BVOCs, insect outbreaks, and the Arctic
Plants continuously release a diverse mixture of biogenic volatile organic compounds (BVOCs) into the atmosphere. These highly reactive compounds play important roles in plant defence and communication. Once released into the atmosphere, BVOCs also influence atmospheric chemistry and contribute to the formation of aerosol particles, with consequences for air quality and the Earth's radiation balance.
In high-latitude ecosystems, BVOC emissions and their composition are expected to change as the Arctic continues to warm. Another consequence of Arctic warming is more frequent and severe insect outbreaks. In northern Fennoscandia, outbreaks of geometrid moth larvae (Figure I) typically occur every decade or even more frequently. These leaf-eating larvae can defoliate large areas of mountain birch forest, with major consequences for ecosystem functioning.
Experimental studies have shown that geometrid feeding can stimulate plants to release increased amounts of BVOCs as part of their defence response. However, while geometrids primarily feed on mountain birch, severe outbreaks can also affect the understory vegetation growing beneath the forest canopy, both through direct herbivory and by altering the understory environment. This led us to ask:
How do BVOC emissions from understory vegetation respond to a natural and severe geometrid outbreak?
Figure I: Video of geometrid moth larvae on understory vegetation during the 2023 outbreak. Video credit: Simon Nyboe Laursen.
Chasing the next geometrid outbreak
Answering this question meant being in the right place at the right time. Natural insect outbreaks are difficult to study because they occur only every decade or so and can be highly patchy across the landscape. The Abisko region in northern Sweden was last affected by a major geometrid outbreak in 2012–2013. In the hope of capturing the next outbreak, we established an experimental site in the area in 2022 (Figure IIa). The site lies in the transition zone between boreal forest and Arctic tundra and is characterised by an open mountain birch canopy above a dense understory of dwarf shrubs, grasses, and mosses (Figure IIb).
Following the outbreak in the field
Throughout the growing seasons of 2022 and 2023, we measured BVOC emissions by placing transparent chambers over small, fixed vegetation plots (Figure III) and collecting the BVOCs released into the chamber air. We focused on three common understory vegetation types dominated by Empetrum nigrum, Vaccinium myrtillus, and graminoids. At the same time, we monitored environmental conditions including temperature, incoming light, soil moisture, and changes in vegetation greenness using digital cameras.
When we began sampling the understory in 2022, geometrid abundance was already elevated, although the forest remained largely green (Figure IV). In 2023, however, it was a completely different story. At times, the field site almost felt like something out of a horror movie, with larval silk threads hanging from trees and shrubs and geometrids crawling across the vegetation. The understory seemed almost alive with them (Figure I).
What the data revealed, and why it matters
We initially expected BVOC emissions to increase during the geometrid outbreak because herbivore feeding can stimulate strong chemical defense emissions. However, when we analysed the data, we found the completely opposite pattern.
Total BVOC emissions declined strongly during the outbreak. Emissions from Empetrum nigrum plots were around 70% lower than in 2022, while Vaccinium myrtillus plots showed a decline of around 52%. Graminoid plots were less affected. This was one of the most surprising results of the study. Interestingly, although total emissions declined, the composition of the emitted BVOCs still carried signs of herbivore stress, with several herbivory-associated compounds becoming relatively more prominent. This suggested that the plants were still responding chemically to the geometrid outbreak, but that another process was limiting their overall BVOC emissions.
The vegetation itself provided an important clue as to why. During the outbreak, we observed a dramatic loss of green vegetation in the understory (Figure IV). At the same time, defoliation of the mountain birch canopy exposed the understory to more sunlight and higher temperatures, while soil moisture declined. Rather than simply stimulating BVOC emissions, this altered microclimate likely imposed additional heat and drought stress on vegetation already affected by herbivory.
Figure IV: Time-lapse video of the study site during the 2022 and 2023 growing seasons, showing the extensive loss of green vegetation during the 2023 outbreak. Video credit: Simon Nyboe Laursen.
Our results therefore suggest that the response of BVOC emissions to insect herbivory is plant-specific and depends strongly on the severity of the disturbance. Under moderate herbivory, plants may increase their emissions while retaining sufficient green vegetation. But when defoliation becomes severe, the loss of BVOC-emitting green vegetation can eventually outweigh this response, simply because there may be too little green plant material left to sustain high overall emissions. This highlights the need to account for structural vegetation loss when predicting BVOC emission responses to increasingly severe geometrid outbreaks in a warming Arctic.
What’s next?
Insect herbivory affects large areas of Arctic and subarctic ecosystems, and outbreaks are expected to become more frequent and widespread under a warming climate. Understanding their broader atmospheric impacts is therefore increasingly important. Our measurements reveal how BVOC emissions changed in small plots of understory vegetation, but the outbreak itself affected the landscape on a much larger scale.
The next challenge is therefore to link ground-based measurements with drone and satellite observations. By mapping the distribution and abundance of green vegetation across the landscape, we can begin to estimate how strongly the outbreak altered BVOC emissions at the ecosystem and regional scale.