From seed to dune: How can a tiny seed help protect the coastline?

Jan-Markus Homberger of Wageningen University & Research: “Most of the existing research on dune development has a focus on the role of sand. But in our study we have approached it from a plant-centric perspective. We set up a large-scale field experiment with 750 plots at two beaches in the Netherlands (Terschelling and The Hague) to see how dunes develop at their earliest stage, and what environmental conditions lead to longer-term plant growth. In these plots we buried more than 190,000 seeds and 6,000 pieces of root, called ‘rhizomes’, from marram grass and sand couch grass.”

Some sand, a lot of rain, and a bit of saltiness

Over the course of one year’s time, the researchers saw the small dunes appear in eleven percent of their experimental plots. Homberger: “We compared environmental conditions, such as changes to the beach bed level (accretion and erosion), salinity and soil moisture to explain these developments. It turned out that a little bit of sand coverage, limited salinity and above-average wet conditions (for example precipitation) are needed for dune-building grasses to be able to thrive. Under these conditions, sufficient little plants emerged in the research plots for sand to be trapped and embryonal dunes to be formed.”

Root pieces and seeds are key

Dune-building grasses are sturdy. Once they have captured some sand and are covered by it, they outgrow their tiny dune and capture more sand. If this process is repeated – grass capturing sand, growing higher, capturing more sand, growing higher again – dunes will eventually develop.

It is often thought that a bunch of dune grass will mainly spread by slow clonal expansion. “But in our research plots, there were no plants yet,” Homberger explains. “We have seen that distribution by seeds and rhizomes (root pieces) in particular does play a major role in the distribution of dune-building grasses as well and can determine the location of dune formation.”

Resilience to storm damage

When a winter storm surge hits a beach, dunes and grasses may suffer from erosion and may be washed away. But apart from damage, this may also have a secondary effect. Pieces of grass (including rhizomes) are transported to new areas by water and wind. There they can sprout again and develop into dunes over time, provided they have landed in a place with the right environmental conditions.

Climate change may however limit these windows of opportunity, because it is likely to cause extreme weather conditions to occur more frequently. This will leave less time for the plants to settle, and thus less time for dune development. Homberger: “In our experiments we saw that grasses prefer above-average wet conditions, which means quite some rain. If a heavy storm is followed by an extremely dry year, it will be much harder for grass seeds and rhizomes to sprout in new places and help to strengthen the coastline. More research is needed to understand when such effects will become a bigger issue for coastal protection. But currently, seeds and rhizomes can indeed help to protect our coast.”

More information

• Read the study in Biogeosciences

Text: Jan-Markus Homberger; Sasja van Rosmalen; Michel Riksen; Juul Limpens
Photos: Jan-Markus Homberger (lead image: dunes developing on a beach); Jürgen Homberger