| 06 Mar 2026
Introducing shrubs enhances the representation of high-latitude vegetation and carbon cycling in the ORCHIDEE land surface model
Abstract. Arctic-Boreal terrestrial ecosystems are rapidly changing under amplified high-latitude warming, including widespread expansion of shrubs, with consequences for regional carbon and energy balances. Yet, high-latitude vegetation diversity and vegetation-climate interactions remain under-represented in many global land surface models. In ORCHIDEE, the land surface component of the IPSL Earth system model, high-latitude vegetation is represented primarily as boreal trees or grasslands, omitting explicit shrubs. Here, we implement three high-latitude shrub plant functional types (PFTs) (tall deciduous, low deciduous, and evergreen dwarf shrubs) in ORCHIDEE (revision 9269). Following literature recommendations, this classification combines phenology and stature to capture key functional contrasts while keeping the number of new PFTs limited. The implementation builds on ORCHIDEE's existing woody vegetation scheme by recalibrating a targeted set of parameters controlling allometry, carbon allocation, recruitment, mortality and phenology. Parameter values are constrained using synthesised pan-Arctic observations to obtain regionally representative shrub traits. Shrub spatial distributions are prescribed with updated PFT maps that combine ESA CCI products with Arctic and regional shrub mapping information. The resulting shrub PFTs reproduce observed ranges of shrub size and biomass allocation across the Arctic–Boreal domain. Introducing shrubs reduces simulated total aboveground biomass in the Arctic-Boreal region from 54 to 46.7 P g C (-13.5 %) and mean annual gross primary productivity from 498 to 481 g C m−2 yr−1 (-3.4 %) over the simulated period 1992-2020, with a stronger reduction in the tundra region (4.6 to 3 P g C (-34.8 %); and 334 to 289 g C m−2 yr−1 (-13.5 %)), increasing agreement with benchmarking datasets. A key strength of our implementation is its simplicity, as it builds on ORCHIDEE's existing woody vegetation framework. In addition, the use of synthesised pan-Arctic observations provides regionally representative observational constraints, making the methodological choices transferable beyond ORCHIDEE. Overall, this work provides a data-constrained shrub representation in ORCHIDEE with minimal added process complexity and establishes a foundation for future development of shrub-climate interactions and dynamic shrubification processes.
The manuscript "Introducing shrubs enhances the representation of high-latitude vegetation and carbon cycling in the ORCHIDEE land surface model" refined PFTs for shrublands and improved a LSM using updated PFT maps for the Arctic region. The study also investigated the influence of shrub PFT incorporation on estimates of biomass and carbon fluxes and used ground-truth data for model evaluation. In general, this manuscript is well written and follows a clear logical flow. The improved PFT methodology can benefit the broader modeling community and should be relevant to the general audience of Biogeosciences. I have a few minor suggestions that may help the authors further improve the manuscript:
L147. Three is a relatively small number, given that more sites could be categorized as shrublands according to the IGBP classification. Please provide more detail on why other sites were excluded based on specific criteria.
L155. Given the importance of the ORCHIDEE model for this study, consider adding a diagram and/or additional description of the model (perhaps in Appendix if space is limited) for a broader audience. This would help readers understand how this model compares with other LSMs and to what extent the findings can inform other modeling efforts.
L177. Provide more detail about the specific grid cell, including its resolution and representativeness, and explain why a single cell is sufficient for sensitivity tests and optimization.
L318-322. The writing in this paragraph is somewhat unclear. Please clarify how the three datasets were selected and what distinct aspects they represent, such as ground-truth data versus data-fusion products. In the later discussion section, the comparison with these datasets could also mention a bit more on uncertainty from the observations/ products.
L336 and L345: Elaborate more on why the simulated range is much smaller than the observed range (Figure 3) and discuss the implications.
Figure 4: It appears that the uncertainty range is truncated for GPP and NEE in some cases.
L405-406: Explain why the modeled trends show a clear increase over time for biomass (Figure 6) and GPP (Figure 7), whereas this pattern is much less evident in the datasets used for comparison.
Table 6. Consider adding a column that specifies the thresholds or definitions associated with these shrub classes, since these definitions can differ substantially across ESMs.