Observational constraints on the response of high-latitude northern forests to warming

31 Since the 1960s, carbon cycling in the high-latitude northern forest (HLNF) has experienced32 dramatic changes: most of the forest is greening and net carbon uptake from the atmosphere has33 increased. During the same time period, the CO2 seasonal cycle amplitude (SCA) has increased by34 ~50% or more. Disentangling complex processes that drive these changes has been challenging.35 In this study, we substitute spatial sensitivity to temperature for time to quantify the impact of36 temperature increase on Gross Primary Production (GPP), total ecosystem respiration (TER), the37 fraction of Photosynthetic Active Radiation (fPAR), and the resulted contribution of these changes38 in amplifying the CO2 SCA over the HLNF since 1960s. We use the spatial heterogeneity of GPP39 inferred from solar-induced chlorophyll Fluorescence in combination with net ecosystem40 exchange (NEE) inferred from column CO2 observations made between 2015 and 2017 from41 NASA’s Orbiting Carbon Observatory -2. We find that three quarters of the spatial variations in42 GPP can be explained by the spatial variation in the growing season mean temperature (GSMT).43 The long term hindcast captures both the magnitude and spatial variability of the trends in observed44 fPAR. We estimate that between 1960 and 2010, the increase in GSMT enhanced both GPP and45 the SCA of NEE by ~20%. The calculated enhancement of NEE due to increase in GSMT46 contributes 56–72% of the trend in the CO2 SCA at high latitudes, much larger than simulations47 by most biogeochemical models.