The alpine timberline zone is very sensitive to climate variability. The rise of temperatures during the vegetation period over long periods induces also a rise of the tree line with higher forest stand density. Temperature reductions, however, will lead to less dense forests and a drop of the timberline. How and how fast do timberline forests react to changing climatic conditions? On this issue researchers of the Institute for Geography at the University of Innsbruck have conducted tree- ring analyses (dendrochronology) in the Tyrolean Central Alps where the timberline region is dominated by Swiss stone pine. According to these studies the distribution area of adult trees and regeneration has moved upwards during the last 150 years (Figure 1).
Fig. 1: In the mid-nineteenth century, the position of the timberline was at an elevation of 2180 m a.s.l. in the Kaunertal (Tyrolean Central Alps) In the meantime, regeneration of Swiss stone pine occurs at elevations around 2370 m.
Timberline rise and today’s occurrence of Swiss stone pine and larch at the tree line has not happened continuously since the mid 19th century. Increased regeneration growth can be found from 1860 onwards. This coincides with a glacial melting phase following a peak in 1855.
During the last 25 years intensive regeneration growth could be found even above the established timberline in the Tyrolean Central Alps. Regeneration of Swiss stone pine reacts directly to improved climate conditions. Providing that the current temperature conditions persist, the timberline will advance around 100 to 150 meters above the one that existed in the middle of the nineteenth century.
Are the present conditions and changes something particular or has there been anything comparable before? For the assessment and classification of present conditions it would be reasonable to refer not only to the last centuries (being well documented) but to the whole post glacial period, which means the last 11.000 years.
Fig 2: For the period between 7000 and 2000 B.C. we can determine the position of the tree and forest line by means of dendrochronological analyses. They were located within or above today’s potential level.
Timberline migration during the post glacial period can be determined now by dendrochronological analyses of tree remnants, some of them dating back thousands of years. These remnants were found in the Central Eastern Alps at and above the elevation of today’s potential timberline. A timely precise and well determined picture of tree line changes over thousands of years could be established (Figure 2). There is evidence for the fact that during the mid-post glacial period the timberline was located above the current potential altitude. For example, between 4674 and 4377 B.C. a tree was growing at 2400 m a.s.l. in the Kaunertal. At this altitude, no tree growth at all would be possible today.
A comparison of the migration of alpine timberline with glacial variations during the last 11.000 years confirms the picture of a climatically favourable early and mid-post glacial period. Today’s average temperatures have not yet reached the average maxima of this period even if the current level is clearly below the average of the past millennium.