(Mis)management of Floodplain Vegetation: The effect of Invasive Species on Vegetation Roughness and Flood Levels

Floodplains are prone to plant invasions, which increase their roughness and decrease their flood conveyance capacity. In recent decades, extremely high floods have occurred in the Tisza River (Hungary) without an increase in discharge. This could be partly explained by land cover changes, as plough fields and pastures have been replaced by forest plantations and invasive plants have become widespread in the Tisza River floodplain. The aims of the present research were (1) to evaluate long-term land cover changes from the point of view of floodplain roughness, (2) to calculate vegetation density with and without the invasive shrub Amorpha fruticosa, and (3) to model (HEC-RAS) the flood conveyance in the case of unmanaged and managed vegetation (eliminating invasive plants). The study was carried out at three floodplain sections of the Tisza and Maros rivers, Hungary. In the eighteenth century, wetlands (61–93%) covered the studied floodplain areas, but as a result of mid-nineteenth-century channel regulation works, pastures and plough fields (42–72%) became widespread, and riparian forests (8–19%) appeared. In the late twentieth century, poplar plantations (43–86%) replaced pastures and plough fields and provided a perfect habitat for invasive plants. As a result of these land cover changes, the mean vegetation roughness of the floodplains increased from 0.021–0.032 (1783) to 0.066–0.092 (2017). However, at-site measurements indicate considerably higher vegetation roughness values (0.093–0.134) when the invasive Amorpha is also considered. Invasive species clearance could decrease the vegetation roughness by 86%. Based on our modelled data, peak flood stages could be decreased by 13–34 cm after the clearance of invasive plants. However, these values are influenced by the floodplain slope and characteristics of the modelled flood wave. The management of longer floodplain sections would have a considerable effect on flood stages, while the clearance of smaller patches would not have this effect