-Radial variations in wood mineral element concentrations (N, P, Ca, Mg, K, Mn, Al) were investigated in 5 oak trees (a ring-porous species with typical heartwood) and 5 beech trees (a diffuse-porous to semi-ring porous wood species lacking typical heartwood) growing on an ochreous brown earth in the Belgian Ardennes. Differences in concentration profiles were consistent with the difference in wood structure. Specifically, oak had markedly higher concentrations in sapwood, suggesting that nutrients are actively resorbed from senescing wood rings, resulting in very low elemental concentrations in heartwood. Similarities between the two species, including outwardly decreasing cation concentrations and a recent increase in Al concentration, might be ascribed to a common environmental influence, i.e. soil leaching by acid rain. However, the decrease in Mn concentration in both species is not consistent with a scenario of decreasing soil pH. Alternative explanations are discussed.dendrochemistry / heartwood / sapwood / Fagus sylvatica / Quercus robur / beech / oak / soil acidification Résumé -Variations radiales des concentrations en éléments minéraux du bois : une comparaison entre hêtre et chêne pédonculé en Ardenne belge. Les variations radiales des teneurs en éléments minéraux (N, P, Ca, Mg, K, Mn, Al) ont été mesurées dans 5 chênes (une essence à anneaux poreux et à bois de coeur bien différencié) et 5 hêtres (une essence à porosité diffuse et dépour-vue de bois de coeur bien différencié) croissant sur sol brun ocreux dans le massif ardennais belge. Les différences dans les profils dendrochimiques sont conformes aux différences structurales du bois. Chez le chêne, les concentrations sont nettement plus élevées dans l'aubier, ce qui suggère une résorption active des éléments minéraux au cours du vieillissement des cernes, avec pour consé-quence des concentrations extrêmement faibles dans le bois de coeur. Des convergences entre les deux espèces -notamment une apparente diminution des concentrations en cations (dans le bois de coeur pour le chêne, tout au long de la chronologie pour le hêtre) accompagnée d'une augmentation récente des teneurs en aluminium -pourraient être attribuées à une influence environnementale commune, par exemple une désaturation du sol sous l'influence de pluies acides. Cependant, la diminution des teneurs en manganèse enregistrée chez les deux essences ne conforte pas l'hypothèse d'une acidification du sol. D'autres hypothèses sont discutées. dendrochimie / bois de coeur / aubier / Fagus sylvatica / Quercus robur / hêtre / chêne / acidification du sol

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A comprehensive understanding of Ca cycling in an ecosystem is desirable because of the role of this element in tree mineral nutrition and its status as a major base cation on the soil exchange complex. The determination of the origin of Ca in forests is particularly indicated in regard of important changes linked to acid inputs and intensive logging. Natural strontium isotopes are increasingly used as tracers of Ca in forest ecosystems for qualitative and quantitative assessments. Nevertheless this method is limited to relatively simple systems with two sources of nutrients. Some recent studies coupled Sr/Ca or Sr/Ba ratios to Sr isotopic measurements in order to solve more complex systems. Such method has however associated with it some uncertainties: this approach assumed that Ca, Sr and Ba behave similarly throughout the ecosystem and does not take into account the Ca biopurification processes occurring in some tree's organs which can alter element ratio. The present work focuses on two deciduous species covering large areas in Europe: European beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.). In order to test the similarity of behaviour between Ca, Sr and Ba, their concentrations were measured extensively in the major compartments of two forest ecosystems. In parallel, the discrimination process inside tree organs was studied in 23 stands for beech and 10 stands for oak. We found that Sr and Ca behave similarly in all soil and tree compartments. By contrast, Ba and Ca appear to have contrasting behaviours, especially in streams, soil solution and soil exchange complex (no correlations between element concentrations). Sr/Ba and Ba/Ca ratios must therefore be used with care as tracer of Ca. The Ca biopurification is absent in roots and slight in bole wood but is large in bark, twigs and leaves. The discrimination factors (DF) between wood and leaves are characteristic of the two species studied and do not change significantly as a function of the soil Ca status (acidic or calcareous soils). Therefore, strontium-calcium DF can be used as a correction factor of the Sr/Ca ratio of leaves when this ratio is used in connection with Sr isotopic ratios. This correction allows to solve systems of tree nutrition with more than two sources of Ca.

The Fore Ãt de Soignes is a beech high forest located near Brussels (Belgium), established on a strongly acidic soil and subjected to atmospheric pollution 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 ®ve 135-year-old trees, variations in ring width in four 40-year-old trees and tested associations with climatic parameters. Growth curves showed a striking increase in mean sensitivity in the last 20 years in the old 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 susceptibility to climatic stress (Fritts, H.C., 1976. Tree Rings and Climate, Academic Press, London, 567pp). May rainfall and, to a lesser extent, soil water recharge (i.e. pooled rainfall from October to May) correlated signi®cantly with ring width. However, two of the ®ve growth depressions in the last 20 years could not readily be accounted for by adverse climatic conditions. Element concentration pro®les were suggestive of a decrease in the availability of Mn, Mg and Ca, but no trend of increasing N was obvious. The possible causes of the recent increase in sensitivity of old beech trees are discussed in terms of interactions between ageing and extreme climatic events, aggravated by soil compaction due to logging traf®c. # 1999 Elsevier Science B.V. All rights reserved.