Impacts of Shrub Coverage for modeled arctic Ecosystem Carbon Uptake and Storage

Shrub vegetation has been expanding across Arctic regions in response to climate warming. However, its effects on terrestrial carbon cycling remain poorly understood. Moreover, shrubs are often underrepresented in land surface models. Here, we incorporated two shrub functional types—deciduous and evergreen, which exhibit distinct strategies from trees—into the nutrient-enabled QUINCY model.

The updated model simulates gross primary production (GPP) at site level in broad agreement with recent observations. Model simulations suggest that shrub expansion into needle-leaved forests or grasslands increases GPP by an average of 13% and 40%, respectively. Shrubs also produce substantial above-ground biomass—lower than in needle-leaved forests but higher than in grasslands—with these differences partly driven by both CO₂ and climate effects.

Our analyses reveal a strong model sensitivity to nitrogen availability. While the model applying unlimited nitrogen supply leads to an overestimation of maximum GPP at most study sites, activating the interactive nitrogen cycle suppresses GPP by up to 50% relative to flux-based observational constraints (ABCfluxnet). An additional sensitivity experiment, introducing permafrost nutrient inputs, improves soil carbon estimates but still results in GPP overestimation.