More fragrant Arctic with stressed plants

- "On one hand, plant volatiles are an essential part of plant defence. Many are toxic to insects that eat the plants or lay eggs on them. Plants can also attract natural enemies of the attacking insects or warn their neighbours of impending danger by airborne communication cues. On the other hand, the same plant volatiles can transform atmospheric chemistry. In air, these compounds produce tiny particles, which scatter solar radiation and thus prevent solar heat from reaching the ground, and can even form clouds. They also contribute to the formation of ground-level ozone, an air pollutant that harms plants, animals and human health," explained Dr. Tao Li, who is the main author for the study. While the multiple functions of plant volatiles in plant ecology and the atmosphere are well-known in the scientific community, their possible role is much less appreciated in connection with the climate change in the Arctic, which is proceeding faster and more pronounced than elsewhere on the planet.

Researchers from the Department of Biology and Center for Permafrost (CENPERM) at the University of Copenhagen have studied stress-triggered release of plant volatiles in the Arctic in a climate change context, and the results are sensational. The release of plant volatiles is known to be largely influenced by temperature and light availability as well as by insect herbivores and diseases. Now the researchers show, that rapid arctic warming can lead to more foliage damage caused by insect feeding, and that this in turn translates to high emissions of plant volatiles over the arctic areas.

It all began in 1999. Field experiments were set up in the tundra near the Abisko Scientific Research Station in Swedish Lapland, with the purpose of monitoring changes in ecosystems caused by climate warming. Small open-top plastic greenhouses were erected every summer to simulate summer warming of the vegetation. In 2017, Dr. Tao Li, a Marie Curie fellow from the research group led by Professor Riikka Rinnan from the Department of Biology, set out to survey insect feeding activity on the vegetation within the open top greenhouses, and investigate the joint impacts of simulated warming and insect feeding on the release of plant volatiles. And this experiment payed off.

The results show that the 2°C warming by the open-top plastic greenhouses increased the plant leaf area consumed by insects by 300%. "To investigate how insect feeding would impact the volatiles released, Tao used a plant hormone that makes the plants believe they are attacked by insects. This hormone treatment led to a drastic increase in the amount of plant volatiles released by the widespread circumpolar dwarf birch, which is a major player in the ongoing Arctic greening. The mimicked insect feeding caused large release of chemicals that we know from the smell of freshly mowed lawn and terpenes known also as essential oils. Surprisingly, a 4-fold increase in terpenes rose to a huge, 11-fold increase under simulated warming, revealing a strong synergy between warming and insect feeding. This can have far-reaching consequences," said Riikka Rinnan.

Rising temperatures are expected to increase the activity of insect pests and lead to more severe insect outbreaks, causing them to periodically demolish the vegetation at a faster rate. This is probably more notable in the Arctic, where insect metabolic rates and population growth have long been constrained by the cold environment. Certainly, increased insect activities will have a tremendous impact on plant volatiles in the Arctic and may tip into a domino effect on volatile-mediated interactions between species as well as on yet to be understood consequences in the atmospheric chemistry.