Category: NordBorn publications

This study by Koivisto et al. investigates how greening is affected by the expansion of shrub species on the Yamal Peninsula in Northwestern Siberia, Russia. Greening, measured with vegetation indices from satellites, has been linked to several changes in vegetation including plant community composition and vegetation structure. Shrub expansion, or shrubification, is one of these notable changes, but how shrubs contribute to satellite observations remains largely unanswered. In addition, more recent multi-index approaches, mostly covered with Normalized Difference Vegetation Index, have been suggested because they can unveil new information on vegetation.

For our methodology, we built a machine learning algorithm to make maps of willow occurrences across the entire study site. We extracted four vegetation index values from Landsat satellite time-series from 1987 to 2023 to see how greening of these indices differs when vegetation is most photosynthetically active. Finally, we analyzed how greening in the satellites relates to willow cover.

The results show that greening is heavily dependent on the used vegetation index, demonstrating that indices do not only measure vegetation but may be disturbed by atmospheric or soil-related factors. We further found that greening was most prominent when willow cover was around 20-60%. We hypothesize that areas with high willow cover have either reached their growth limit or that vegetation indices can not capture willow growth past a certain point. Absent greening in low willow cover could be a result of herbivory by the locally abundant reindeer in summer months, that may hinder shrub expansion.

Reference: Koivisto, E., Kuzmin, A., Berner, L., Kerby, J. T., Verdonen, M., Skarin, A., Kolari, T. H. M., Tahvanainen, T., Korpelainen, P., Kumpula, T. (2026). Multi-index analysis reveals complexity of tundra greening and shrubification on Yamal Peninsula. Environmental Research: Ecology, 5(3), e035006. https://doi.org/10.1088/2752-664X/aea99e

Text: Elias Koivisto, University of Eastern Finland (UEF), Photo: Timo Kumpula & Pasi Korpelainen

This study explores how a tundra plant community at Zackenberg in NE Greenland is influenced by both grazing animals and climate warming. The Arctic is a nutrient‑poor environment, so plants are strongly affected by changes in temperature and by herbivores such as muskoxen, which feed heavily on vegetation. As the Arctic warms, plant growth is increasing, but at the same time the numbers and movements of large grazing animals are changing. Understanding how these two forces interact is important for predicting future ecosystem changes.

To investigate this, we conducted a 13‑year experiment in a wet Arctic fen, where we fenced off small areas to exclude muskoxen while leaving nearby areas open to grazing and trampling. Initially, the effects of excluding muskoxen were clear. After only five years, fenced areas held much more plant biomass and nitrogen, indicating that muskox grazing and trampling limit vegetation growth.

Over the longer term, however, these differences faded. Muskox numbers in the region declined during the study, reducing grazing pressure even in unfenced areas. Hence, 13 years after the establishment of the experiment, plant biomass and nitrogen pools had increased both inside and outside the fenced areas, and the treatment effect was no longer evident.

The declining muskox population at Zackenberg reduces its influence on the tundra vegetation, allowing climate warming to drive widespread increases in plant growth. These changes are likely to affect how nutrients and energy move through Arctic ecosystems, with possible knock-on effects in freshwater and coastal ecosystems.

Reference: Brockmann, F. K., Michelsen, A., Stewart, L., van Beest, F. M., Hansson, S. V., & Schmidt, N. M. (2026). Herbivore decline switches a high Arctic plant community from top-down to bottom-up control. Journal of Ecology, 114, e70308. https://doi.org/10.1111/1365-2745.70308

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Text: Niels Martin Schmidt, Aarhus University / Picture: Muskox exclosures at Zackenberg, Lars Holst Hansen.

We are happy to start the new year with the publication of the first topical review from the Nordic Borealization Network, where we introduce the concept of tundra borealization to describe northward and upward shifts of boreal species into tundra ecosystems — a key yet still underrecognized dimension of climate-driven ecological change. 

In this synthesis, we: 

• clarify how borealization is defined across disciplines and environments 
• summarise evidence of plant and animal community changes in tundra systems 
• review methods for quantifying borealization 
• discuss how climate and land-use change drive species reorganization 

By bridging ecology, biogeography, climate science, and socio-ecological research, this work advances theoretical understanding of ecosystem responses to global environmental change. Given the implications for biodiversity, ecosystem functioning, climate feedbacks, and Arctic livelihoods, we hope it will support both research and adaptation efforts. 

Reference: Verdonen, M., Barrio, I. C., Barbero-Palacios, L., López-Blanco, E., Speed, J. D. M., Defourneaux, M., García Criado, M., Moullec, M. le, Mellard, J. P., Salazar, A., Bakke Westergaard, K., Guðmundsson, J., Schmidt, N. M., Sigurdsson, B. D., Austrheim, G., Bjorkman, A. D., Björnsdóttir, K., Jepsen, J. U., Pagneux, E. P., Trascau, B.M., Williamson, D.R., Aradóttir, Á.L., Barry, A., Bernard, T., Koivisto, E., Kumpula, T., Villoslada, M., Myers-Smith, I. H. (2026). Borealization of tundra ecosystems with climate and land-use change. Environmental Research Letters, 21(1), 013002. https://doi.org/10.1088/1748-9326/ae2e18 

Photo: Birch trees growing on a ridge in Kilpisjärvi, Finland. Credit: Mariana García Criado