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.