The mountain birch [Betula pubescens var. pumila (L.)] forest in the Subarctic is periodically exposed to insect outbreaks,
which are expected to intensify due to climate change. To mitigate abiotic and biotic stresses, plants have evolved
chemical defenses, including volatile organic compounds (VOCs) and non-volatile specialized compounds (NVSCs).
Constitutive and induced production of these compounds, however, are poorly studied in Subarctic populations of
mountain birch. Here, we assessed the joint effects of insect herbivory, elevation and season on foliar VOC emissions
and NVSC contents of mountain birch. The VOCs were sampled in situ by an enclosure technique and analyzed by
gas chromatography–mass spectrometry. NVSCs were analyzed by liquid chromatography–mass spectrometry using an
untargeted approach. At low elevation, experimental herbivory by winter moth larvae (Operophtera brumata) increased
emissions of monoterpenes and homoterpenes over the 3-week feeding period, and sesquiterpenes and green leaf
volatiles at the end of the feeding period. At high elevation, however, herbivory augmented only homoterpene emissions.
The more pronounced herbivory effects at low elevation were likely due to higher herbivory intensity. Of the individual
compounds, linalool, ocimene, 4,8-dimethylnona-1,3,7-triene, 2-methyl butanenitrile and benzyl nitrile were among
the most responsive compounds in herbivory treatments. Herbivory also altered foliar NVSC profiles at both low and
high elevations, with the most responsive compounds likely belonging to fatty acyl glycosides and terpene glycosides.
Additionally, VOC emissions from non-infested branches were higher at high than low elevation, particularly during
the early season, which was mainly driven by phenological differences. The VOC emissions varied substantially over
the season, largely reflecting the seasonal variations in temperature and light levels. Our results suggest that if insect
herbivory pressure continues to rise in the mountain birch forest with ongoing climate change, it will significantly increase
VOC emissions with important consequences for local trophic interactions and climate.
biotic stress, geometrid moth, global change, plant–insect interactions, secondary metabolites, volatile organic
compounds.
