Box 23C European alluvial forests endangered

Alluvial forests are among the richest and most complex ecosystems of Europe. They present an exceptional diversity of tree species, such as European oak (Quercus robur), elm (Ulmus campestris), poplars (Populus spp) and ash (Fraxinus spp). The forest along the Rhine contains 24 different trees, 24 shrubs and 3 climbers belonging to 21 families and 34 genera (Schnitzler-Lenoble and Carbiener, 1993). The Shannon index (which takes into account the total number of species as well as the frequency of each species), a good indicator of biodiversity, ranges from 0.6 to 0.8 for the terrestrial temperate forest and from 3 to 4 in the alluvial temperate one. The presence of species here which have disappeared in other European forests is due to the richness of the soil in mineral particles, and the abundance of light in the understorey which allows a complex stratification to take place. Alluvial forests are also rich in animal species; in the Rhine forest, 38 bird species have been recorded with about 200 pairs per 10 hectares of forest, which is twice the number of pairs to be found in temperate oak­beech forest.

Since the middle of the twentieth century, these forests have been endangered through the diking of major European rivers and reduction of the natural flooding zone, deforestation, pollution of air, water and soil, bad management of water resources and non-adapted silvicultural practices. Most of the original riparian forest has disappeared along the four major European rivers (Danube, Po, Rhine, Rhone). Forest used to cover about 2000 km2 along the Rhine. Nowadays, it is very much fragmented and covers a total of 150 km2, of which less than 1.5 km2 is still semi-natural. Only some parts of the Danube (in Hungary, Serbia-Montenegro and Romania) survive under a relatively natural regime, due to a delay in technological development. The perturbation of the flooding regime has altered the delivery of nutrients to the riparian forests. Flood water, rich in mineral particles (such as phosphates) and rainfall, is no longer filtered by soils and trees, causing eutrophication of groundwater.