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.

中文翻译：

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冬蛾幼虫（Operophtera brumata）诱导山桦（Betula pubescens var. pumila）特殊化合物的季节和海拔变化

亚北极的山桦（Betula pubescens var. pumila (L.)）森林定期遭受昆虫爆发，预计由于气候变化，这种情况将会加剧。为了减轻非生物和生物胁迫，植物进化出了化学防御能力，包括挥发性有机化合物（VOC）和非挥发性特殊化合物（NVSC）。然而，在亚北极山桦种群中，这些化合物的组成和诱导产生却很少被研究。在这里，我们评估了昆虫食草、海拔和季节对山桦叶 VOC 排放和 NVSC 含量的联合影响。通过封闭技术对挥发性有机化合物进行原位采样，并通过气相色谱-质谱法进行分析。使用非靶向方法通过液相色谱-质谱法对 NVSC 进行分析。在低海拔地区，冬蛾幼虫（Operophtera brumata）的实验性草食行为在三周的取食期内增加了单萜和同萜的排放，并在取食期结束时增加了倍半萜和绿叶挥发物的排放。然而，在高海拔地区，食草动物仅增加同萜排放。低海拔地区更明显的食草效应可能是由于食草强度较高。在各个化合物中，芳樟醇、罗勒烯、4,8-二甲基壬二烯-1,3,7-三烯、2-甲基丁腈和苄基腈是草食处理中最敏感的化合物。草食动物还改变了低海拔和高海拔地区的叶 NVSC 特征，其中反应最灵敏的化合物可能属于脂肪酰糖苷和萜烯糖苷。此外，高海拔地区未受侵染枝条的挥发性有机化合物排放量高于低海拔地区，尤其是在早季，这主要是由物候差异驱动的。VOC 排放量随季节变化很大，很大程度上反映了温度和光照水平的季节性变化。我们的研究结果表明，如果随着气候变化的持续，山地白桦林中昆虫食草压力继续上升，它将显着增加挥发性有机化合物的排放，对当地的营养相互作用和气候产生重要影响。