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

In many countries worldwide, measures have been taken in floodplains for flood prevention and nature rehabilitation. In the Netherlands, floodplains are lowered by excavating to enlarge the discharge capacity and to create opportunities for development of river habitats such as forest. As forest can obstruct the water flow through the floodplain, their development has to be controlled in some cases. In many floodplains, vegetation development is controlled by cattle and horses. We carried out an exclosure experiment over a twelve year period in a partly excavated and year-round grazed floodplain along a lowland river in the Netherlands. We focussed on the thorny shrub hawthorn (Crataegus monogyna Jacq.) as it plays an important role in the obstruction of the water flow and in the wood-pasture cycle. Most hawthorn shrubs established on the excavated part of the floodplain with low cover of tall herbs. The total number of established hawthorn 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 establishment and growth of hawthorn. Although lowering the floodplain by excavation will increase discharge capacity of the floodplain in the short term, it will decrease in the long term as excavation also increases opportunities for 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 of the measures and grazing management is needed.