An emergent constraint on the thermal sensitivity of photosynthesis and greenness in the high latitude northern forests

Creators: Liu, Junjie; Wennberg, Paul O.¹

1. California Institute of Technology

Abstract

Despite the general consensus that the warming over the high latitudes northern forests (HLNF) has led to enhanced photosynthetic activity and contributed to the greening trend, isolating the impact of temperature increase on photosynthesis and greenness has been difficult due to the concurring influence of the CO₂ fertilization effect. Here, using an ensemble of simulations from biogeochemical models that have contributed to the Trends in Net Land Atmosphere Carbon Exchange project (TRENDY), we identify an emergent relationship between the simulation of the climate-driven temporal changes in both gross primary productivity (GPP) and greenness (Leaf Area Index, LAI) and the model’s spatial sensitivity of these quantities to growing-season (GS) temperature. Combined with spatially-resolved observations of LAI and GPP, we estimate that GS-LAI and GS-GPP increase by 17.0 ± 2.4% and 24.0 ± 3.0% per degree of warming, respectively. The observationally-derived sensitivities of LAI and GPP to temperature are about 40% and 71% higher, respectively, than the mean of the ensemble of simulations from TRENDY, primarily due to the model underestimation of the sensitivity of light use efficiency to temperature. We estimate that the regional mean GS-GPP increased 28.2 ± 5.1% between 1983–1986 and 2013–2016, much larger than the 5.8 ± 1.4% increase from the CO₂ fertilization effect implied by Wenzel et al. This suggests that warming, not CO₂ fertilization, is primarily responsible for the observed dramatic changes in the HLNF biosphere over the last century.

Copyright and License

© The Author(s) 2024. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

Acknowledgement

We thank James Randerson, Dave Schimel, and TRENDY modelers (Stephen Sitch, Pierre Friedlingstein, Vivek Arora, Atul Jain, Markus Kautz, Danica Lobardozzi, Sebastia Lienert, Julia Nabel, Benjamin Poulter, Nicolas Vuichard, Andy Wiltshire, and Ning Zeng) for their comments and suggestions in preparing this paper. We also thank Dr. Ranga Myneni and Dr. Martin Jung for providing the GIMMS and FLUXCOM data. Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advance Supercomputing (NAS) division at Ames Research Center.

Funding

Part of the research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration, (80NM0018D0004). P.W. acknowledge the support from Caltech’s Resnick Sustainability Institute.

Contributions

Conceptualization and methodology: J.L.; Investigation: J.L. and P.O.W.; Visualization: J.L.; Writing—original draft: J.L. and P.O.W.

Data Availability

All data used to support the findings of this study are publicly available. TRENDY model simulations and its met drivers are available on request from TRENDY coordinator Dr. S. Sitch (s.a.sitch@exeter.ac.uk). The GIMMS AVHRR and MODIS LAI data are available upon request from by Dr. Ranga Myneni (rmyneni@bu.edu). The NOAA AVHRR LAI data is available at: https://www.ncei.noaa.gov/access/metadata/landing-page/bin/iso?id=gov.noaa.ncdc:C01559. The MCD-MODIS is available at: https://lpdaac.usgs.gov/products/mcd15a2hv006/. The FLUXCOM GPP dataset was obtained from https://www.bgc-jena.mpg.de/geodb/projects/Data.php. The OCO-2 SIF data is publicly available at https://disc.gsfc.nasa.gov/datasets/OCO2_L2_Lite_SIF_10r/summary?keywords=oco2%20sif%20lite The CERES data is available at https://asdc.larc.nasa.gov/project/CERES.

Conflict of Interest

The authors declare no competing interests.

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