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 accepted a five-subzone version of the Russian zonal approach. The subzone boundaries are somewhat modified from the phytogeographic subzones of Yurtsev (1994). 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. In Alaska, woody plants occur as 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.
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.