Responses of needle terpene concentrations and characteristics of resin canals to different warming treatments in Scots pine and Norway spruce seedlings grown in a field experiment

We studied responses of needle terpene concentrations and resin canal characteristics to warming in Scots pine ( Pinus sylvestris) and Norway spruce ( Picea abies) seedlings grown in a controlled field set-up in eastern Finland. Warming was simulated during the growing seasons using infrared heaters that increased air temperature in study 1 (2016–2019) by 1 °C and in study 2 (2019–2020) by 2 and 4 °C, compared to ambient conditions. Terpenes were sampled in study 1 from non-matured current year and matured previous year needles in June 2019, and study 2 from mature current year needles in August 2020. In study 1, we also studied resin canal anatomy. We found that 1 °C elevation of temperature caused two-fold increase in concentrations of total terpenes, oxygenated monoterpenes, and sesquiterpenes of non-matured current year needles of Norway spruce. Further, it caused 1.3–1.8-fold increases in sesquiterpene concentrations both in unmatured and matured needles of Scots pine. It also decreased resin canal diameter of mature needles in Norway spruce. In study 2, the stronger warming treatments did not affect terpene concentrations of matured current year needles in either species. Based on our findings, even minor elevation of temperature may affect terpene concentrations of non-mature needles in boreal conifers.