Do beech and spruce change their carbon allocation under future environmental conditions? Lessons learned from a three-year growth chamber experiment.

Plant carbon (C) allocation describes the distribution of carbon among different organs and processes and is sensitive to environmental conditions. As climate change proceeds, it will introduce additional uncertainties to the forest’s function as a crucial terrestrial carbon sink. Previous studies have explored the effect of single environmental variables on plant carbon allocation. Still, they cannot provide insights into the combined effects of changing environment in the future. To understand how European tree species will alter their C allocation after acclimating to the future environment, beech and spruce seedlings were grown in controlled environment facilities (CEFs) of different scenarios for three years. The scenarios represent the present condition of 1987 to 2016 (PC) and, in accordance with the IPCC scenarios, a mitigation scenario (RCP2.6) and a worst-case scenario (RCP8.5) of 2017 to 2100. This implies an increase in air temperature by approximately 1°C (RCP2.6) and 3°C (RCP8.5), an increase in CO2 concentration by approximately 30 ppm (RCP2.6) and 500 ppm (RCP8.5), and changes in other variables over the three years, including the irradiance, the relative humidity, and the O₃ concentration. After three years of treatment, the plants were labeled with ¹³C-enriched CO₂ for three days to understand the allocation and turnover of new photoassimilates. Both beech and spruce had greater biomass under RCP8.5 compared to RCP2.6 and PC, accompanied by enhanced allocation to belowground biomass. The concentration of non-structural carbohydrates (NSC) showed no significant difference across the scenarios, neither in leaves nor fine roots. Yet, the mean residence time of carbon (MRT) of the soil respiratory pool had shortened in the RCP scenarios in both species. Specifically for beech, a compartmental model showed an increased pool size of mobile carbon and confirmed the shortened MRT of the mobile carbon pool under RCP8.5. The unchanged NSC concentration in the sink organ with the shortened MRT has indicated a more rapid carbon turnover under both RCP scenarios accompanied by more substantial C allocation to the immediate respiration. Additionally, the fixed C was substantially invested in biomass growth, i.e., structural carbon, only under RCP8.5, which indicates that the doubled CO₂ concentration has alleviated the environmental stress. In contrast, the minor increase in CO2 concentration under RCP2.6 had no such effect. We thus recommend that the relationship between C fixation and biomass growth should be interpreted more cautiously under changing environmental conditions in the future.