changes in winter climate.” said study lead author Noora Kantola, a PhD student at Oulu.

Why did only the 55-year exclosure at Kevo swing with winter snow? The researchers point to the carpet at your feet. Decades without grazing let lichen layers recover. Those mats can change how warmth and water move between air and soil. 

“It is possible that at Kevo, the substantially recovered lichen cover has influenced soil temperature and moisture conditions. Together with changing snow depth, these factors may affect soil decomposers and thereby the amount of carbon released,” Kantola continued.

Reindeer grazing limits carbon release

The broader message is that reindeer can dampen winter’s knock-on effects. By cropping lichens and reshaping understory composition and structure, grazing appears to keep summer carbon release within a narrower range, even after very different winters. 

“Our results show that reindeer grazing can buffer ecosystem functions, such as carbon exchange, under changing climate conditions,” said senior author Maria Väisänen, a postdoctoral fellow at Oulu.

That buffering speaks to more than carbon. Reindeer shape biodiversity in northern forests, from the ground layer up. 

“The lichen-mediated impacts of grazing on the understory carbon exchange of winter pastures can be significant,” added co-author Jeffrey Welker, a professor at Oulu.

Trees feel the herd, too. Grazing can influence sapling survival, crown growth, and the timing and vigor of photosynthesis. 

How snow and grazing interact 

Ongoing projects at the University of Oulu are probing how decades of grazing and climate trends together show up in tree rings and ecophysiology. 

The goal is a whole-forest view of carbon – how much is stored in wood, how much cycles through leaves and litter, and how much escapes back to the air.

Understanding how snow and grazing interact helps land managers anticipate surprises. If long-protected areas carry thick lichen blankets, winters with unusual snow may swing their summer carbon release more. 

Where herds are present, the same snow shocks may translate to smaller summer changes. Those insights could inform choices in forests where lichen cover has been altered by grazing, forestry, or both – what to expect, and where to watch.

Part of a bigger picture

These experiments are nested within EcoClimate, a system of studies probing how snow controls northern ecosystems. 

Earlier work has shown that reindeer can also blunt summer climate impacts on tundra carbon cycling. Taken together, the message is consistent: animals and winter work together to set the stage for summer function.

Winter climate change is not a one-note story for boreal forests. Snow depths and durations are shifting, and so are the carpets of lichens that blanket the ground. Where reindeer graze, those living ground layers are thinner and the system runs steadier, even after odd winters. 

Where grazing has been absent for decades, a plush lichen mat can make summer carbon release more sensitive to how winter plays out. In a warming North, the herd on the horizon may be part of what keeps the forest’s carbon heartbeat even.

The study is published in the journal Science of the Total Environment.

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