NEWS
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.
Website: http://alunsalt.com
Reprinted From: http://bit.ly/2QnJPOQ
S
and dunes are one of the coastal
features that help protect shorelines
from storms, but they’re under con-
stant 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 erod-
ing the sediment, and then comes back
taking away the sediment. So, generally,
the swash has less power than its corre-
sponding 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 25 cm × 25 cm × 25 cm. In the pa-
per the process of gathering the cores is
covered by “Cores were collected…” but
De Battisti said that getting the cores right
was not simple.
“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 com-
pactness 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.”
19
Alun Salt
“After the core was inserted at the re-
quired 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 labora-
tory 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 in-
serted the spade in each side of the core
and hit the spade few time to cut possi-
ble rhizomes connection with other plant
clones.”
“At this point it was possible to extract and
lift, by hand, the core and bring it to the
vehicle. In total, it took me about 30 min-
utes 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 annu-
al plants were eroding less than the bare
cores, although at that point it was more
an intuition. Then, when I was cleaning
the plants from the sediment, I clearly saw
that annual plants had few roots but abun-
dant shoots buried under the sediment.”
“At that point it was clear that, if the ero-
sion reduction would have been signifi-
cant in annual plants, then buried shoots
should have had a strong role for sedi-
ment 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 po-
tential importance that annual plants can
have for sediment stabilisation in sand
dunes. In particular, our findings could
lead managers to integrate the use of an-
nual plants into management schemes.”
“Furthermore, this paper shows the im-
portance in considering the context of the
study. In terrestrial systems, sedimentation
is (generally speaking) a negligible factor
and therefore roots are the main plant or-
gans presents in the sediment. Clearly, in
erosion studies researchers have focused
on the role of roots for sediment stabili-
sation. In contrast, in systems where sedi-
mentation 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.
Figure 1: Dune cores. Photo: Davide De Battisti.
Grassroots
Vol 19
No 4
November 2019