The role of climate on present and past vitality of silver fir forests in the Vosges mountains of northeastern France

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Abstract
A dendroecological study was carried out on 196 sites distributed througout the whole natural range of silver fir (Abies alba Miller) in the Vosges mountained of northeastern France. At each site, six dominant trees were bored to the pith. Stand age varied, intentionally, from 40 to 180 years. All tree rings were measured and crossdated; data were then standardized with reference to the mean curve ring width vs. cambial age. The growth indices obtained in this way were studied (setting aside the cambial age) and reveal a great increase in mean vigour from 1830 to 1930-1940 (+ 70%) and a slight decrease from 1930-1940 to the precent (-10%). Using meteorological data from Strasbourg (monthly precipitation and temperature data available from 1881) and a stepwise multiple linear regression, a climatic model was created that explains 79% of the variance. This amount included not only the monthly parameters of years y (year of ring formation) and y - 1 in the model, but also the parameters of years y - 2 to y-6. Thus, the notion of structural autocorrelation loses much of its credibility in comparison with the notion of climatic aftereffects. The statistical validation of the model distinguishes a calibration period (1881-1960) and a verification period (1961-1983). The model reconstructs the long-term trends satisfactorily, as well as periodic severe growth declines of silver fir in 1917-1923, 1943-1951, and 1976-1983. These phenomena are mainly explained by climatic factors. The possible aggravating role of air pollution is but forward, in particular, the role of the CO2 increases during the last century.