Category: new publications

Floral communities in tundra and montane grasslands are shaped by both grazing and soil nutrients. Our study, conducted at two northern sites as part of the Nutrient Network, shows how bottom-up and top-down forces interact to influence flower abundance and diversity.

Nitrogen addition generally reduced flower abundance, while phosphorus and potassium increased it. Excluding grazers boosted flower abundance, especially when combined with nutrients. Yet grazing also maintained species richness and diversity by keeping tall competitive plants in check.

These results highlight that management of grazing and nutrient inputs can strongly influence floral resources, with likely consequences for pollinators and the stability of high-latitude ecosystems. We are currently investigating the consequences of nutrient- and grazing-driven changes in floral resources on flower visitor assemblages.

The paper was recently published in Oikos Volume 2025, Issue 7.

Reference: Johanson, N., Olde Venterink, H., Carvalheiro, L. G., Eskelinen, A., & Virtanen, R. (2025). Long‐term nutrient addition and grazing exclusion determine flower abundance, diversity and community composition in high‐latitude grasslands. Oikos, e11562. https://doi.org/10.1002/oik.11562

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Text and photograph: Nicolina Johanson, 2024

Large herbivores play a crucial role in maintaining vegetation structure and composition of Arctic ecosystems in times of global change by grazing and trampling on plants and moving nutrients across the landscape. However, the role of herbivores in shaping the size of ecological communities and overall biodiversity in the Arctic is poorly understood. 

Measuring ecological communities and biodiversity is a challenge difficult to overcome because it requires the extensive participation of researchers with deep taxonomic knowledge to conduct field inventories. An alternative approach entails employing existing indexes that indicate the number of organisms that interact with a particular plant species. In this study, we coupled vegetation data from a network of plots scattered across the Fennoscandian tundra with an index developed for Swedish flora to understand how herbivores shape ecological communities and overall biodiversity.

We found that herbivores reduce the size of ecological communities by grazing on taller plants which commonly interact with a higher number of species. Our study highlights the importance of preserving reindeer and moose populations in the Arctic to conserve the vegetation structure and biodiversity of the tundra.

The article is Open Access and you can find the full text here.

Reference: Ramirez, J.I., Sundqvist, M., Lindén, E., Björk, R.G., Forbes, B.C., Suominen, O., Tyler, T., Virtanen, R. and Olofsson, J. (2024), Reindeer grazing reduces climate-driven vegetation changes and shifts trophic interactions in the Fennoscandian tundra. Oikos e10595. https://doi.org/10.1111/oik.10595

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Text: Juan Ignacio Ramirez; Image generated by AI.

Range shifts and changes in dominance of species in communities are among the major predicted impacts of climate change on ecosystems, supported by numerous modeling studies. While climate is changing particularly rapidly in the Arctic, little observational data is available to document predicted changes in the composition of communities, in particular from the large Russian tundra areas.

In their recent article in Global Change Biology, Natalia Sokolova and coauthors outlined changes over 60 years in occurrence of nine species of small rodents along a latitudinal gradient spanning from the forest-tundra ecotone to the high Arctic tundra on Yamal Peninsula.

They found that the occurrence of lemmings, specialized arctic endemics, decreased in the southern parts of the peninsula, whereas the occurrence of voles, representing boreal or wide-spread species, increased and expanded northwards.

Reference: Sokolova, N. A., Fufachev, I. A., Ehrich, D., Shtro, V. G., Sokolov, V. A., & Sokolov, A. A. (2024). Expansion of voles and retraction of lemmings over 60 years along a latitudinal gradient on Yamal Peninsula. Global Change Biology, 30, e17161. https://doi.org/10.1111/gcb.17161

Small rodent population cycles characterize northern ecosystems, and the cause of these cycles has been a long-lasting central topic in ecology. While some researchers have rejected plant–herbivore interactions as a cause of rodent cycles, others have continued to research their potential roles.

In their recent article in Biological Reviews, Eeva Soininen and Mage Neby outlined four different pathways on how plants could create population cycles in rodents. They reviewed the existing scientific literature, assessing the support to these pathways.

They found studies from the temperate biome to the tundra, but studies were scattered across different plant-rodent combinations, and only a few specific topics were studied repeatedly. They concluded that the literature is currently insufficient to discard with confidence any of the four potential hypotheses for plant-rodent cycles.

Reference: Soininen, E.M. and Neby, M., 2023. Small rodent population cycles and plants–after 70 years, where do we go?. Biological Reviews. https://doi.org/10.1111/brv.13021

Photo: L. Johnson

Trees are the main carbon (C) stocks in boreal forests, but the understory vegetation can contribute significantly to total C balance. In northern boreal pine forests, understories consist of variable habitats with different roles for C balance: often, the understory is a combination of warmer and drier sunlit patches dominated by lichens and more shaded and moister patches dominated by mosses and dwarf shrubs. These habitats are influenced by herbivores that may control canopy and understory vegetation, soil C cycling and, consequently, forest C balance.

We studied how excluding large herbivores (Rangifer tarandus and Alces alces) for short and longer period affects understory CO₂ fluxes across sunlit and shaded habitats in a boreal forest. We measured understory CO₂ fluxes with manual and automated chamber methods across sunlit and shaded habitats in grazed and ungrazed areas over the growing seasons of 2019 and 2020. We used fences that had excluded large herbivores for one year and for 25 years alongside the adjacent grazed area at Oulanka research station in northeastern Finland.

We found that CO₂ release increased in shaded habitats when large herbivores were excluded for one year. On the other hand, when large herbivores were excluded for over two decades, CO₂ fluxes decreased to some extent, and this was independent of a habitat type. Our findings suggest that impacts of large herbivores on CO₂ fluxes may vary over time and be opposite in short term compared to long term. Considering these temporal variations in grazing impacts may help to forecast C fluxes more accurately, which may be relevant for informed climate solutions.

