All plants emit volatile organic compounds (VOCs); chemicals they use for defense, communication, and recovery from stress. VOCs are released in greater abundance when a plant is exposed to biotic or abiotic stressors, although very little research has analyzed the effects of multiple stressors. While insect herbivory and increased temperature have both been shown individually to increase VOC emissions in plants, the interaction of these two stressors is currently unknown. It has been predicted that the simultaneous presence of both insect herbivory and increased temperature would result in an enhanced, but non-additive increase in VOC production relative to plants exposed to either one of these factors individually. To test this hypothesis, we grew replicates of two different aspen (Populus tremuloides) genotypes in controlled environments at ambient and elevated temperatures, both with and without gypsy moth larvae (Lymantria dispar), an important herbivore of this species. We then collected volatiles from each tree using dynamic headspace methods and analyzed them using gas chromatography-mass spectrometry.
We found that plants exposed to either insect herbivory or elevated temperature emitted higher levels of VOCs than controls. Furthermore, plants subjected to both stressors emitted more VOCs than plants exposed to either stressor individually. In addition, the two aspen genotypes showed different VOC profiles. This study helps establish the interaction between the stress factors of insect herbivory and temperature and their separate and combined effects on plant VOC production. In the context of current and future global climate change, this study can contribute to understanding how warming will affect mediation of herbivory by plant volatiles and consequently overall forest ecosystem function.