On 7 June, Sija Stofberg obtained her doctorate at Wageningen University & Research for her research into the possible exposure of plants to salt via groundwater in deep polders with saline groundwater, and via surface water in lowland fen nature areas. Among the subjects she studied was the impact of salt exposure on different plant species in these nature areas. She also researched the hydrology of floating root mats. For agricultural areas with freshwater lenses, she described a user-friendly approach to assess the risk of the disappearance of freshwater lenses and of salt reaching the plant roots. Her thesis supervisors were Sjoerd van der Zee (WUR) and Flip Witte (KWR/VU Amsterdam).
Freshwater lenses can disappear
A variety of processes can make both deep agricultural polders and the more elevated nature areas subject to salinisation. Freshwater lenses form in deep polders where there is an upwelling of groundwater. If these lenses are very thin, they can disappear during periods of drought, which can lead to the salinisation of the root zone. Sija combined a few simple approaches into a tool to assess the risk that such a lens will disappear and the salt reach the root zone. The approach can be applied without the use of numerical models, which has the advantages of user-friendliness, rapid calculation and transparency; it can also be used by water managers for the regional prioritisation of freshwater allocation.
Hydrology of terrestrial lowland fens and impact on plants
Nature areas in the lowland fen landscape are regularly supplied with surface water to maintain their water levels constant. The transported water comes from rivers or (indirectly) from other polders, and might contain elevated salt concentrations, particularly during very dry summers, which will probably become increasingly common in the future. This might make it possible for salt to reach the root mats, which (partially) float in the water. A large part of Sija’s doctoral research focused on the hydrology of these root mats. Field measurements and computer simulations revealed which of their parts were important in this regard. She also studied how plant species in this environment react to higher salt concentrations. She concluded that a number of these species are sensitive to relatively low salt concentrations. The research results can be used by water managers to underpin measures to prevent the salinisation of lowland fens. Moreover, the insights into the hydrology of root mats offer guidance for nature management, for example, in addressing the desiccation and salinisation of terrestrial lowland fens.