The role of wind and splash erosion in inland drift-sand areas in the Netherlands.
European drift-sand areas reached their maximum size in the 19th century. From the late 19th until the mid-20th century most drift-sand areas in Europe were successfully reforested. Today drift-sand formation can still be studied in the Netherlands. However, due to change in land-use, their limited size, eutrophication and colonisation by invasive exotics, the area occupied by active drift sand is rapidly declining and with it its unique fauna and vegetation. Between 2001 and 2005, experiments and observations were carried out at the inland drift-sand area Kootwijkerzand (central Netherlands) to know more about the extent, the intensity and the role of wind and splash erosion in a drift-sand ecosystem. Measurements were carried out on the horizontal sediment flux, the vertical accumulation flux, the raising and lowering of the surface, the organic matter content of the topsoil, the saltation activity and the weather conditions. An attempt was also made to evaluate the effect of various measures to re-activate erosion. Without sufficient erosion the scattered drift-sand vegetation will rapidly change into closed, grass-dominated vegetation. The role of erosion in the drift-sand ecosystem is therefore crucial: erosion slows down the succession rate because it causes the regular deposition of nutrient-poor sediment, and facilitates the regular renewal of the vegetation by creating new locations with extremely poor conditions for colonisation (blow-outs). On bare deflation plains wind erosion is the dominant process. Here the transport generated by splash drift can be of the same magnitude as during a moderate wind erosion event, but splash drift events are less frequent and also shorter than wind erosion events. They normally represent less than 10% of the total sediment transport. In the scattered drift-sand vegetation areas, on the other hand, splash erosion and splash drift dominate. Here the organic matter content of the transported (and later deposited) sediment is equal to the organic matter content of the parent soil, showing that splash (drift) has been the main transport mechanism. Biological crusts developed in these areas may later reduce the influence of splash. High transport rates are needed to reactivate wind erosion in vegetated areas. Under the present conditions the erosion activity in most drift-sand areas is small. Heavy wind erosion is only observed during extreme storms.