Coastal dunes may have a tipping point — and it lives in the grass. A new study of young dune fields on the Dutch coast found that patches of beach grass gain sand when the grass is dense enough, but lose sand once cover drops below a certain level, and that loss can feed on itself.

The research, published in the Proceedings of the National Academy of Sciences, was led by ecologist Paul Berghuis of Utrecht University with colleagues from the Royal Netherlands Institute for Sea Research. The authors concluded that below this critical density, decline becomes self-reinforcing.

That has a practical edge for beachgoers. If disturbance such as trampling or storm damage knocks grass cover below the threshold, a patch could start losing sand and keep losing it, according to a summary of the study by Earth.com. The study did not measure foot traffic directly, so that is a plausible risk rather than a tested one.

Grass Patches That Grow Together

Dunes start small — often as a single clump of grass. Wind blows sand across a beach, and clumps of grass slow it down so the sand drops and piles up. Over years, those mounds grow and merge into the ridges that protect towns behind them.

The researchers set out to learn what rule governs that growth. Using aerial photos sharp enough to resolve about 4 inches (10 centimeters) per pixel, they identified more than 10,000 grass patches across three young dune fields on the islands of Ameland and Texel and at the Sand Engine, an engineered sand peninsula near The Hague.

They also tracked the height of the dunes at Texel from 2014 to 2023. As patches merged there, the typical distance between clusters grew from about 2.3 meters (7.5 feet) to about 4.8 meters (16 feet).

"There is no fixed size. Sand is moved by the wind again and again before it stays somewhere," Berghuis told Earth.com, explaining why dunes keep coarsening rather than settling into neat rows. The pattern matched a theory proposed in 2023 by co-author Koen Siteur, now at the Dutch National Institute for Public Health and the Environment. Patches did not settle into a regular spacing. Instead, small patches fused into larger ones, and the largest dunes accounted for most of the growth over time.

A Threshold in the Grass

The central finding is a tipping point in grass density. Sparsely vegetated areas tended to lose sand, while densely vegetated areas gained it, and the gap grew larger over time.

"According to the theory, damage to the pattern does not heal itself once it exceeds a critical threshold value," Berghuis said. "Our measurements show exactly such a threshold value in the amount of marram grass."

The authors describe the process as density-dependent aggregation. In plain terms, sand and grass tend to collect where there is already more sand and grass. That is good news for building dunes, because strong patches get stronger. But it also means weak patches can slide in the other direction.

The team's earlier work, published in March in Nature Communications, found that neighboring grass patches start working together to trap sand once they are less than about 4.5 meters (15 feet) apart, according to Utrecht University. In connected groups, sand was captured and held up to twice as efficiently.

Lessons for U.S. Beaches

Dunes are a first line of defense for many U.S. coastal communities, from the Outer Banks to the Jersey Shore and Florida's Atlantic coast. They absorb wave energy during storms and help protect roads and homes behind them.

Many beach towns already ask visitors to stay off dunes and use marked walkovers, and the new findings offer a scientific reason that rule may matter more than it looks. Beach grass has deep roots that hold sand, but stems can be crushed by repeated foot traffic, and bare paths can become channels for wind and water.

Beachgoers can help by using boardwalks and designated paths, keeping pets and children off dune vegetation, and never pulling up or trampling grass. Homeowners near the coast who want to restore dunes should check with local officials or coastal programs before planting, since many areas regulate work on dunes.

The study also hints at a restoration strategy. If dunes grow fastest once grass passes a certain density, planting grass in clustered groups rather than scattered rows might help new dunes reach that point sooner. The authors suggested in their earlier work that strategic planting could lower restoration costs.

Limits Behind the Findings

The research has real limits. All three study sites were wide Dutch beaches with little erosion or human disturbance, so the threshold may behave differently on narrow, heavily used or eroding U.S. beaches.

The grass also matters. The Dutch dunes are dominated by marram grass, which grows upright and traps sand differently from the sea oats common on southeastern U.S. beaches or the creeping vines on many tropical dunes. The amount of cover needed to cross the threshold may vary by species and location.

The study was observational, based on photos and elevation maps rather than controlled experiments, and the height record covers about eight years. The article summaries do not give a single numeric threshold that would apply to any beach. And the study did not test trampling directly, so the link between foot traffic and dune collapse remains a reasonable inference that needs field testing.

For now, the findings add weight to familiar beach etiquette. Staying off dune grass costs a visitor a few extra steps. The study suggests that on some dunes, it may help keep a natural barrier from tipping into decline.

What Readers Want to Know

What did the dune study find?

Patches of beach grass gain sand when grass cover is dense, but lose sand below a critical threshold, and that decline can become self-reinforcing.

Where was the research done?

On young dune fields on the Dutch islands of Ameland and Texel and at the Sand Engine near The Hague.

Can walking on dune grass damage a dune?

It could. Researchers say disturbance such as trampling could push grass cover below the threshold, but the study did not test foot traffic directly.

Does this apply to U.S. beaches?

Possibly, but the threshold may differ because U.S. dunes often have different grasses, narrower beaches and more erosion.

What can beachgoers do?

Use boardwalks and marked paths, keep off dune vegetation, and follow local beach rules.

Could the findings help dune restoration?

The researchers suggest planting grass in clusters may help new dunes reach the growth threshold sooner and at lower cost.