Preprints
https://doi.org/10.5194/egusphere-2025-3382
https://doi.org/10.5194/egusphere-2025-3382
25 Sep 2025
 | 25 Sep 2025

Representing dynamic grass density in the land surface model ORCHIDEE r9010

Siqing Xu, Sebastiaan Luyssaert, Yves Balkanski, Philippe Ciais, Nicolas Viovy, Liang Wan, and Jean Sciare

Abstract. In semi-arid regions, grasses and shrubs often form spatial heterogeneous patterns interspersed with bare soil, optimizing resource use and productivity. Accurately representing the matrix of vegetation and bare soil in global land surface models is essential for advancing the understanding of the carbon, water, and dust cycles. This study focuses on grasslands using the land surface model ORCHIDEE (ORganizing Carbon and Hydrology In Dynamic EcosystEms), which originally assumes a globally fixed maximum grass density. This assumption, referred to as the fixed maximum density approach, limits the model’s ability to capture grassland responses to environmental changes, resulting in unsustainable productivity and unrealistically frequent mortality events, particularly in resource-limited regions. To address these limitations, we introduced a dynamic density approach that simulates grassland density based on indicators of vegetation growth, such as reserve and labile carbon content in the grass. The emerging positive correlation between precipitation and simulated grass density supported the validity of the approach. Compared to the fixed maximum density approach, the new approach substantially reduced simulated mortality events, raised the aridity threshold for frequent mortality, and maintained realistic grassland productivity in regions where the presence of grassland is indicated by the observed leaf area index (LAI). This study not only demonstrates that simulating grass density as a function of carbon availability improves ORCHIDEE’s capacity to capture grassland dynamics under environmental variability, but also provides a promising foundation for investigating land–atmosphere feedbacks in (semi-)arid regions.

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Journal article(s) based on this preprint

05 Jan 2026
Representing dynamic grassland density in the land surface model ORCHIDEE r9010
Siqing Xu, Sebastiaan Luyssaert, Yves Balkanski, Philippe Ciais, Nicolas Viovy, Liang Wan, and Jean Sciare
Geosci. Model Dev., 19, 1–25, https://doi.org/10.5194/gmd-19-1-2026,https://doi.org/10.5194/gmd-19-1-2026, 2026
Short summary
Siqing Xu, Sebastiaan Luyssaert, Yves Balkanski, Philippe Ciais, Nicolas Viovy, Liang Wan, and Jean Sciare

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2025-3382', Anonymous Referee #1, 30 Sep 2025
    • AC1: 'Reply on RC1', Siqing Xu, 20 Nov 2025
  • RC2: 'Comment on egusphere-2025-3382', Anonymous Referee #2, 31 Oct 2025
    • AC2: 'Reply on RC2', Siqing Xu, 20 Nov 2025

Interactive discussion

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2025-3382', Anonymous Referee #1, 30 Sep 2025
    • AC1: 'Reply on RC1', Siqing Xu, 20 Nov 2025
  • RC2: 'Comment on egusphere-2025-3382', Anonymous Referee #2, 31 Oct 2025
    • AC2: 'Reply on RC2', Siqing Xu, 20 Nov 2025

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload
AR by Siqing Xu on behalf of the Authors (21 Nov 2025)  Author's response   Author's tracked changes   Manuscript 
ED: Referee Nomination & Report Request started (21 Nov 2025) by Nathaniel Chaney
RR by Anonymous Referee #1 (23 Nov 2025)
RR by Anonymous Referee #2 (08 Dec 2025)
ED: Publish as is (08 Dec 2025) by Nathaniel Chaney
AR by Siqing Xu on behalf of the Authors (15 Dec 2025)  Author's response   Manuscript 

Journal article(s) based on this preprint

05 Jan 2026
Representing dynamic grassland density in the land surface model ORCHIDEE r9010
Siqing Xu, Sebastiaan Luyssaert, Yves Balkanski, Philippe Ciais, Nicolas Viovy, Liang Wan, and Jean Sciare
Geosci. Model Dev., 19, 1–25, https://doi.org/10.5194/gmd-19-1-2026,https://doi.org/10.5194/gmd-19-1-2026, 2026
Short summary
Siqing Xu, Sebastiaan Luyssaert, Yves Balkanski, Philippe Ciais, Nicolas Viovy, Liang Wan, and Jean Sciare

Model code and software

ORCHIDEE code for the submitted paper: Representing dynamic grass density in the land surface model ORCHIDEE r9010 Siqing Xu https://doi.org/10.5281/zenodo.15723740

Siqing Xu, Sebastiaan Luyssaert, Yves Balkanski, Philippe Ciais, Nicolas Viovy, Liang Wan, and Jean Sciare

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Short summary
Prescribing a fixed maximum grass density in the ORCHIDEE model limits its ability to capture grassland dynamics, leading to unrealistic mortality, especially in semi-arid grasslands. We proposed a dynamic density approach where a positive density-precipitation relationship emerges. This method significantly reduces mortality, sustains productivity, and raises the aridity threshold above which frequent mortality events occur in grasslands.
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