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Belowground biomass reduces erosion rates in sand dunes

New research shows that it's the parts of the plants you don't see that help preserve dunes from erosion.

Sand dunes are one of the coastal features that help protect shorelines from storms, but they’re under constant attack from the sea. Why doesn’t the sand wash away? New research from De Battisti and Griffin investigates how three plants help fight the erosion of the shore.

The big problem is swash. It’s a word I only know as half of -buckler. De Battisti explained that the problem isn’t piracy; it’s frequency. “Basically, the swash is the amount of water that runs up the shore after a wave breaks on the beach. The swash hits the dune toe, partially eroding the sediment, and then comes back taking away the sediment. So, generally, the swash has less power than its corresponding wave (i.e. the wave that broke and created the swash) because part of the energy wave has been dissipated in the breaking process and along the beach slope before reaching the dune.”

“However, waves can directly hit the dunes only during a big storm surge. In contrast, the swash can attack and erode dunes more frequently, exactly because it expand further away from the waves breaking points. Thus, the swash has a strong role in sand dune erosion.”

To see how vegetation influenced erosion, De Battisti and Griffin collected cores of dunes and tested them in a flume. Careful analysis showed how the roots, rhizomes and buried shoots contributed to erosion resistance. A core, in this case, is a block of dune 25cm × 25cm × 25cm. In the paper the process of gathering the cores is covered by “Cores were collected…” but De Battisti said that getting the cores right was not simple.

Dune cores. Photo: Davide De Battisti.

“The cores extraction required a lot of work. For inserting the core, I needed to hammer the core inside the sediment, which was not easy because of the compactness of sand. I used a wood plank that I placed on top of the core, and I hammered the wood to avoid hitting the core directly and damaging it. I had to be careful when striking because I didn’t want to disrupt the sediment, although it was something that was not possible to avoid altogether.”

“After the core was inserted at the required depth, I inserted the metal plate in the front part of the core (the core was designed with one side open to facilitate the extraction of the sample in the laboratory for the flume test). Then, I dug a hole in front of the core and inserted another metal plate at the bottom of the core. This closed the core itself and avoided losing sediment during transport to the laboratory. For Ammophila arenaria, I inserted the spade in each side of the core and hit the spade few time to cut possible rhizomes connection with other plant clones.”

“At this point was possible to extract and lift, by hand, the core and bring it to the vehicle. In total, it took me about 30 minutes for extracting each core. Also, there were times that I had to extract the cores during bad weather, which increased the time and effort to extract each core.”

One of the surprises in the paper is that annual plants can contribute to the fight against erosion. De Battisti said that the experiment had changed his ideas of how dune erosion works. “I had the first clue that buried shoots were important for sediment stabilisation during the flume experiment. During the experiment, I started to recognise that cores with annual plants were eroding less than the bare cores, although at that point was more an intuition. Then, when I was cleaning the plants from the sediment, I clearly saw that annual plants had few roots but abundant shoots buried under the sediment.”

“At that point was clear that, if the erosion reduction would have been significant in annual plants, then buried shoots should have had a strong role for sediment stability. Yet, at that time, I was still convinced that roots would have been the most important organ for sediment stabilisation.”

The results have relevance for dune restoration, De Battisti said. “I believe that this paper indicates the potential importance that annual plants can have for sediment stabilisation in sand dunes. In particular, our findings could lead managers to integrate the use of annual plants into management schemes.”

“Furthermore, this paper shows the importance in considering the context of the study. In terrestrial systems, sedimentation is (generally speaking) a negligible factor and therefore roots are the main plant organs presents in the sediment. Cleary, in erosion studies researchers have focused on the role of roots for sediment stabilisation. In contrast, in systems where sedimentation is high, such as sand dunes, researchers need to take this into account and thus incorporate each plant part that is found under the sediment.”

De Battisti and Griffin’s conclusions point to the importance of adding annual plants into dune restoration, and that variety helps promote sedimentation of sites.

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