Combined Warming and Herbivory Stress Intensifies Isoprene Emissions and Alters CO2 Exchange in Arctic Shrubs

Arctic regions are experiencing rapid and disproportionate warming, with consequences for plant ecophysiology and the frequency and severity of insect herbivory outbreaks. Although the individual effects of warming and herbivory on Arctic vegetation and associated biosphere–atmosphere feedbacks have received increasing attention, their combined impacts remain insufficiently understood. Because climatic and biotic stressors frequently co‑occur, identifying whether their effects are additive, synergistic, or antagonistic is critical for predicting future ecosystem functioning.

We conducted a 13-week mesocosm experiment in controlled climate chambers using shrub-dominated plant communities (Salix spp.). The experiment included a 2-week acclimation, an 8-week treatment phase, and a 3-week recovery period. During the treatment phase, we applied four conditions: (1) ambient control, (2) warming (+5 °C, including a +5 °C heat wave in week 8), (3) herbivory by geometrid moth larvae (15 larvae added between weeks 5–7), and (4) combined warming and herbivory. CO₂ exchange was measured continuously throughout the experiment, while volatile organic compound (VOC) emissions via sorbent cartridges were quantified at four time points: weeks 8, 9, and 10 during treatments, and once in week 12, during the recovery period. Plant phenology was continuously monitored using greenness indices before, during, and after the 13-week experimental period.

Combined warming and herbivory strongly enhanced isoprene emissions by ~13-fold, whereas neither stressor alone produced a significant effect on VOC emissions. Isoprene emissions were highest in week 8, followed by a gradual decline during the following weeks, suggesting a transient stress-induced response. Phenological dynamics showed limited treatment sensitivity, although control plants exhibited a slower decline in greenness late in the season, suggesting that stressed plants may enter senescence earlier. CO₂ flux measurements indicated treatment-related trends in carbon exchange, though further analysis is ongoing.

Our results demonstrate that simultaneous climatic and biotic stressors can intensify VOC emissions and influence CO₂ exchange in Arctic shrubs, with potential consequences for atmospheric chemistry and carbon cycling. Incorporating multi‑stressor interactions into ecosystem models will be essential for accurately predicting vegetation–atmosphere feedbacks in a rapidly changing Arctic.