Invaders in the Dunes

The spread of American beach grass could raise risks for structures along the Northwest coast, such as these new houses at Ocean Shores, Washington. (Photo: Phoebe Zarnetske)

Unnoticed by most beach–goers, a showdown is under way in Oregon’s coastal dunes, and the winner could pack increased risks for coastal property, especially during winter storms.

OSU scientists have documented a slow but steady takeover by American beach grass (Ammophila breviligulata), an invasive species from the East Coast and Great Lakes. They have found that protective “foredunes” covered by the new species are only about half as high as those created by the European species of grass (Ammophila arenaria), another non–native that was dominant. And they are initiating research to understand what gives the American variety the edge and what that might mean for coastal property owners and native plant restoration.

The takeover has already occurred from Ocean Shores, Washington, to Pacific City, Oregon, and it’s continuing. “This decrease in dune height may translate into a significant decrease in coastal protection from storms and tsunamis,” says Eric Seabloom, an OSU assistant professor of zoology. Historically, the dunes were more open than they are today, hosting plants such as wild rye and relatives of morning glory, buckwheat and other wildflowers. The European grass has stabilized Oregon dunes since it was first introduced for this purpose around 1900. “It did its job extremely well,” says Sally Hacker, OSU associate professor of zoology and an expert on estuaries. “Without it, the sand would cover towns and roads.”

It was so successful that by the 1930s it had spread along the entire Oregon coast and created an extensive “foredune” system, large protective sand hills in front of almost every sandy beach. These dunes can provide significant protection for homes, roads, towns and other infrastructure, and serve as a barrier against flooding during storm surges.

The second invasion, by American beach grass, went practically undetected for 50 years. Introduced near the mouth of the Columbia River in the mid–1930s, also to stabilize beaches, it out–competes its European cousin. It wasn’t until a survey in the late 1980s by Seabloom and a colleague at Evergreen State College that scientists realized how far it had spread, south to Tillamook Head and north to the Olympic Peninsula.

Coastal surveys have now determined that from Pacific City north, American beach
grass has nearly replaced the European variety. “Lower dune heights, increasing
wave heights that have been observed over the last 50 years and global climate change
could create a scenario in which the dunes no longer serve a coastal protection function,”
Hacker says.

With funding from Oregon Sea Grant, zoology Ph.D. student Phoebe Zarnetske of
Storrs, Connecticut, is teasing out the story behind these trends. In experiments at the
Hatfield Marine Science Center in Newport and at the O.H. Hinsdale Wave Research
Laboratory on the OSU campus, she is subjecting the two grasses to varying rates
of sand deposition to see which one thrives. She has visited practically every sandy
beach in Oregon and Washington to survey beach grass conditions. And as a student in OSU’s Ecosystem Informatics IGERT program, she will develop a mathematical model to explain how this dynamic system is changing.

Beyond the protection concerns, there are other ecological issues in play as well. While
the foredune system created by European beach grass is good for coastal landowners, it is not so good for endangered native plant species and the federally threatened
Western snowy plover. As more sand accumulated in growing stands of the European
grass, the land behind the dune tended to get turned into wetlands and forest habitats.
“The willows and other trees and larger shrubs you often see behind the dunes
are an indication that wetlands are being formed in the mini–valley behind the
dunes,” says Hacker.

As European grass advanced, beach habitat disappeared, taking with it the
plovers’ critical nesting grounds. The southward march of the American beach
grass could reverse the trend. Hacker and Seabloom are also working
with Peter Ruggiero, a coastal geomorphologist in the OSU Department of Geosciences,
to understand how coastal sediment supply and nearshore oceanographic
conditions influence beach grass competition and the coastal protection capabilities
of dunes. The researchers plan to meet with coastal property owners in 2008 to
discuss the results of their work.