In this post Bianca R. Charbonneau discusses her recent paper ‘A species effect on storm erosion: Invasive sedge stabilized dunes more than native grass during Hurricane Sandy‘
You can also read another blog post about this paper here: ‘The “bright side” of invasive species – with Portuguese and Spanish translations‘
Coastal zones are arguably the most dynamic terrestrial habitats worldwide by nature of their location as the interface between land and sea, making them unusually susceptible to erosion and damage from wind, water and other forces of nature. However, nature tends to safe-guard itself, and these geologically unstable areas have one natural defence – dunes. Dunes are natural buffers that absorb the destructive energy of high tides and storms by physically blocking waves and storm surges from propagating inland. We tend to consider dune height and depth as the most relevant factors for preventing erosion and causing infrastructure damage. However, in doing so we overlook plants – the most pivotal player in storm abatement, and for the genesis, development, and maintenance of these unique dune systems.
Though under some situations, dunes form spontaneously without the activity of plants, in harsh coastal areas, plants form the backbone that allows coastal dunes to grow and tolerate storms. Above-ground leaves trap sand particles blowing in the wind and bouncing along the sand surface; when the grains hit the leaves, forward progress is halted and the grain falls to the base of the plant, allowing dunes to grow taller and wider over time. Underground roots act like a net, growing dense both vertically and horizontally through the sand and thereby providing stabilization to an otherwise unstable mound of individual particles. Worldwide, only a handful of plants can tolerate the harsh conditions of the first dune mound closest to the ocean, the foredune. However, these pioneer plants and their habitat are immensely important for habitat creation in backdune areas and as the first and best line of defence during storms.
Though only a handful of plants can live where land meets sea, all dune plants are not created equal; species vary in morphology and physiology, and we thought that this should logically translate into a difference in their potential to combat collision dune erosion (erosion to the dune face from waves and surge crashing in) in a storm. Hurricane Sandy (in Fall 2012) offered the opportunity to test this idea. Using pre- and post-Sandy aerial images, in conjunction with ground truthing, we georeferenced dune features Island Beach State Park, NJ to compare Sandy erosion as a function of species; data collection was no easy task as we logged over a marathon of walking in the sand over a 10-km barrier island, with over 700 km of georeferencing in ArcGIS, but it is a good thing that we enjoy long walks on the beach… Specifically, we compared monoculture stretches of foredune dominated by and stabilized by either native American beach grass (Ammophila breviligulata) or invasive Asiatic sand sedge (Carex kobomugi). We quantified erosion with a new linear metric, dune crest transgression, which we found is related to the classic erosion metric, volumetric loss, can be applied to other habitats, and is accessible for conservation managers to calculate and use with a GPS and time! Dune crest transgression accounts for variability inherent in the habitat, and can be collected in a manner that controls for variable abiotic factors among comparisons, such as different wind conditions across a shoreline.
As if coastal management was not complicated enough already, we found that there is a species-effect on erosion. Specifically, we found that native Ammophila-dominated dunes suffered more erosion, about 3 m more dune transgression, and more volumetric loss, than the invasive Carex species. Similarly, dune height and beach width were the same among species such that we can account the differences in erosion to a species-effect. We recognize that these results further muddy the waters of invasive species management and the coastal management decision-making process. This species of Carex has been shown to have significant negative effects on dune community dynamics, and it is hard if not impossible to eradicate and prevent spreading along the coast. Thus, while its benefits in terms of dune stabilization may make its presence on dunes desirable to managers working to protect human infrastructure, its negative ecological consequences may outweigh those positives in terms of day to day management implementation.
The foredune is a phalanx defense, like that of the Spartans, where the line cannot be broken and is arguably only as strong as its weakest link! The Carex is still an invasive and should be treated as such, but ‘a’ plant, is better than none, such that bare sand should not be favoured over any vegetation. Understanding the way in which natural dunes, respond to storms is an important piece of the puzzle for buffering our coastlines as ‘safe’ geologically unstable areas, multi-functioning as homes, recreational areas, and biological habitats. We hope that this research will enrich and inform the decisions made by coastal managers and be considered without a grain of sand as they plan coastal dune management initiatives. Biotic components, like plants, need to be key components of management discussions that can otherwise be very engineering-centric in this day and age. Understanding the impacts of different plant communities on dune stability is one more piece in the complex puzzle that is dune management today!