Ring width and element concentrations in beech (Fagus sylvatica L.) from aperiurban forest in central Belgium

The Foret de Soignes is a beech high forest located near Brussels (Belgium) , established on a strongly acidic soil and subjected to atmospheric pollut ion and recreational pressure. We investigated variations in ring width and mineral element concentrations (N, P, K, Ca, Mg, Mn, Al) over the last 95 years in five 135-year-old trees, variations in ring width in four 40-year- old trees and tested associations with climatic parameters. Growth curves s howed a striking increase in mean sensitivity in the last 20 years in the o ld trees but not in the young ones, starting with the 1976 summer drought. Mean sensitivity is a statistical measure of the mean relative variability between adjacent ring widths within a tree, which is correlated to suscepti bility to climatic stress (Fritts, H.C., 1976. Tree Rings and Climate, Acad emic Press, London, 567pp). May rainfall and, to a lesser extent, soil wate r recharge (i.e. pooled rainfall from October to May) correlated significan tly with ring width. However, two of the five growth depressions in the las t 20 years could not readily be accounted for by adverse climatic condition s. Element concentration profiles were suggestive of a decrease in the avai lability of Mn, Mg and Ca, but no trend of increasing N was obvious. The po ssible causes of the recent increase in sensitivity of old beech trees are discussed in terms of interactions between ageing and extreme climatic even ts, aggravated by soil compaction due to logging traffic. (C) 1999 Elsevier Science B.V. All rights reserved.