Forest development in a restored floodplain: Effects of grazing, inundation and vegetation

Abstract

In many countries worldwide, measures have been taken in floodplains for flood prevention and nature rehabilitation. In the Netherlands, floodplains are lowered by excavating in order to enlarge the discharge capacity and create opportunities for the development of river habitats such as forest. As forest can obstruct the flow of water through a floodplain, their development has to be controlled in some cases. In many floodplains, the growth of the vegetation is controlled by cattle and horses. We carried out an exclosure experiment over a twelveyear period in a partly excavated and year-round grazed floodplain along a lowland river in the Netherlands. We focussed on hawthorn (Crataegus monogyna Jacq.) as it plays an important role in the obstruction of water flow and in the wood-pasture cycle. Most hawthorn shrubs were recorded on the excavated part of the floodplain with a low cover of tall herbaceous plants. The total number of hawthorn plants was negatively related to inundation on the lower parts of the excavated sites and positively related to inundation on the higher parts of the excavated sites. The herbivores negatively affected the establishment and growth of hawthorn. Although lowering the floodplain by excavation will increase the discharge capacity of the floodplain in the short term, it will decrease in the long term as excavation also increases opportunities for the development floodplain forest. If flood prevention and nature rehabilitation are both goals to be achieved in a floodplain, hawthorn encroachment can be controlled by a clever design and management of grazing.