The Upper Forest Limit in the Mountains of the Boreal Zone of the USSR^∗

Abstract

The high-mountain areas of the Boreal Zone of the USSR may be subdivided, according to the composition of physiognomic types of the upper forest limit, into three sectors: the western or Atlantic moderately continental sector with oceanic influences, the central or Siberian strongly continental sector, and the eastern or Pacific monsoon sector. On the extreme flanks of this zone, in the western and eastern sectors, the upper forest limit is formed by deciduous summer-green trees (respectively, Betula tortuosa and B. ermani). In the central sector, the summer-green coniferous trees (Larix sibirica, L. sibirica var. sukaczewii, L. dahurica) form the upper forest limit, but in some southern regions of this sector, where the climate is warmer and more humid, some evergreen coniferous trees (Picea obovata, Pinus sibirica, Abies sibirica) reach the upper forest limit. The fluctuations of the upper forest limit as a response to cyclic climatic change are determined through the application of a complex of different methods. These include evaluation of the vitality of trees, of changes in their growth patterns, and of natural regeneration of high-mountain forests. The 60-to-80- and 140-to-160-yr climatic cycles have caused the most important influence on the position of the upper forest limit. A tendency for increase in this upper limit has been observed in most parts of the Boreal Zone during the last few decades.