How salt-tolerant floodplain forests can continue to protect us

Salt intrusion is a growing concern worldwide. Eleonora Saccon, who completed a master's degree in climate change ecology in her native Italy, studied the effects of salty surface water at the NIOZ branch in Zeeland. ‘Seawater is flowing further and further into the rivers. This happens during storm surges and when rivers supply less fresh water during prolonged droughts. For example, during westerly storms, seawater can flow kilometres into the Rotterdam Waterway.’

A big issue in Italy

Eleonora Saccon grew up in the Italian Po valley, in an area with natural springs. ‘Salt intrusion receives more attention in Italy than in the Netherlands.’ The Po Delta is the most well-known case of saltwater intrusion in Italy, she explains. ‘This phenomenon is becoming increasingly frequent and intense. For example, in 2022, saltwater moved 40 kilometres upstream compared to the usual five kilometres. This area is important for agriculture, which is increasingly being affected.’ Among the causes of salt intrusion, there is reduced precipitation, the melting of alpine glaciers, and groundwater abstractions leading to reduced availability of freshwater. While sea level rise increases the pressure from the sea, saltwater is pushed further upstream.

Water purification

In addition to Dutch dykes and dunes, coastal and riverine wetlands protect this country from flooding from the sea or rivers. These wetlands can be tidal marshes and floodplain forests: Saccon studied both these types of wetlands in relation to salination. ‘All tidal marshes in the Netherlands flood twice per day. While upstream along the river, where tides are not present, the wetlands along the rivers are flooded occasionally only during river floods.’ ‘These areas not only protect against flooding, but also purify the water and store much carbon, which helps to mitigate the effects of climate change.”

Salinisation of freshwater wetlands could throw a spanner in the works. Within the NWO project SALTISolutions, Saccon investigated two nature-based solutions that can help keep tidal marshes effective even in the face of salinisation.

White willow and black alder

In containers in which she varied the salt concentration, she investigated the salt tolerance of two tree species: black alder and white willow. ‘Traditionally, these wetlands were willow plantations. In the past, people used wood from this fast-growing species to reinforce dykes.’ Before people started replacing natural forests with willow plantations around the 14^th century, black alders were quite common in floodplain forests. These trees proved to be more resistant to salt than the planted willows.

Saccon tested salt concentration and salt stress duration and the effect of seasonality on salt stress. ‘Is a salt intrusion event in summer more damaging than a salt intrusion event in winter? In my containers, I exposed the two species to periods of saltier water for a year. Temporary salt intrusion in winter is not a problem to either species. If it lasted longer, the alders simply sprouted again after a winter of salt water, but the willows were in bad condition. Willows suffer and die after temporary salt intrusion in spring and summer, while alder resprouted.’ So, should we plant black alders en masse? ‘No, what we need is variety of trees, forbs, grasses, shrubs… That way, you create a resilient system. Otherwise, a drought or an alder disease could wipe out an entire forest in one go.’

Fractal shapes in the landscape

For both tree species, temporary salt intrusion is not a problem if the area is refreshed afterwards. Saccon investigated the refreshing capacity of shape structures in the soil. ‘In a salt marsh such as the drowned land of Saeftinge in Zeeuws-Vlaanderen, you can see a whole system of meandering creeks from above. A kind of fractal shape that branches out into smaller and smaller sections. Water flows in quickly, but also flows back again quickly as soon as this is possible. So much silt is deposited here that this place, where a village once drowned, is now the highest point in Zeeland, dykes and dunes excluded.’ You would want to achieve this in more places with land subsidence.

In many places in Europe, space is being made for rivers by opening the dike to allow the water to flood the area behind it. They often place new dikes behind or reinforce existing ones. ‘This managed realignment is often flat land that was formerly agricultural land. Sometimes straight channels are dug into it.’ Due to the increasing salinisation of rivers, it is important that such landscapes allow salt water that flows into the area during storms or droughts to flow back quickly. Otherwise, the plants will suffer.

Resilient, adaptive landscape

Using computer simulations, Saccon compared how different types of landscape collect water and allow it to flow back into the river. She compared a flat area with straight channels in it, a willow plantation with straight rows of willows and a natural creek landscape.

‘The naturally developed marsh landscape proved to be the most effective, having the shortest duration of salt intrusion of all landscapes.’ Unfortunately, there are also practical objections to creating the optimal circumstances. ‘Such a landscape also takes up a lot of space, which is often not available. Every piece of land is already being used.’ Also, creating an ingenious creek landscape is not easy, maybe even impossible. ‘The clay soil is tough and unyielding. The best solution is really to combat salinisation by slowing down climate change and raising the groundwater level.’