Impacts of shrub coverage for arctic ecosystem carbon uptake and storage

Although shrubs employ different water and carbon-use strategies compared to trees and are expanding rapidly across warming tundra and grassland, they are not adequately represented in global land surface models. In this study, we incorporated two types of shrubs, deciduous and evergreen, into the QUINCY nutrient-enabled terrestrial biosphere model, which features a state-of-the-art treatment of soil nutrient dynamics and carbon exchange. We investigated changes in carbon (C) fluxes and storage due to shrub cover, it's response to climate and CO ₂ fertilization effect, as well as the role of nitrogen availability. With this new implementation, shrubs exhibited reasonable seasonal cycle of gross primary production (GPP) at 50 % of the Arctic study sites. The model achieved mean R ² values of 0.5 on average, when compared with field measurements and remote sensing products, which represents an improvement for evergreen shrubs compared with the old model parametrization. However, the model-measurement agreement is lower at sites with heterogeneous local vegetation, which is not captured by the model hydrology or represented by the shrub plant functional type. The modelled gross primary production of shrubs is similarly distributed compared to needleleaf evergreen forest, with no significant difference in the median. Compared to graminoids, shrubs' carbon fluxes are 40 % higher. Shrubs produce substantial above-ground biomass, albeit lower than needleleaf trees, and exhibit distinct phenological patterns. While CO ₂ fertilization generally benefits all plant types, shrubs appear to have particularly robust growth under elevated CO ₂ concentrations. Providing the plants with unlimited nitrogen doubled gross primary production at most sites and improved the model-measurement agreement by 15 % whereas the modelled deciduous shrubs substantially reduce their nitrogen sources more than evergreen shrubs. A similar effect occurred when nitrogen and carbon contents were initialized based on permafrost profiles, which partly alleviated nitrogen limitation in the model. Overall, this study demonstrates the impact of evergreen and deciduous shrub vegetation on the Arctic carbon cycle and ongoing changes. It recommends including shrub plant functional types in global Earth system models. However, the strong nitrogen limitation of Arctic shrub productivity when using standard model parameterizations suggests that global models underestimate the Arctic's contribution to global land carbon.