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Herbivore-induced emissions of biogenic volatiles from arctic plants under climate warming

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Insects, climate change and plant defences – unravelling the connection

Climate change is causing profound impacts on Arctic ecosystems. An EU-funded project investigated the links between higher temperatures, insects and the chemical gases emitted by plants.

Climate Change and Environment

The Arctic is warming at twice the speed of the global average. Climate change is altering the region in ways that are unprecedented in recent times, and scientists are trying to figure out what it will all mean. One perceived change is on the level of volatile organic compounds (VOCs). These are chemicals released by plants to protect themselves from stressors, such as high temperatures and attacks from herbivorous insects. In the Arctic, VOCs have important implications for aerosol formation. This is key for climate models, as aerosols actually scatter radiation and can counteract climate warming. Previous studies have shown warming to increase Arctic VOCs. “But note that the cooling impact is estimated to be much less than the warming impact from greenhouse gas emissions,” says Tao Li, assistant professor of Terrestrial Ecology at the University of Copenhagen, Denmark and Marie Skłodowska-Curie postdoc fellow. Yet the root causes of the changes are still unknown. To this end, the HIVOL project investigated links between Arctic VOCs, warming temperatures and insect herbivory. The team worked on several, long-term climate change experiments, which have been running from a few months to several decades. In these experiments, small areas of tundra are heated by open-topped greenhouses attempting to mimic future climate. “By comparing measurements from the heated areas and the adjacent environments, we can assess climate-induced changes,” explains Riikka Rinnan, professor in the Department of Biology and head of the Rinnan Lab at the University of Copenhagen. Specifically, the goals were to measure how warming is changing insect attacks on woody plants in the Arctic, how this changes VOC emissions, and how the impacts of insect herbivory should be included in emission models to extrapolate this information to a larger scale. “Our research shows that warming not only directly increases VOC emissions, but also has important indirect impacts, by increasing insect feeding pressure, for example,” adds Li, lead researcher on the HIVOL project.

Uncovering the links

The impacts of insect feeding varied depending on the plant and insect species in question. Interestingly, the levels of feeding by leaf-chewing insects had stronger effects on VOC emissions than climatic warming itself. “Surprisingly, warming further enhances the impacts of chewing insects on VOC emissions, and such effects appear to be consistent irrespective of warming duration,” notes Li. In the boreal mountain birch forest, the team found that the emission rates increased linearly with the extent of leaf damage caused by caterpillar feeding. Also, some specific compounds were only released when moth caterpillars were feeding on the leaves. These new results could be translated into further research avenues, and for use in monitoring ecosystem changes and informing pest management strategies. “VOCs could be potentially used as biomarkers for non-invasive detection and monitoring of caterpillar populations in the mountain birch forest,” explains Li.

What’s next?

The Rinnan Lab will continue with further experiments assessing responses in mountain birch forests with different herbivore outbreak histories. “We will also expand to a larger scale and measure VOC emissions from a whole mountain birch forest, while monitoring insect densities. Our activities in upscaling the responses and modelling will continue benefiting from the results of HIVOL,” Rinnan adds.

Keywords

HIVOL, herbivory, VOC, climate change, emissions, Arctic, plants, insects

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