Alluvial forest vegetation in an active and inactive alpine floodplain - a case study from River Ammer (Bavaria)

Most alpine floodplains are heavily modified and have lost their natural habitat dynamics and biodiversity. However, little is known about the consequences for floodplain forests. To study this topic, the Bavarian River Ammer was selected. An analysis of the effects of river regulation was possible within the ‘Schnalzaue’ where 55 years ago a weir and a dam had been built separating an active and inactive section of the floodplain. A comparison between both sections helps assessing the effect of the dam on the alluvial grey-alder forests (Alnetum incanae), as basis for future restoration. We expected lower disturbance’ lower moisture and higher nutrient supply in the inactive floodplain and thus’ a transition to a maple-ash forest (Adoxo-Aceretum). Therefore, we analysed species composition, functional plant traits of the leaf-height-seed scheme (leaf dry matter content, plant height, seed mass) and Ellenberg indicators (indicator values for moisture and nutrients, indicator species for flooding and for periodical wet conditions). The ordination results indicated different species compositions in both floodplain sections, while this was poorly reflected by the results of a syntaxonomic analysis. The latter suggested presence of a rarely flooded Asarum-subassociation of grey-alder forest in both floodplain sections. In the sections, similar site conditions prevailed regarding moisture and nutrient supply, while plant height indicated slightly more disturbance in the active floodplain. Overall, the results reveal low disturbance in both sections, even though the active one is connected with the river. A reason for this finding could be increased erosion of River Ammer downstream the weir with respective lower water levels. Accordingly, restoration should improve river dynamics in the entire floodplain to allow more frequent disturbance as a measure to maintain the grey-alder forest in the long term.