Origin, activity and environmental acclimation of stem secondary tissues of the polar willow ( Salix polaris ) in high-Arctic Spitsbergen

High Arctic is an intensively explored region in relation to predicted global warming. Its long-term impact is analysed in dwarf-shrubs, such as a widely distributed and pioneering species Salix polaris (polar willow). Understanding the functioning of lateral meristems in these dwarf-shrubs is however limited, affecting interpretation of the data. The presented study focused on the formation and anatomy of the derived secondary tissues in the stems of S. polaris from polar habitats. The entire branches of the polar willow were collected in five vegetation types of tundra located in SW Spitsbergen (Svalbard) and analysed on the series of transverse and longitudinal sections. Our analyses showed that the periderm developed in two or three-year old stems as the secondary protective tissue, and that the multiple epidermis, typical of many willow species, was not formed in S. polaris . The secondary xylem in the polar willow stems showed considerable differences in diameter and length of vessel elements within a single stem. Importantly, the size of the vessel elements differed also depending on the tundra vegetation type. Our findings suggest that the soil moisture was a main factor determining the size of vessels, and that their size was not correlated to the nitrogen supply. Our study thus demonstrated the usefulness of different anatomical features of wood, in above-ground organs, in High Arctic research.