The Arctic is being overrun by shrubs that can access nitrogen even when the ground is frozen

Arctic tundra plants can take up nitrogen during winter, even when the ground is frozen and the landscape appears dormant. 

A new study from East Greenland shows that this hidden winter activity may help explain why shrubs gain ground as the Arctic warms. It also suggests that climate models need to pay closer attention to what happens during the long cold season.

A hidden race for winter nutrients

Nitrogen is a key nutrient for plant growth. In the Arctic, it is often in short supply. This means plants, roots and soil microbes compete for every small release of available nitrogen.

 The new study followed that competition from the moment tundra soils began to freeze through to late winter. It looked at tundra heath from four sites along an East Greenland climate gradient, from High Arctic conditions in the north to milder Subarctic conditions in the south.

 iC3 researcher Laura Helene Rasmussen, lead author of the study, says the results challenge a simple view of winter.

“The cold season is not just a waiting room before spring,” Laura says. “There is still movement of nutrients below the snow, and some plants are ready to use it.” 

Credit: Laura Rasmussen (UiT, iC3).

Microbes took the biggest share

The clearest result was that microbes dominated winter nitrogen uptake. Across the experiment, microbes held far more of the tagged nitrogen than plants did. Roots and above-ground plant parts together retained only 5-8 %.

But the plant uptake still mattered for the plant competition. And it varied from north to south. Plants from the southern Subarctic site held the most nitrogen in above-ground tissues of all sites. Much of this was stored in the stems of deciduous shrubs. That could give these plants a head start when spring arrives and new leaves need to be built quickly.

The northern High Arctic plants behaved differently. They have the coldest winters and took up the least nitrogen overall. They were also most affected by a simulated winter warming event. 

In that part of the experiment, some mesocosms were moved into warmer conditions for six days in late winter. In the High Arctic samples, plants ended up with less tagged nitrogen, while microbes held more.

“That was one of the most striking findings,” Laura says. “A short warm spell did not release more nitrogen available to plants. At the coldest site, and only at the coldest site, it shifted the competition towards microbes.”

Not all shrubs behave the same

The study also shows that broad functional plant groups are not enough to explain tundra change. Evergreen shrubs, as a group, retained the most winter-released nitrogen per square metre. They could store nitrogen in leaves as well as stems.

But when the researchers looked at uptake per unit of plant mass, two deciduous shrub species stood out. Dwarf birch and Arctic willow were especially effective at acquiring winter nitrogen.

This matters because shrub expansion is one of the most visible signs of Arctic change. Many parts of the tundra are becoming taller, darker and woodier. That can affect snow cover, soil temperature, wildlife habitat, carbon storage and how much sunlight the land surface reflects.

“Our results suggest that winter nutrient access could be one more piece of the shrubification puzzle,” Laura says. “It is not only about warmer summers. The degree to which different plant species are able to access nitrogen in winter may also shape future tundra communities.”

Credit: Laura Rasmussen (UiT, iC3).

Following nitrogen through the cold season

 The team collected 34 intact tundra blocks, including plants, roots, soil and surface 

organisms, from four East Greenland sites.

These blocks were brought to climate chambers, where the researchers simulated the transition from autumn into winter. Just before freeze-in, they added a small amount of tagged nitrogen to the soil. This made it possible to track where the nitrogen went.

They then sampled soil water, microbes, roots, stems and leaves four times: in early winter, two points in midwinter, and late winter. By separating plant species and plant parts, the team could see not only whether plants took up nitrogen, but which plants did so, and where they stored it.

Why winter needs more attention

 The findings add to a small, but growing body of research showing that Arctic winter is biologically active.

This has big implications. Many models still focus mainly on the short growing season. But if plants and microbes are competing for nutrients during freeze-in, under snow and during winter warm spells, then winter processes may affect spring growth, carbon uptake and long-term vegetation change.

For policy makers and land managers, the study reinforces the need for year-round Arctic monitoring. Rapid winter warming, rain-on-snow events and unstable snow cover can all alter the way nutrients move through ecosystems – and thereby which plant types that compete better.

Find out more

The study “Winter climate or plant functional type? Controls on tundra winter nitrogen uptake along an Arctic climate gradient in East Greenland” is available open access in Plant and Soil.

Lead author Laura Helene Rasmussen is an MSCA postdoctoral fellow at the iC3 Polar Research Hub. Her work focuses on nitrogen dynamics in Arctic landscapes, especially how nitrogen is released, transformed and transported through soils, sediments, rivers and fjords. The work links closely to iC3 research’s wider research on nutrients in Arctic landscapes, including how newly exposed soils and sediments may release nitrogen as glaciers retreat. Laura recently supervised a master’s student looking at biogeochemical cycles and nitrogen mobilization in the Lyngen Alps of Arctic Norway. She is based at the Department of Geosciences of UiT The Arctic University of Norway, located in Tromsø.