, Volume 9, Issue 4, pp 181-189

Increase in tree-ring width in subalpine Pinus cembra from the central Alps that may be CO2-related

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Abstract

It has been suggested many times that elevated atmospheric CO2 levels should stimulate radial increment of stem growth. However, interpretation of dendrochronologies with respect to a CO2 signal is a difficult task, since a multitude of environmental and tree factors influence the growth of stems. Here we provide a data set from subalpine stone pine which covers the period from 1750 to 1988, and from which growth rings of the 80- to 90-year age class were analysed. The most common climatological effects are taken into consideration. We found a steady and significant increase of mean ring width for the considered age class from approximately 1 mm per year in the middle of the last century to about 1.4 mm per year at present. Selected periods of equal mean summer temperatures in the last century and in more recent decades still yield a mean stimulation of about 25% for which atmospheric CO2 enrichment appears to be the most plausible explanation. The recent dramatic increase of atmospheric N-deposition could confound this interpretation, but chronologies of the last 2 decades during which wet and dry deposition of N-compounds showed the most dramatic increase exhibit no deviation from the long term trend. In contrast to the so far conflicting evidence of tree-ring responses to atmospheric changes the clear signal obtained here may be explained as follows: (1) stone pine produces little late season wood and moisture is never a limiting factor (particularly not in the early season); (2) comparatively good climatic records permitted the selection of thermally comparable periods; (3) trees grew under little spatial competition, (4) cores were collected well below the upper altitudinal range-limit of stone pine, leaving enough physiological leeway under episodic climatic stress, but (5) trees grew at altitudes high enough so that the reduction of the partial pressure of CO2 could be expected to cause CO2 to become relatively more limiting than at low elevations.