Winter Climate Change and Reindeer Grazing: A Complex Interplay Shaping Northern Forest Carbon Dynamics
In the expansive boreal forests of the northern hemisphere, climate change is reshaping ecological processes in profound and often unexpected ways. A recent groundbreaking study led by researchers at the University of Oulu in Finland has unveiled new insights into how winter climate change influences the carbon cycle in northern coniferous forests, revealing a critical and previously underappreciated role for reindeer grazing in these dynamics. This research highlights the nuanced interaction between snow cover variations, vegetation, soil processes, and herbivory, offering a vital piece of the puzzle in understanding northern ecosystem responses to a changing world.
Northern coniferous forests encompass vast tracts of the Arctic-boreal region and are responsible for storing approximately one-third of the Earth’s terrestrial carbon. These forests act as immense carbon sinks, sequestering atmospheric carbon dioxide and thereby mitigating global climate change. The carbon dynamics within these ecosystems are governed not only by the dominant trees but also by the understory vegetation and intricate soil microbial communities, which together regulate organic matter decomposition and carbon fluxes. Winter conditions, particularly snow depth and duration, play a pivotal role in shaping these carbon exchange processes, with effects that cascade into the critical growing seasons.
Winter climate change is perturbing snow cover patterns across northern latitudes, leading to alterations in snow depth and snow season length. Changes in snow insulate soil and impact soil temperature and moisture regimes, thereby influencing the activity of soil decomposers. The understory vegetation, comprising largely of lichens, mosses, and dwarf shrubs, is sensitive to these shifts. Meanwhile, intensive reindeer grazing exerts a strong pressure on understory plant composition, particularly by reducing lichen cover, which is a keystone nutritional resource for reindeer during winter months. Understanding how these factors intersect offers new pathways to predict carbon cycle outcomes under future climate scenarios.
The University of Oulu team conducted an extensive multi-year field experiment from 2019 to 2023 at two ecologically distinct sites in northern Finland: Oulanka in the east and Kevo in the far north. These sites featured paired plots where reindeer grazing was either actively ongoing or had been excluded for substantial periods, 25 years at Oulanka and 55 years at Kevo. The research design also incorporated snow manipulation experiments, artificially increasing or decreasing snow depth to isolate snow effects from grazing impacts. Through rigorous measurements of carbon dioxide (CO₂) fluxes from the understory and soil layers, the study sought to dissect the respective and combined influences of winter snow conditions and herbivory on forest carbon exchange.
One of the most striking findings emerged from the 55-year grazing exclosure at Kevo, where areas deprived of reindeer influence showed a distinct sensitivity of carbon dynamics to snow depth. Here, shallow snow cover was associated with increased carbon release from the understory and soil, indicative of enhanced decomposition and respiration under less insulated winter conditions. Conversely, deeper snow reduced carbon emissions, suggesting a buffering effect through soil temperature stabilization and moisture conservation. This pattern contrasts sharply with observations from grazed sites and the shorter-term exclosure at Oulanka, where carbon release remained remarkably stable across varying snow depths.
The apparent insensitivity of carbon exchange to snow depth in grazed areas suggests a potential buffering role of reindeer grazing against winter climate variability. The reduction of lichen cover by grazing may alter soil microclimate conditions and vegetation structure, thereby modulating the soil’s thermal and moisture regime and microbial activity. The recovered lichen populations observed in the 55-year exclosure likely influence these parameters differently, potentially accounting for the contrasting snow effects on carbon release at Kevo. These nuanced effects underscore the importance of herbivory as a biological regulator that mediates ecosystem responses to abiotic stressors such as snow variation.
According to Dr. Noora Kantola, the lead doctoral researcher on the project, these findings “indicate that northern coniferous forests may possess a degree of resilience to short-term winter climate changes, mediated through the complex interplay of vegetation, soil, and herbivory.” The study contributes substantially to the understanding of winter season processes, which have historically received less attention compared to summer growing season dynamics in ecosystem carbon research.
Postdoctoral researcher Maria Väisänen further elaborates that reindeer grazing “can buffer critical ecosystem functions, such as carbon exchange, under changing climatic conditions.” The role of large herbivores in shaping vegetation and belowground processes is increasingly recognized as a key component in ecosystem functioning, moving beyond simplistic views of grazing as merely consumptive pressure. The ecological interactions in these northern systems reveal how grazing can modulate the effects of abiotic environmental change, potentially maintaining carbon sink function despite the challenges posed by global warming.
The broader implications of this research are significant. The findings suggest that land management practices, including reindeer husbandry and forestry, must be carefully considered in the context of ecosystem carbon balance and climate mitigation strategies. Alterations in lichen cover caused by different land uses can influence soil microclimate, microbial activity, and carbon fluxes, thereby affecting the overall carbon sequestration capacity of these forests. Understanding these relationships is essential for informed decision-making in boreal forest management and conservation.
Additionally, the ongoing projects at the University of Oulu are investigating how long-term climate change and grazing interact to influence tree growth and physiological responses, such as tree ring development and photosynthetic activity. These studies aim to provide a comprehensive picture of how northern forest ecosystems will function in an era marked by rapid warming, fluctuating snow regimes, and dynamic herbivore populations.
The experimental sites are part of the broader EcoClimate system, a long-term research infrastructure that examines the impacts of changing snow conditions on northern ecosystems. These efforts complement prior findings that reindeer grazing mitigates the impacts of summer climate variability on tundra carbon cycling, building a consistent understanding across seasons and ecosystem types.
Published in the July 2025 issue of Science of the Total Environment, this study represents a landmark contribution to Arctic-boreal ecosystem science. The article titled “Impacts of winter climate change on northern forest understory carbon dioxide exchange determined by reindeer grazing” synthesizes data integrating climate manipulation, herbivore ecology, and ecosystem carbon flux measurement to reveal intricate feedback mechanisms governing carbon cycling in cold regions.
The dynamic interplay illuminated by the University of Oulu team emphasizes the critical need to incorporate biotic factors such as herbivory into models predicting carbon cycle responses to climate change. By revealing how reindeer grazing can modulate soil and understory responses to winter snow variability, this research advances both fundamental ecological theory and applied environmental management in a warming world.
As the Arctic and subarctic regions face unprecedented climatic shifts, the stewardship of reindeer populations and their grazing landscapes emerges not only as a cultural and economic concern but also as a pivotal element in sustaining global carbon balance. Through studies such as this, science continues to uncover the delicate interdependencies that underpin ecosystem resilience and highlights the profound impacts that even subtle biological interactions can have on Earth’s climate system.
Subject of Research: Impacts of winter climate change and reindeer grazing on carbon cycle dynamics in northern coniferous forests
Article Title: Impacts of winter climate change on northern forest understory carbon dioxide exchange determined by reindeer grazing
News Publication Date: July 2025
Web References: https://doi.org/10.1016/j.scitotenv.2025.180089
References: Kantola, N., Welker, J. M., Leffler, A. J., Lämsä, J., Paavola, R., Suominen O., and Väisänen, M. (2025). Impacts of winter climate change on northern forest understory carbon dioxide exchange determined by reindeer grazing. Science of the Total Environment, 180089.
Image Credits: Noora Kantola / University of Oulu
