Seasonal and elevational variability in the induction of specialized compounds from mountain birch (Betula pubescens var. pumila) by winter moth larvae (Operophtera brumata)

Abstract

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.

Keywords: biotic stress; geometrid moth; global change; plant–insect interactions; secondary metabolites; volatile organic compounds.

Figure 1.

Leaf damage on herbivory treated branches at low and high elevations. The label (ExT ^*) denotes a significant interaction between elevation and measurement time (P < 0.05, ANOVA). The asterisks above the bars indicates a significant difference between the elevations (P < 0.01, T-test). Letters above bars denotes significant differences between measurement times (P < 0.05, Tukey’s post-hoc test). Bars represent mean ± SE (n = 16).

Figure 2.

Total VOC emissions from control and herbivore-damaged branches at low and high elevations. The mean PPFD and enclosure temperature (A) and total VOC emission from control and herbivory treated branches (B) measured five times during the growing season at low and high elevations. The label (ExT ^**) denotes a significant interaction between elevation and measurement time (P < 0.01, ANOVA). The asterisks above the bars indicate a significant difference between the control and herbivory treatment within each measurement time and elevation (P < 0.01, T-test). Bars labeled with different lowercase letters indicate significant differences between measurement times at the same elevation (P < 0.05, Tukey’s post-hoc test). Bars labeled with different uppercase letters denotes significant differences between elevations within each sampling time (P < 0.05, T-test). Bars represent mean ± SE (n = 16; n = 15 for the control at high elevation 2–6 June). Dashed lines indicate the defoliation period.

Figure 3.

MT, HT, SQT and GLV emissions from control and herbivore-damaged branches at low and high elevations. MT (A), HT (B), SQT (C) and GLV (D) emissions from control and herbivory treated branches at five measurement times during the growing season at low and high elevations. Significant effects and interactions for herbivory (H), elevation (E) and measurement time (T) are shown (^*P < 0.05; ^**P < 0.01; ^***P < 0.001; ANOVA). The asterisks above the bars indicate a significant difference between the control and herbivory treatment within each sampling time and elevation (^*P < 0.05; ^**P < 0.01; ^***P < 0.001; T-test). Bars labeled with different lowercase letters indicate significant differences between measurement times at the same elevation (P < 0.05, Tukey’s post-hoc test). Bars labeled with different uppercase letters denotes significant differences between elevations within each sampling time (P < 0.05, T-test). Bars represent mean ± SE (n = 16; n = 15 for the control at high elevation 2–6 June). Dashed lines indicate the defoliation period.

Figure 4.

Metric multi-dimensional scaling (MDS) plots of the results of the RF analyses on VOCs. MDS plots of the proximity matrices showing the separation of the samples according to the VOC blends, between the control and herbivory treatment in the end of the defoliation period, i.e., 1–5 July, at the low elevation (A), between low and high elevations across herbivory treatments in the early growing season, i.e., 2–6 Jun (B), and among measurement times across both herbivory treatments and elevations (C).

Figure 5.

Metric multi-dimensional scaling plots of the results of the RF analyses on NVSCs. MDS plots of the proximity matrices showing the separation of the samples according to the NVSC blends, between the control and herbivory treatment after the defoliation period, i.e., 2–6 July, at the low elevation (A), between low and high elevations across both herbivory treatments and measurement times (B), and between measurement times across both herbivory treatments and elevations (C).