Does Marram Grass Encourage Deeper Erosion?

The more I look the more I see evidence that recent erosion is strongest (ie leaving the tallest erosion scarp) in areas where the dunes are heavily covered by marram grass. Compared with areas where only runners of coastal spinifex, appear to leave a partially covered and “less well protected” dune front. These two photos where taken from the same location and the difference is very clear.

The photo on the right is looking slightly north to an area of heavy marram grass on the dune, and two recent erosion events. Further the beach infront of this is still showing its steeper curves winter shape. Compare this with the photo on the right, just taken straight-on here only a few runners of the coastal spinifex have regrown in front of previous erosion, but the slope is already flatter and the beach infront of this point has already started to develop its summer shape. This is often termed the summer berm. The beach above the high water mark has started to dry out and the curve of the upper becah is being flattened by wind erosion, leaving the characteristic wind blown ripple marks, with extra seaweed & other heavier debris to collect sand with wind gutters around the base and often a wind shadow of dry sand. It is the development of this summer berm that provides the best home for beach nesting birds, a rest place for animals and migrating bird (like penguins, terns and seals).

For a long time I have assumed that the taller erosions scarps are associated with deeper gutters out in the sand bars, and I still see that there will be a distinct gutter or channel in the vicinity of fresh erosion but the relationship with marram grass on the foredunes does seem to be becoming more compelling.

So  the big question is -
Does marram grass encourage deeper erosion of the dunes?

Still Eroding

 

Having been away for a couple of what I have come to know as the most active erosion months (August and September, presumably because of the strong swells and storms are common in these months). I was surprised that maybe there had not been as much erosion as I was expecting. Still quiet a lot of the beach shows an erosion scarp of 1 to 2m. I was also surprised to see so much seaweed on the beach (perhaps the swells have also disturbed the kelp forest off shore).

This location (just south of the beach one ramp) also shows clearly something I had noted before. The fenced “rehabilitation” area, has mainly coastal spinifex, where as the eroded area to the south is dominated by marram grass. Perhaps the flatter slope angle of the dunes with coastal spinifex lets the occasional high waves run up their surface rather than starting an erosion notch.

Tumbling Tumble Weed

Australia doesn’t really have the true (by which I mean the American Western Movie Style) tumble weed but the seed of the humble Spinifex, Spinifex sericeus comes very close. This grass also known as the beach spinifex is crucial for dune stability. It usually forms tussocks and has long rhizomes that spread out through the sand and hold the dunes in place. It can easily be propogated from rhizome cuttings planted in damp sand. Interestingly this spinifex has separate male and female plants. The above photo shows the dried flower, from a female plant, now holding the fertile seed. It blows along the beach following the shifting sand and thus populating newly formed dunes. A very clever design!

The introduced marram grass, is now competing with native spinifex all along the Venus Bay beach and may eventually replace all the native spinifex. The marram grass also helps stabilize the dunes, that is why it was originally introduced, but its dense growth may contribute to steeper dunes by entrapping more sand, at the expense of the upper beach zone (ie summer berm) This may be one factor in an increased susceptibility to direct wave erosion.  The dense grass mat also inhibits other dune pioneer plants and may reduce the traditional nesting and foraging habitat available for some small animals and birds. For example at Summerland Beach on Phillip Island the change in dune shape bought about by marram grass replacing spinifex has meant special temporary penguin tracks have had to be constructed up the dunes to give access to the little penguins breeding areas.