Improving high latitude vegetation representation in the ORCHIDEE land surface model by introducing shrub PFTs

High-latitude terrestrial ecosystems are significantly affected by anthropogenic climate change. One of the most notable observed ecological responses is an expansion of shrubs across tundra ecosystems. These shifts in plant composition influence tundra carbon and energy balances, modify snow and soil dynamics, and have broader implications for regional and global climate systems. However, due to multiple interacting processes involving ecosystem CO₂ and energy fluxes, permafrost, soil moisture, nutrient availability and interactions with snow cover, the net climate impact of shrubification, including its feedback potential and future trajectory, remain highly uncertain.

Land surface models can contribute to reducing those uncertainties and improving understanding of interactions, drivers and responses of tundra shrubification, but this requires an adequate representation of the involved ecosystems and processes in the models. However, the diversity of high-latitude ecosystems and processes is underrepresented in many global land surface models, including the ORCHIDEE land surface model. The current ORCHIDEE model version lacks key tundra plant types such as shrubs, limiting its ability to account for their role in high-latitude carbon and energy budgets, as well as to simulate tundra vegetation shifts and climate feedback processes, including shrubification. Instead, boreal trees are simulated in areas where shrubs dominate, resulting in a significant overestimation of aboveground biomass in high latitudes.

This work introduces two new plant functional types (PFTs) into the ORCHIDEE model—tall deciduous shrubs and evergreen dwarf shrubs - enhancing its representation of tundra vegetation. Their implementation is heavily based on observational data of shrub plant traits, growth form, biomass and CO₂ fluxes across the tundra region, which are used for calibration of model parameters and validation. The successful introduction of two shrub plant functional types with realistic growth form, carbon allocation and carbon fluxes into the ORCHIDEE model considerably improves its representation of high latitude vegetation, including its estimate of carbon stored in tundra biomass. Furthermore, it lays the foundation to simulate observed and future shrubification processes, their interactions with snow and permafrost dynamics and their climate impacts and feedbacks, which will be an important contribution to improve understanding of drivers and impacts of tundra vegetation change.