Species
Cassiope tetragona (L.) D. Don., common name Arctic Bell Heather, is an evergreen dwarf shrub of the Ericaceae family. The species has a hemi-prostrate growth form and reaches heights of up to 10-15 cm at the research site, but can grow up to 30 cm tall when supported by, for example, rocks. It is the dominant species in prostrate/ hemi-prostrate dwarf-shrub tundra, but less so in the erect dwarf-shrub tundra as found at the research site, and has a circumarctic distribution . It occurs in the northernmost polar desert at 83 °N in northern Greenland , where it is the most common heath-forming species . In western Greenland it is found as far south as 63.35 °N, but towards the south only at alpine sites or on north-facing slopes . The species’ leaves are attached to its stems in opposite pairs, which alternate in 90° angles resulting in four rows of leaves (hence tetragona ). Leaf lengths rapidly increase at the beginning of the growing season and decrease thereafter, resulting in a wave-like pattern with each wave representing a single growing season . The discovery of so-called wintermarksepta , i.e. dark bands coinciding with lows in leaf lengths visible in lateral cross-sections of C. tetragona branches, has enabled the construction of >century-long growth chronologies . WMS consist of green and with increasing age, brown lignified meristem cells (see Fig. S1).