Stand density in the last millennium at the upper tree-line ecotone in the Polar Ural Mountains

Abstract

Significant spatiotemporal changes in the establishment, mortality, and abundance of trees have taken place in the upper tree-line ecotone in the Polar Ural Mountains over the last millennium. Until now, these forests have developed mainly under the influence of natural factors. A large number of well-preserved tree remains can be found up to 60–80 m above the current tree line, some dating to as early as a maximum of 1300 years ago. The research reported here extends the work begun by S.G. Shiyatov, who examined evidence of tree growth dynamics along a transect on the eastern slope of the Polar Ural Mountains in the 1960s. For this study, 769 discs from dead trees and 378 increment cores from living trees were collected along an altitudinal transect 860 m long and 80 m wide. The positions of all living trees, fallen dead trees, and wood remains were mapped. Dimensions of each living tree, including saplings and understory plants, were measured: basal diameter, diameter at breast height, crown diameter, and height of stems. Calendar years of tree germination and death were estimated using dendrochronological techniques. The earliest distinct maximum in stand density occurred in the 11th to 13th centuries, coincident with Medieval climate warming. Climate warming in the 18th century appears to have resulted in a second stand-density maximum. The recent temperature increase observed in the 20th century is reflected in the high number of young trees observed.