Invasion Of New Beach Grass Could Weaken Shoreline Protection

Scientists at Oregon State University have documented a slow but steady takeover by this beach grass, an invasive species. They found that protective “foredunes” covered by the new grass species are only about half as high as those created by the European species of grass that were formerly dominant.

This phenomenon has already occurred from Long Beach, Wash., to Pacific City, Ore., and is continuing to spread, the researchers say.

“This decrease in dune height may translate into a significant decrease in coastal protection from storms and tsunamis,” said Eric Seabloom, an OSU assistant professor of zoology.

In continuing studies, scientists plan to use oceanographic models to show just how much protection is lost when European beach grass is replaced by American beach grass.

The European grass – also an invasive species – has been dominant since it was first introduced to the area around the turn of the 20th century, to help stabilize blowing sand on the coast.

“It did its job extremely well,” said Sally Hacker, an OSU associate professor of zoology and expert on marine and estuarine communities. “Without it, the sand would cover towns and roads.”

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

But the second invasion by the American beach grass species had gone practically undetected. Introduced near the mouth of the Columbia River in the mid-1930s, also to stabilize beaches, American beach grass tends to out-compete its European cousin. The status of this beach grass variety went unnoticed for more than 50 years, until Seabloom and a colleague discovered it had crept as far south as Tillamook Head and as far north as the Olympic Peninsula.

Surveys of the entire Oregon coast have determined that the current range of domination of American beach grass extends from Long Beach, Wash., to Pacific City, Ore. But even beyond that, from Pacific City south, most of the beach grass is the American beach grass, with just a few pockets of European beach grass.

“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 said.

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 beach plant species and the federally-threatened Western snowy plover, scientists say. As more sand accumulates in growing stands of beach grass, the land behind the dune tends to get “terrestrialized,” or 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,” Hacker said.

As that process advances, beach habitat disappears, taking with it the plovers’ critical nesting grounds. The southward march of the American beach grass species could reverse the terrestrialization trend, as the American variety creates a much smaller foredune.

Hacker and Seabloom have received funding from Oregon Sea Grant to study the impacts and interactions of these invasive grasses on the Oregon coast. They are also working with Peter Ruggiero, a geomorphologist in the Department of Geosciences at OSU, to understand the coastal protection capabilities of dunes along the coast.