A three-season field study in northern Sweden tracked volatile compounds from birch branches as insect pressure increased. During the outbreak, some emissions were strongest early in leaf development, while the mixture became more diverse at peak feeding. The peer-reviewed observations connect seasonal leaf growth with herbivory; they do not calculate the forest’s net warming or cooling effect.
Birch branches in the Stordalen forest of northern Sweden released different amounts and mixtures of volatile organic compounds as insect feeding intensified. The peer-reviewed field study followed three growing seasons, from 2021 through 2023, with 256 independent tree measurements around a 250 m transect. Leaf damage, caterpillar numbers and canopy greenness were assessed alongside the compounds collected from branch enclosures.[1]
Average larval density rose from two per metre of branch in 2021 to ten in 2022 and 24 in the outbreak year, 2023. Some terpene and nitrogen-containing compounds increased between years, though chemical groups responded differently. Within the outbreak season, some emissions were higher during early leaf development than at peak feeding. The mixture at the peak was more chemically diverse, indicating that amount and composition changed on different schedules.[1]
After the outbreak, birches grew a second flush of leaves. Some compound groups continued to be emitted while others declined. Leaf development itself contributes to the seasonal pattern, so early high levels cannot all be assigned to insect pressure. Each tree supplied one branch sample on one occasion. The natural outbreak was observed rather than experimentally assigned, and the work did not calculate the emissions’ total climate effect.[1]
