Carboxylation capacity is the main limitation of carbon assimilation in High Arctic shrubs.

Increases in shrub height, biomass and canopy cover are key whole-plant features of warming-induced vegetation change in tundra. We investigated leaf functional traits underlying photosynthetic capacity of Arctic shrub species, particularly its main limiting processes such as mesophyll conductance. In this nutrient-limited ecosystem, we expect leaf nitrogen concentration to be the main limiting factor for photosynthesis. We measured the net photosynthetic rate at saturated light (A sat ) in three Salix species throughout a glacial valley in High-Arctic tundra and used a causal approach to test relationships between leaf stomatal and mesophyll conductances (g sc , g m ), carboxylation capacity (Vc max ), nitrogen and phosphorus concentration (N area , P area ) and leaf mass ratio (LMA). Arctic Salix species showed no difference in A sat compared to a global data set, while being characterized by higher N area , P area and LMA. Vc max , g sc and g m independently increased A sat , with Vc max as its main limitation. We highlighted a nitrogen-influenced pathway for increasing photosynthesis in the two prostrate mesic habitat species. In contrast, the erect wetland habitat Salix richardsonii mainly increased A sat with increasing g sc . Overall, our study revealed high photosynthetic capacities of Arctic Salix species but contrasting regulatory pathways that may influence shrub ability to respond to environmental changes in High Arctic tundra.