Flooding tolerance of trees in water retention basins and riparian forests. A joint research project of the Forestry Faculty Freiburg

Runoff retention basins designed to mitigate flood hazards entail impacts in habitats that so far were poorly documented. This applies especially to ecological implications in forest stands irregularly impounded. Some damages observed in such a forested area near Freiburg in South West Germany have prompted a more detailed investigation. whose purpose is to assess patterns and associated processes displayed by various tree species in this basin. For comparison purposes another site has been selected in a riparian forest along the Rhin River (Ile de Rhinau/Alsace), in stands exposed to floodings on a more natural basis.

This joint project is a common effort of four institutes of the Forestry Faculty of Freiburg. Designed as an interdisciplinary approach of the flooding tolerance of trees, the studies include in situ observations of damages, the assessment of stem radial changes under various flooding conditions, mycological and histological studies to harming symptoms as well as ecophysiological observations in the field and in the laboratory.

Preliminary observations carried out as a contribution by the Institute for Landscape Management showed that after brief periods of exposure to flooding (5 days) not only species already known as littler tolerant to flooding such as beeches (Fagus sylvatica) or red oaks (Quercus rubra), but also such ones rather considered as tolerant like ashes (Fraxinus excelsior) showed some symptoms of injury. There is some evidence suggesting that in retention basins levels of flooding tolerance of woody species are markedly below such known from riparian forests.

A follow up project that should also include polders that will be established along the Rhine River is now in the planning phase. By extending observations on additional sites and on a longer period, this european joint programme (Interreg III/NW Europe) promises to provide new insights in ecological and social implications of forested areas used for flood mitigation purposes.