Successional status of trees in gallery forest along the river Rhine

Abstract

Abstract. 20 alluvial forest stands of different ages along the river Rhine in central Alsace, France, are described. A natural complex landscape occurs which is formed by erosion activity of the river. Recent human impact (canal construction, cutting) has affected river hydrology: flooding is eliminated over large areas and the underground water levels are stabilized. The forest vegetation varies in species diversity and structure, from young pioneer to older, mid‐successional forests. The forests were classified into four associations: Salici‐Populetum nigrae, Ligustro‐Populetum nigrae, Fraxino‐Populetum albae and Querco‐Ulmetum minoris. The first three communities are ‘softwood’ because of the dominance of Salix and Populus, the fourth, dominated by Quercus robur, Fraxinus excelsior and Ulmus minor, is ‘hardwood’.Differences in structure, species composition and diversity in 10 widely varying stands in 30‐yr and 150‐yr old forests are quantified and interpreted in relation to the processes and gradients (moisture and texture) involved.A model of forest succession is developed as follows: Whatever the topographic level, Salix and Populus species are the most competitive in colonizing bare sediments. Under natural conditions, pioneer softwoods are generally replaced by hardwoods in less than 100 yr. Old Querco‐Ulmetum is basically the terminal stage of the alluvial succession. Old softwood forests result from an interruption of the natural course of succession. Softwoods may be an intermediate or late‐successional phase depending on the interruption. Successional processes change according to hydrological and edaphic gradients. Allogenic processes of flooding are fundamental in the space‐time species pattern. Allogenic processes are responsible for the high species and community diversity.