Observational Constraints on the Response of High‐Latitude Northern Forests to Warming. Issue 4 (24th November 2020)

• Record Type: Journal Article
• Authors: Liu, Junjie; Wennberg, Paul O.; Parazoo, Nicholas C.; Yin, Yi; Frankenberg, Christian
• Is Part Of: AGU advances. Volume 1:Issue 4(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-11-24
• Subjects: OCO‐2 -- SIF -- GPP; Earth sciences -- Periodicals; Space sciences -- Periodicals; 550
• DOI: 10.1029/2020AV000228
• Languages: English
• ISSNs: 2576-604X

Abstract

Abstract: Since the 1960s, carbon cycling in the high‐latitude northern forest (HLNF) has experienced dramatic changes: Most of the forest is greening and net carbon uptake from the atmosphere has increased. During the same time period, the CO2 seasonal cycle amplitude (SCA) has increased by ~50% or more. Disentangling complex processes that drive these changes has been challenging. In this study, we substitute spatial sensitivity to temperature for time to quantify the impact of temperature increase on gross primary production (GPP), total ecosystem respiration (TER), the fraction of Photosynthetic Active Radiation (fPAR), and the resulted contribution of these changes in amplifying the CO2 SCA over the HLNF since 1960s. We use the spatial heterogeneity of GPP inferred from solar‐induced chlorophyll Fluorescence in combination with net ecosystem exchange (NEE) inferred from column CO2 observations made between 2015 and 2017 from NASA's Orbiting Carbon Observatory‐2. We find that three quarters of the spatial variations in GPP can be explained by the spatial variation in the growing season mean temperature (GSMT). The long term hindcast captures both the magnitude and spatial variability of the trends in observed fPAR. We estimate that between 1960 and 2010, the increase in GSMT enhanced both GPP and the SCA of NEE by ~20%. The calculated enhancement of NEE due to increase in GSMT contributes 56–72% of the trend in the CO2 SCA at high latitudes, much larger than simulations by most biogeochemical models. Plain Language Summary: The carbon cycling in the high‐latitude northern forest has experienced dramatic changes during the last 5–6 decades along with temperature increase almost double that of the global mean temperature. Using observations, we quantify the impact of temperature increase alone on the changes of gross primary production, greenness, and CO2 seasonal cycle amplitude. Our results show a dominant temperature effect on the changes of carbon cycle over the region. Our results imply that an additional 2–5°C temperature increase by 2, 100 (e.g., RCP6.0 and IPCC AR5) would dramatically alter the distribution of tree species over the region: Evergreen trees will invade the shrublands, and much of the evergreen forests will transition to deciduous trees. GPP over the region has the potential to double. Key Points: Three fourths of the spatial variations in productivity of high‐latitude northern forests can be explained by growing season mean temperature (GSMT) Substituting space for time, we estimate that the increase in GSMT between 1960 and 2010 enhanced productivity and net carbon uptake by ~20% The calculated enhancement of net ecosystem exchange due to temperature alone contributes 56–72% of the trend in the CO2 seasonal cycle amplitude at high latitudes