Impacts of shrub coverage for arctic ecosystem carbon uptake and storage

Abstract. Although shrubs employ distinct water- and carbon-use strategies compared to trees and are increasingly expanding across warming tundra and grassland, they remain insufficiently represented in global land surface models. Here, we incorporated two shrub types, deciduous and evergreen, into the nutrient-enabled terrestrial biosphere model QUINCY, which features a state-of-the-art treatment of soil nutrient dynamics and carbon exchange. We investigate the change in carbon fluxes and storage due to shrub cover, it's response to climate and CO2 fertilization effect and the role of nitrogen availability. With this new implementation, shrubs showed reasonable seasonal cycle of gross primary production (GPP) at 50 % of the Arctic study sites. The model achieved mean R2 values of 0.5 and 0.6, when compared with in situ measurements and remote sensing products for modeled shrubs. However, at 50 % of the study sites the model underestimated observed GPP due to too strong simulated nitrogen limitation. Compared to needle leaved evergreen forest the modeled gross primary production of shrubs is similarly distributed with a non-significant difference in the median. Compared to graminoids the carbon fluxes of shrubs are 40 % higher. Shrubs produce a substantial, though lower, above-ground biomass than needle leaved trees and show phenological patterns that are distinct from those of trees. Although CO2 fertilization generally benefits all plant types, shrubs appear to maintain a particularly strong growth response under elevated CO2 concentrations. We also demonstrated that the modeled deciduous shrubs reduce their nitrogen sources substantially more than evergreen shrubs, generally resulting in a 50 % decrease in gross primary production. Providing the plants with unlimited nitrogen and thus doubling gross primary production at most sites improved the model-measurement agreement by 15 %. A similar effect occurred when initializing nitrogen and carbon contents best on permafrost profiles, resulting in partly alleviating nitrogen limitation in the model. These finding underlines the importance of including evergreen and deciduous shrub PFTs in global land surface models to accurately predict ongoing changes in the Arctic carbon cycle. However, the strong nitrogen limitation of Arctic shrub productivity when using the standard model parametrizations suggests that the Arctic contribution to global land carbon is underestimated by global models.