CHANGES IN THE ACTIVE FLOODPLAIN VEGETATION OF THE SZIGETK Z

The Szigetk z is situated in the northwest part of Hungary. In the late 19th century, a flood protection dam was built, which divided the original floodplain into an active part and an inundation-free part. In 1992, most of the water in the main Danube channel was diverted into a canal built to supply the hydroelectric power plant at B?s. This large-scale modification led to – among other things – the decline of surface water level in the active floodplain as well as the drop of groundwater depth beyond the dikes. The region’s wetland vegetation was characteristically rich in species due to a favorable water supply and the wealth of propagules (seeds, fruits, shoots with live buds) dispersed over the area by recurring floods. The area supports only few rare species. Instead, its real value lies in the vast richness of species compositions: montane and lowland species often grow side by side. The degree of naturalness varies greatly for the different parts of the Szigetk z ranging from intensively managed arable fields under constant human influence to pristine wilderness proposed to be placed under strict legal protection. Plantations of hybrid poplar also cover extensive areas. The most severe water level decline took place on the active floodplain at the section of the Great Danube channel right upstream of the canal supplying the hydroelectric power plant. Even beyond the flood protection dams there are scattered patches of considerable botanical value, despite most of the land there being under cultivation. These precious habitat islands are dispersed across the area, and their water supply is largely ensured already or can be easily remedied due to their proximity to canals or former river channels. Terrestrial plants have very quickly established in the dried-up riverbed of the Old Danube channel. In the first years, the exposed gravel substrate abounded in usual riparian plant species otherwise common on bars. On the new shoreline, a 30 m wide belt of white willow (Salix alba) has developed. Above this, a zone of water-demanding tall forb community has established which tolerates temporary inundation. Further away up to the original shoreline, a strip of vegetation composed of box elder (Acer negundo) has appeared. Unlike the white willow belt, this zone developed slowly. At first, knee-high box elder saplings vegetated in the dry grassland, but once their roots has reached permanently wet soil layers, their growth has greatly accelerated. In the former riverbed, the mass appearance of invasive plants (Acer negundo, Ailanthus altissima, Solidago