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Global climatic changes affect fluvial processes, such as erosion, on forested slopes and riverbanks. Both the continuous and discontinuous erosion of riverbanks often lead to destabilized and uprooted tree stems. Dating of the stem-wood grown along riverbanks in combination with dendrogeomorphic analysis help to reconstruct the erosion dynamics. The current study analyses the anatomical wood features of exposed roots from European ash (Fraxinus excelsior L.). First results show different reactions in vessel and fiber cell growth: fiber cells show a distinct decrease in lumen width due to the exposure, whereas vessel lumens show a higher variation in width. The results demonstrate that with European ash it is possible to determine the time of exposure at an annual resolution.

Geomorphic events such as debris flows and avalanches are likely to increase in magnitude and frequency due to the influence of global warming. Such processes usually have an impact on the growth of affected trees. As a consequence, dendroecological techniques have been developed to determine the frequency of these events. It is, however, rarely possible to retrieve information on the type and intensity of the geomorphic event. A com-prehensive wood anatomical analysis of reaction wood has the potential to obtain more complete information on this matter. This paper presents the wood anatomical analysis of four European tree species. In growth ex-periments the impacts of the most common geomorphic events were realistically imitated. It was shown that in all four tree species the treated trees exhibited significant growth reactions compared to the untreated refer-ence trees. The treatments are significantly different only in some cases but trends in the data can be identified. The stronger the bending was the more severe reaction wood was formed. Additional treatments to the apex of the trees resulted in a weakening of the reaction wood formation. The results showed that the accuracy of dendrogeomorphic reconstructions profits from further wood anatomical investigations, which should be ap-plied if an intraseasonal reconstruction is aimed for.Keywords: Wood anatomy, reaction wood, geomorphic processes, growth experimentsdoi: 10.3188/szf.2007.0058* Wilhelm-Johnen-Strasse, D-52425 Julich, E-Mail i.heinrich@fz-juelich.de