Net Ecosystem CO2 Exchange of a Subalpine Spruce Forest in Switzerland Over 26 Years: Effects of Phenology and Contributions of Abiotic Drivers at Daily Time Scales

Climate change affects carbon sequestration dynamics and phenology in forests, especially in alpine and subalpine regions. Here, long-term trends in climate, net ecosystem CO₂ exchange (NEE), net carbon uptake period (CUP_net) and their drivers were investigated, using 26 years of flux measurements in a subalpine spruce forest (CH-Dav, Switzerland; 1997 to 2022). CUP_net length, start (SOS) and end of season (EOS) were extracted from smoothed daily NEE time series. We used machine learning to determine the importance of environmental drivers on daily NEE and CUP_net. Annual mean and maximum air temperatures (T_air) increased, while soil water content (SWC) decreased significantly between 1997 and 2022. Annual C sinks increased from 1997 to 2012, leveled off between 2012 and 2015, followed by a decline. Annual NEE was strongly related to CUP_net length, SOS, and EOS. No significant trends in CUP_net, SOS, or EOS were detected, most likely indicating ecophysiological acclimation, that is, physiological adjustments to changing environmental conditions over the past 26 years. We identified 48 days with significant negative trends in mean daily NEE over the 26 years, that is, stronger net C uptake or weaker net C loss, particularly in spring and autumn, but no significant positive trends. Daylength, incoming shortwave radiation (Rg), SWC, and minimum T_air were the main drivers of daily NEE. SOS was mainly driven by daylength and T_air, EOS by daylength and Rg. Thus, the spruce forest benefited from higher temperature between autumn and spring, with higher net C uptake during favorable conditions and reduced C loss when winter photosynthesis compensated respiration. However, high summer temperatures increasingly limited NEE, suggesting adverse effects for subalpine Picea abies forests in the future. Our study demonstrated that identifying driver contributions to NEE dynamics at daily time scales allows better understanding of the complexity of climate change impacts on forest C dynamics.