Polar Willows Frozen Then Warmed Outperform Controls - An Arctic Paradox

Each January for five years beginning in 2016, a team from the Norwegian University of Science and Technology traveled to a small valley outside Longyearbyen, Svalbard, carrying large jugs of liquid water. Their task was unusual: to pour the water over selected patches of tundra and freeze the plants inside blocks of ice, simulating what happens when winter rain falls on Arctic ground and then refreezes into a solid icy cap that can persist until spring.

The polar willow (Salix polaris) was the target. This low-growing shrub, rarely more than a few centimeters tall, dominates many plant communities across the Svalbard archipelago and serves as a primary forage source for Svalbard reindeer year-round. As the Arctic warms at roughly four times the global average rate, winter rain-on-snow events are becoming more frequent - the kind of weather that creates exactly the conditions the researchers were replicating. Understanding what those conditions do to polar willows is therefore not an abstract ecological question.

The Core Finding: Resilience

Over five years of deliberate icing, the plant communities showed no accumulated damage. "Over the five years, we don't see any accumulated effect, which means that the community is actually very resilient to icing," said Mathilde Le Moullec, a researcher at the Greenland Institute of Natural Resources and NTNU, and first author of the study published in the Journal of Ecology.

This result is in some ways reassuring. Polar willows appear capable of surviving winter ice encasement - at least at the intensities and durations that could be experimentally imposed. The concern that increasing rain-on-snow events would simply kill these communities outright is not borne out by the data.

Plants in the icing-only treatment were delayed in opening their leaves, because the soil took longer to thaw. They eventually caught up, but produced smaller and thinner leaves than control plants. Flower production was also reduced, and seed production came later in the season. In an Arctic environment where the growing season spans only a few weeks, delays in phenology - the timing of biological events - can matter substantially for reproduction and energy reserves.

The Warming Countereffect

The more striking result involved the combined treatment. A subset of plant plots was enclosed in small open-top plexiglass greenhouses during summer, simulating the higher temperatures projected for Svalbard as the Arctic continues to warm. Plants that had been iced in winter and then warmed in summer did not simply recover to control levels - they exceeded them.

"If you sum up what happened throughout the entire growing season, you actually had more above-ground production than plants in the control situation," Le Moullec said. This supercharged growth - more biomass, earlier leaf-out than even the warming-only plants, and advanced seed dispersal - suggests that the stress of winter icing, followed by the stimulus of summer warmth, triggers a compensatory response that overshoots normal production levels.

Biologists measure above-ground production as everything a plant creates above the soil surface during a growing season: leaves, branches, flowers, seeds. More production means more reserves for winter survival, more ground coverage, and more offspring. The plants that experienced both stressors became, paradoxically, the most productive in the experiment.

Implications for Reindeer Forage

The reindeer connection matters. Svalbard reindeer (Rangifer tarandus platyrhynchus) rely on polar willows as a primary food source throughout the year, including in winter when they must dig through snow to reach plants. Ice encasement directly impedes winter grazing - the animals cannot break through thick ice to reach vegetation beneath. The finding that summer warming after winter icing produces more above-ground plant material could mean more forage available during the grazing season, potentially helping reindeer accumulate the fat reserves needed to survive winter. The net effect on reindeer populations depends on the balance between summer forage gain and winter forage inaccessibility.

"More above-ground production also means more forage for reindeer in summer and fall, allowing them in turn to build up reserves to survive the winter," said Brage Bremset Hansen, professor at NTNU's Gjaervoll Centre and senior researcher at NINA, the Norwegian Institute for Nature Research.

Methodological Notes and Limitations

The icing experiment used liquid water poured by hand over specific plots - a controlled but artificial approximation of rain-on-snow events. Natural icing events vary in thickness, timing, and duration in ways that the experiment could not fully replicate across five years. The open-top greenhouses produce warming that is somewhat different in character from the diffuse atmospheric warming projected under climate change scenarios. The study was conducted in a single valley outside Longyearbyen and may not represent all polar willow communities across Svalbard's diverse terrain.

Five years is a relatively short period for detecting long-term vegetation dynamics in a system where individual plants can live for decades. The absence of accumulated negative effects over five years does not guarantee resilience under more intense or prolonged icing scenarios.