An atlas of megadroughts in Europe and in the Mediterranean Basin during the Common Era provides insights into climate variability.

-The aim of the present work was to provide a synopsis of the scientific literature concerning the effects of different tree species on soil and to quantify the effect of common European temperate forest species on soil fertility. The scientific literature dealing with the tree species effect on soil has been reviewed. The composition of forest overstory has an impact on the chemical, physical and biological characteristics of soil. This impact was highest in the topsoil. Different tree species had significantly different effects on water balance and microclimate. The physical characteristics of soils also were modified depending on the overstory species, probably through modifications of the soil fauna. The rates of organic matter mineralization and nitrification seem to be dependent on tree species. A coniferous species, Picea abies, had negative input-output budgets for some nutrients, such as Ca and Mg. This species promoted a higher soil acidification and a decrease in pH. Thus, it should not be planted in very poor soils in areas affected by acidic atmospheric depositions. Nevertheless, the effect of the canopy species on soil fertility was rarely significant enough to promote forest decline. The impact of a tree species on soil fertility varied depending on the type of bedrock, climate and forest management.forest soils / tree species / fertility / sustainability / resiliency Résumé -Effet des principales essences des forêts tempérées sur la fertilité des sols. L'objectif de cet article est de fournir une synthèse bibliographique au sujet de l'effet des essences sur le sol, et, en particulier, de l'effet des principales essences utilisées en foresterie tempérée. La composition du couvert arboré a une influence importante sur les propriétés physiques, chimiques et biologiques du sol. Cet impact est le plus important dans les horizons superficiels. L'effet des essences se traduit au niveau du pédoclimat, modifiant fortement le bilan hydrique du sol. La modification des paramètres physiques est liée à l'activité biologique, elle même dépendant de nombreux paramètres chimiques et biochimiques. La dégradation de la matière organique (minéralisation) et la nitrification semblent dépendre des essences. L'épicéa commun conduit à une acidification substantielle du sol qui se traduit parfois au niveau du pH ; les bilans d'éléments nutritifs calculés pour cette essence sont le plus souvent négatifs pour des éléments tels Ca et Mg. Cette essence ne doit pas être introduite sur des sols trop pauvres ou affectés par des apports atmosphériques acidifiants. Il faut cependant insister sur le fait que le seul effet des essences n'est jamais tel qu'il puisse conduire au dépérissement des forêts. L'impact des essences sur la fertilité du sol dépend du type de sol, du climat et des aménagements forestiers (essences et traitement).sols forestiers / essences forestières / fertilité des sols / durabilité / résilience Ann. For. Sci. 59 (2002) 233

During the vegetation periods 1994 and 1995, net uptake of nitrate and ammonium by roots of adult spruce (Picea abies (L.) Karst) and beech (Fagus sylvatica L.) trees was studied at a field site exposed to high loads of N (' Ho$ glwald ', Germany). In addition, uptake experiments were carried out under controlled conditions with young spruce and beech trees grown at normal N supply.In the field, nitrate was not taken up by the roots of spruce trees in appreciable amounts. This was also true for beech except during September 1995. Apparently, beech trees was capable of taking up nitrate, but the environmental condition prevailing at the field site usually prevented net uptake. Net uptake of ammonium in both tree species showed a seasonal course, with maximum rates in mid summer. Rates of ammonium uptake by both species correlated with soil temperature at the field site.Laboratory experiments on the influence of root temperature on uptake of nitrate indicated that uptake rates at temperatures found in the field were low compared with the uptake capacity at optimum temperature. At temperatures of 10 and 15 mC, frequently found in the soil at the field site, net uptake of nitrate by spruce and beech amounted to c. 16 % and 11 %, respectively, of maximum uptake at 25 mC. By contrast, net uptake of ammonium at 10 mC reached 73 % and 31 % of the maximum uptake for spruce and beech trees, respectively. Independent of temperature, rates of nitrate uptake were considerably lower than those of ammonium. In young spruce and beech trees, net uptake of nitrate was significantly inhibited by ammonium at nitrate : ammonium ratios found in the soil solution at the forest site. Preincubation of roots of both species, with amino acids present in the phloem of adult trees at the field site, led to an increase in the amino acid pool in the roots. For spruce trees a correlation between inhibition of uptake of nitrate and enrichment of the roots with the amino compounds Glu, γ-amino butyric acid (Gaba), Gln, and Asn was observed. In beech trees, enrichment of Asp and Gln in the roots correlated with a decrease in net uptake of nitrate. The results of laboratory experiments on the effects of temperature, the nitrate to ammonium ratio in the nutrient solution, and amino acid enrichment in the roots are discussed with special emphasis on the patterns of net uptake of ammonium and nitrate observed in the field.Key words : Nitrate uptake, ammonium uptake, soil temperature, nitrate to ammonium ratio, amino acids. In forest ecosystems, nitrate and ammonium are the most abundant N compounds available to the roots * To whom correspondence should be addressed. E-mail : here!sun2.ruf.uni-freiburg.de of trees. Total amounts of nitrate and ammonium, and the ratio of nitrate to ammonia depend on quality and quantity of N input and on the balance of ammonification, nitrification, immobilization and denitrification processes in the soil (Haynes & Goh,

The future performance of native tree species under climate change conditions is frequently discussed, since increasingly severe and more frequent drought events are expected to become a major risk for forest ecosystems. To improve our understanding of the drought tolerance of the three common European temperate forest tree species Norway spruce, silver fir and common beech, we tested the influence of climate and tree-specific traits on the inter and intrasite variability in drought responses of these species. Basal area increment data from a large tree-ring network in Southern Germany and Alpine Austria along a climatic cline from warm-dry to cool-wet conditions were used to calculate indices of tolerance to drought events and their variability at the level of individual trees and populations. General patterns of tolerance indicated a high vulnerability of Norway spruce in comparison to fir and beech and a strong influence of bioclimatic conditions on drought response for all species. On the level of individual trees, low-growth rates prior to drought events, high competitive status and low age favored resilience in growth response to drought. Consequently, drought events led to heterogeneous and variable response patterns in forests stands. These findings may support the idea of deliberately using spontaneous selection and adaption effects as a passive strategy of forest management under climate change conditions, especially a strong directional selection for more tolerant individuals when frequency and intensity of summer droughts will increase in the course of global climate change.

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