The article is open access and you can find the full text here.

Reference: Kantola, N., Väisänen, M. Leffler, A.J., Welker, J.M. Contrasting impacts of short- and long-term large herbivore exclusion on understory net CO₂ exchange in a boreal forest. Ecography
https://doi.org/10.1111/ecog.06724

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Text and photograph: Noora Kantola, University of Oulu

Migration is a key life-history strategy that allows animals to exploit seasonal food peaks while avoiding periods of scarcity. The success of a migratory strategy hinges on the balance between reproductive output and survival. Long-lived species, such as Arctic-breeding geese, can trade off survival and reproductive success by skipping breeding in years when conditions for reproduction are unfavourable. Breeding propensity — the likelihood of mature females breeding in a given year — therefore plays a crucial role in population dynamics.

We studied how spring conditions (onset of spring) affect breeding propensity and nesting success of barnacle geese (Branta leucopsis) breeding along a latitudinal gradient. We collated tracking data of barnacle geese breeding from the temperate region in the Netherlands, to Novaya Zemlya in the high Arctic. By using a novel technique to derive breeding attempts from high-resolution tracking data, our study avoids the bias introduced by overlooking non-breeding birds, providing a comprehensive understanding of breeding dynamics across the entire breeding range.

The results revealed that breeding propensity is significantly influenced by spring phenology in Arctic regions. At latitudes >66° N, breeding propensity decreased markedly in late springs compared to early springs. Nesting success, mirrored the pattern of breeding propensity, declining in later springs at Arctic latitudes. However, temperate latitudes exhibit consistently high breeding propensity and nesting success, seemingly unaffected by spring phenology. The relationship between breeding propensity and spring phenology at Arctic latitudes aligns with the idea that local food availability and conditions upon arrival play a crucial role. At temperate latitudes, food and nesting site availability is usually not limited by snow cover, and timing of spring might therefore be less important. Furthermore, agricultural intensification provides a stable and nutrient-rich food supply which could facilitate high breeding propensity regardless of spring phenology.

Earlier Arctic springs, driven by climate warming, have mostly been considered to have negative impacts on reproductive success through phenological mismatches. However, our study suggests that higher breeding propensity and nesting success in early springs may partially offset these negative impacts. This underscores the intricate relationship between spring phenology and reproductive success in Arctic breeding geese and other species. As climate change continues to influence spring timing, understanding these relationships becomes crucial for assessing the broader implications on population viability.

You can find the article here.

Reference: Boom, M. P., Schreven, K. H., Buitendijk, N. H., Moonen, S., Nolet, B. A., Eichhorn, G., van der Jeugd, H.P. & Lameris, T. K. (2023). Earlier springs increase goose breeding propensity and nesting success at Arctic but not at temperate latitudes. Journal of Animal Ecology. Early view

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Text and photo credit: Michiel Boom,Dutch Centre for Avian Migration and Demography (NIOO-KNAW)

We use a simple model of coupled carbon and nitrogen cycles in terrestrial ecosystems to examine how explicitly representing grazers versus having grazer effects implicitly aggregated in with other biogeochemical processes in the model alters predicted responses to elevated carbon dioxide and warming. The aggregated approach can affect model predictions because grazer-mediated processes can respond differently to changes in climate from the processes with which they are typically aggregated. We use small-mammal grazers in arctic tundra as an example. We conclude that implicitly aggregating the effects of small-mammal grazers with other processes results in an underestimation of ecosystem response to climate change relative to estimations in which the grazer effects are explicitly represented. The magnitude of this underestimation increases with grazer density. We therefore recommend that grazing effects be incorporated explicitly when applying models of ecosystem response to global change.

Reference: Rastetter, E.B., Griffin, K.L., Rowe, R.J., Gough, L., McLaren, J.R. and Boelman, N.T., 2022. Model responses to CO2 and warming are underestimated without explicit representation of Arctic small‐mammal grazing. Ecological Applications, 32(1), p.e02478. Link to publisher

In arctic tundra, large and small mammalian herbivores have substantial impacts on the vegetation community and consequently can affect the magnitude of carbon cycling. However, herbivores are often absent from modern carbon cycle models, partly because relatively few field studies focus on herbivore impacts on carbon cycling. Our objectives were to quantify the impact of 21 years of large herbivore and large and small herbivore exclusion on carbon cycling during peak growing season in a dry heath tundra community.

When herbivores were excluded, we observed a significantly greater leaf area index as well as greater vascular plant abundance. While we did not observe significant differences in deciduous dwarf shrub abundance across treatments, evergreen dwarf shrub abundance was greater where large and small herbivores were excluded. Both foliose and fruticose lichen abundance were higher in the large herbivore, but not the small and large herbivore exclosures. Net ecosystem exchange (NEE) likewise indicated the highest carbon uptake in the exclosure treatments and lowest uptake in the control (CT), suggesting that herbivory decreased the capacity of dry heath tundra to take up carbon. Moreover, our calculated NEE for average light and temperature conditions for July 2017, when our measurements were taken, indicated that the tundra was a carbon source in CT, but was a carbon sink in both exclosure treatments, indicating removal of grazing pressure can change the carbon balance of dry heath tundra. Collectively, these findings suggest that herbivore absence can lead to changes in plant community structure of dry heath tundra that in turn can increase its capacity to take up carbon.

Reference: Min, E., Wilcots, M.E., Naeem, S., Gough, L., McLaren, J.R., Rowe, R.J., Rastetter, E.B., Boelman, N.T. and Griffin, K.L., 2021. Herbivore absence can shift dry heath tundra from carbon source to sink during peak growing season. Environmental Research Letters, 16(2), p.024027. Link to publisher