CALM UNDER FIRE : THE EFFECTS OF A SIMULATED HEATWAVE ON SOIL- AND ROOT-ASSOCIATED FUNGAL COMMUNITIES IN SUBARCTIC ERICACEOUS SHRUBS

Abstract

Extreme climate events, such as heatwaves, are predicted to increase in frequency, intensity, and duration in the near future. Subarctic and boreal climate zones are expected to be especially negatively impacted, as recent shifts in temperature regimes are already particularly strong compared with more southern biomes. While many studies have focused on gradual warming and on both acute and chronic drought, the effects of acute heatwaves on soil and plant-associated fungal communities in these systems remain poorly understood. In this project, we aimed to determine how simulated acute heatwaves affect fungal communities associated with three abundant ericaceous shrub species (Empetrum nigrum, Vaccinium myrtillus, and Vaccinium vitis-idaea), which form a prominent part of the subarctic ground vegetation around the Abisko area in northern Sweden. After a short-term (two month) in situ heatwave simulation, we collected and analysed bulk soil, rhizosphere, and fine root samples from all three shrub species, in order to assess changes in fungal species diversity, community composition and variability, and functional composition using high-throughput amplicon sequencing of a fungal taxonomic marker. We observed a strong effect of the heatwave on plant-associated fungal communities in the rhizosphere and fine roots, with marked shifts in diversity and in taxonomic and functional composition, including a relative increase in mycorrhizal and endophytic fungi, which suggests a potential role of these mutualists in plant resistance to heatwave events. In contrast, bulk soil fungal communities remained comparatively less affected, indicating that heatwave effects on fungal communities are substantially mediated through changes in plant physiological properties that indirectly impact plant-associated fungi. This interpretation is further supported by the fact that the plant species that resisted the heatwave most strongly in terms of leaf biochemical composition and survival, V. myrtillus, also exhibited the weakest changes in its associated fungal communities. Collectively, our results contribute to emerging evidence that plants act as mediators of soil and root-associated fungal community responses to acute abiotic disturbances such as heatwaves.

Popular Abstract

Some of us can handle the heat

Climate change is reshaping ecosystems around the world, but its effects are particularly pronounced in the north. As temperatures are rising increasingly faster in subarctic and arctic regions, plants and the microscopic organisms living around them are facing new and unfamiliar challenges. Among the most threatening are heatwaves — short, intense bursts of high temperature that are predicted to become far more common in the coming decades. While scientists have an overall strong understanding about how gradual warming and drought affect ecosystems, we still know surprisingly little about how sudden heatwaves influence the soil and plant-associated microbiome.

Fungi play a vital role in nature. Depending on their role in the environment, they may be helping plants take up water and nutrients, or recycling dead organic matter, or possibly causing diseases. Regardless of whether they are living on the surface, within the roots, or unattached in the soil, many fungi depend on the carbon provided by the surrounding plants, as does the rest of the system depend on the fungi to recycle it. But! What happens to these delicately established processes when plants are exposed to extreme stress, like a heatwave, particularly in a subarctic environment?

In this project, I utilised samples collected a year prior (2024), when an experiment was conducted in Abisko, northern Sweden, over the summer months during which an acute heatwave was simulated on four random plots. I examined how the fungi living in bulk soil, shrub rhizosphere, and fine roots responded to an extreme warming event, compared to their normal behaviour from control plots. For this, I used three common shrub species that are widespread in boreal and subarctic regions: Empetrum nigrum, Vaccinium myrtillus, and Vaccinium vitis-idaea, and all form relationships with ericoid mycorrhizal fungi. Using DNA sequencing, I analysed how fungal community richness and diversity, fungal community composition, beta dispersion, and functional groups changed after the heatwave. I also measured plant tissue chemistry to see how well each shrub species coped with the extreme conditions. And lastly, I used microscopy to determine mycorrhization of the fine roots of all three shrub species, with and without the effects of the heatwave. Together, these approaches allowed me to explore not only whether heatwaves affect fungi, but also whether those effects are caused directly by temperature or indirectly through changes in plant physiology.

The results showed that heatwaves can significantly reshape fungal communities, but mainly those living closest to plants. In the rhizosphere and inside fine roots, fungal diversity and composition shifted noticeably after the heatwave. Some fungal groups became less common, while others increased. Bulk soil fungi, in contrast, were much less affected, suggesting that the presence and condition of the host plant play a major role in determining how fungal communities respond to sudden environmental stress. Some of the results proved to be quite interesting, as the fungi that increased in abundance included several types of mycorrhizal and endophytic fungi, which was contrary to our assumptions. Similarly, we observed no increase in plant pathogenic fungi, which we assumed would grow more abundant due to shrubs’ weakened immunity. All of the above would suggest that under certain circumstances, mutualistic fungi may help plants cope with heat stress, or that stressed plants may change their carbon allocation in ways that favour these fungi. Another intriguing observation was in how the responses from the three shrub species differed in the reactions of their associated fungal communities. Vaccinium myrtillus proved the most resilient plant species during the heatwave, showing the smallest changes in leaf chemistry and the least disturbance in its fungal community. Empetrum nigrum and Vaccinium vitis-idaea displayed stronger signs of stress, and their fungal communities shifted more dramatically, all of which suggests the plant species intrinsical traits also having a role in mitigating the negative effects of acute heatwaves.

This study highlights the importance of examining both plants and their microbial partners when assessing the effects of climate change. Heatwaves are expected to become more frequent in northern ecosystems, and their consequences may be more complex than previously assumed. Because fungi play essential roles in nutrient cycling, soil carbon storage, and plant health, changes in their communities could have long-term consequences for ecosystem stability. Ultimately, understanding these interactions can help us better predict how subarctic ecosystems will function in a warmer, more variable climate.

Master’s Degree Project in Conservation Biology, 45 credits 2025
Department of Biology, Lund University
Advisor: François Maillard
Functional Ecology, Lund University

• author: Ivicic, Iva
• supervisor: Francois Maillard; Johannes Rousk
• organization: Degree Projects in Biology
• course: BION02 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 9218716
• date added to LUP: 2026-01-14 15:43:59
• date last changed: 2026-01-14 15:43:59