Altitudinal and polar treelines in the northern hemisphere causes and response to climate change

Abstract

This paper provides an overview on the main treeline-controlling factors and on the regional variety as well as on heterogeneity and response to changing environmental conditions of both altitudinal and northern treelines. From a global viewpoint, treeline position can be attributed to heat deficiency. At smaller scales however, treeline position, spatial pattern and dynamics depend on multiple and often elusive interactions due to many natural factors and human impact. After the end of the Little Ice Age climate warming initiated tree establishment within the treeline ecotone and beyond the upper and northern tree limit. Tree establishment peaked from the 1920s to the 1940s and resumed again in the 1970s. Regional and local variations occur. In most areas, tree recruitment has been most successful in the treeline ecotone while new trees are still sporadic in the adjacent alpine or northern tundra. Lack of local seed sources has often delayed tree advance to higher elevation. New trees established above the present seed trees and ancient solitary trees far above the present forest limit which survived cool periods without being able to reproduce from seeds may now become effective seed sources and encourage further upward shift of the treeline ecotone. At exposed sites warming cannot compensate for growth disturbances caused mainly by winter injury and biotic factors. Extreme climatic events may cause lasting setbacks at any time. Microsite facilitation is of particular importance for the survival of seedlings and saplings above the closed mountain forest stands. The relative importance of microsite facilitation will increase in parallel with the upslope migration of the tree limit into a much windier environment. Treeline migration to higher elevations and a more northern position is bringing about fundamental changes in high mountain environments and at the tundra-taiga interface which will also influence the local economy.