Winter warming response of gas-exchange and growth of Abies alba and Picea abies seedlings

Climate change is significantly altering the carbon sequestration potential of European forest ecosystems. Elevated winter temperatures at high latitudes can have a negative impact on the carbon balance and subsequent growth of evergreen trees. This study aimed to test the hypothesis of a negative impact of elevated winter temperature on winter respiration, carbon balance and growth of evergreen coniferous trees in Denmark. Seedlings of Abies alba and Picea abies were exposed to elevated temperatures (2-8degree celsius increase) for one month during the winter of 2020 using outdoor infrared heaters and greenhouses. Both species showed a significant increase in respiration under higher temperatures, possibly negatively affecting their total leaf carbon balance. Dark respiration increased by 15-16% per 1 degree celsius increase in the monthly average temperature for both species. Neither Abies alba nor Picea abies showed downregulation of dark respiration under prolonged elevated temperatures. Radial and height growth in the following season was negatively correlated with winter temperature treatment for Picea abies, but not for Abies alba. Unfortunately, the Picea abies seedlings were attacked by aphids which could affect the growth as well. Further research focused on the impact of winter warming on carbohydrate reserves is needed to fully understand why warming winters negatively affect the growth of some evergreen conifers, but not others.