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. 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 three-week feeding period, and sesquiterpenes and green leaf volatile in 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. 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.

Synergistic effects of insect herbivory and changing climate on plant volatile emissions in the subarctic tundra

“…The individual VOCs that increased most strongly in response to herbivory in the shading treatment included (E)‐ and (Z)‐DMNT, (E)‐β‐ocimene, linalool, indole, benzyl nitrile, and 2‐methylbutanenitrile. All these compounds have earlier been reported to be induced by herbivory in birch and other species, and play a role in direct and indirect plant defense (Dicke & Baldwin, 2010 ; Irmisch et al, 2014 ; McCormick et al, 2019 ; Rieksta et al, 2020 ; Ryde et al, 2021 ).…”

“…All these compounds have earlier been reported to be induced by herbivory in birch and other species, and play a role in direct and indirect plant defense (Dicke & Baldwin, 2010;Irmisch et al, 2014;McCormick et al, 2019;Rieksta et al, 2020;Ryde et al, 2021).…”

“…The direct effect of climate warming has been shown to substantially increase VOC emissions from Arctic ecosystems (Faubert et al, 2010 ; Kramshøj et al, 2016 ; Lindwall et al, 2016 ). Additionally, climate warming is accelerating the range expansion of insect species into higher altitudes and latitudes (Ammunét et al, 2015 ; Jepsen et al, 2008 ), thus leading to increased insect herbivore pressure on plants, which also leads to increased VOC emissions from Arctic plants during periods of active insect herbivory (Li et al, 2019 ; Rieksta et al, 2020 ; Ryde et al, 2021 ). Moreover, reduced light availability resulting from the increasing cloudiness predicted for the Arctic (Boucher et al, 2013 ; Norris et al, 2016 ; Young et al, 2019 ) may decrease VOC emissions (Kramshøj et al, 2016 ).…”

“…Few studies have analyzed VOC emissions in response to insect herbivory in high-latitude ecosystems (Li et al, 2019;Mäntylä et al, 2008;Rieksta et al, 2020;Ryde et al, 2021). To date, there is only one study that has assessed the interactive effects of warming and mimicked insect herbivory (methyl jasmonate application) on VOC emissions from B. nana (Li et al, 2019).…”

Plant Secondary Metabolites Produced in Response to Abiotic Stresses Has Potential Application in Pharmaceutical Product Development

“…Some PSMs are specific to certain related plant taxa [ 21 ], and their concentrations can vary between populations and individual plants with plant ontogeny and tissue type [ 22 , 23 ]. These PSM variations can be due to genetic variability, but their concentrations are affected by environmental abiotic factors (growth conditions) such as those expected to intensify with climate change (e.g., heat stress, drought, UV radiation, and O 3 ) [ 24 ], and herbivore and pathogen attacks [ 25 , 26 ]. Based on a biosynthetic pathway and chemical structure, PSMs have broadly been categorized into three major groups: (i) terpenoids (plant volatiles, sterols, carotenoids, saponins, and glycosides), (ii) phenolic compounds (flavonoids, phenolic acids, lignin, lignans, coumarins, stilbenes, and tannins), and (iii) nitrogen-containing compounds (alkaloids, glucosinolates, and cyanogenic glycosides) [ 27 , 28 , 29 , 30 ].…”