A fundamental problem for the CAVM is how to characterize the transitions in vegetation that occur across the roughly 10 °C mean July temperature gradient from the tree line to the coldest parts of the Arctic. Various authors, working with different geobotanical traditions, have divided the Arctic into bioclimatic regions using a variety of terminologies. The origins of these different terms and approaches have been reviewed by the Panarctic Flora (PAF) initiative (Elvebakk 1999). The PAF and CAVM have accepted a five-subzone version of the Russian zonal approach. The subzone boundaries are somewhat modified from the phytogeographic subzones of Yurtsev (1994) based on recent information from a variety of sources. Subzone A is the coldest subzone whereas Subzone E is warmest.
Warmer summer temperatures cause the size, horizontal cover, abundance, productivity and variety of plants to increase. Woody plants and sedges are absent in Subzone A, where mean July temperatures are less than 3 C. Woody plants first occur in Subzone B (mean July temperatures about 3-5 C) as prostrate (creeping) dwarf shrubs, and increase in stature to hemiprostrate dwarf shrubs (<15 cm tall) in Subzone C (mean July temperatures about 5-7 C, erect dwarf shrubs (<40 cm tall) in Subzone D (mean July temperature about 7-9 C), and low shrubs (40-200 cm tall) in Subzone E (mean July temperature about 9-12 C. At treeline, where the mean July temperatures are between 10 and 12 C, woody shrubs up to 2 meters tall are abundant. The number of plants in local floras available to form plant communities increases from fewer than 50 species in the coldest parts of the Arctic to as many as 500 species near treeline.
Elvebakk, A. 1999. Bioclimate delimitation and subdivisions of the Arctic. Pages 81-112 in I. Nordal and V. Y. Razzhivin, editors. The Species Concept in the High North - A Panarctic Flora Initiative. The Norwegian Academy of Science and Letters, Oslo.
Yurtsev, B. A. 1994. Floristic divisions of the Arctic. Journal of Vegetation Science 5:765-776.