Reefs made up of trees cut down to save marine life

Sunken discarded fruit trees in the Wadden Sea north of the Netherlands increase the local diversity and abundance of marine life

Reefs, whether natural or artificial, are breeding grounds for marine biodiversity. But reefs, particularly in soft-bottomed seas, have become scarce as many hard substrates have been removed by overfishing of shellfish, dredging, trawling and deep-sea mining. How can we restore this lost biodiversity, as promoted by the UN Decade for Restoring Ecosystems and the EU Biodiversity Strategy?

Researchers have shown that seaborne waste fruit trees are a cheap and effective way to restore reefs and improve local diversity and abundance of marine life. The study, published in Frontiers in Marine Science, was conducted in the Wadden Sea, a UNESCO World Heritage Site and the world’s largest salt marsh system.

“Here we show that native marine biodiversity can be restored in a severely degraded ecosystem like the Dutch Wadden Sea by using trees as reefs,” says Jon Dickson, lead author of the study and a PhD student at the Royal Netherlands Institute for Marine Research.

“Before humans tamed the landscape through agriculture, logging, and river control, trees fell into rivers in large numbers and were washed into the sea. We know that submerged timber has been present in marine ecosystems since the Jurassic and provides a home, shelter and food for marine life.”

slaughtered pear trees

In April 2022, Dickson et al. They built 32 pyramid structures from 192 felled pear trees that were past their economic useful life and transported them by boat to the open waters between the Dutch barrier islands of Texel and Vlieland. There, the “reef trees” were rooted in concrete feet and sank to the soft seabed in four different places, about three to four meters deep.

reef trees

One of the tree reefs after five months in the Wadden Sea. CC By: Jon Dickson

Four months later, they were briefly brought aboard a boat so researchers could count the number of different types of organisms attached to them, such as mollusks, algae or polyps. They were placed back on the seabed and allowed to accumulate more biodiversity there for another two months. Three fish traps were then set around each reef block and at nearby checkpoints and retrieved 24 hours later. All fish and crustaceans in the traps were counted and measured and their species identified before being safely released.

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Abundance of sea creatures

“Within six months, the reefs were covered with an abundance of sessile animals and algae, and harbored more fish than the surrounding control areas,” explains Dickson.

In total, the researchers found 15 species of sessile organisms: mostly barnacles and hydroid polyps, but also bryozoans, sea grapes, sea lettuce and starfish. Each of these taxa tended to specialize at a different elevation range from the seafloor.

Six fish species (such as Norway pout, goby and European eel) and four crustaceans were caught in the forest areas, compared to only two fish and five crustacean species in the control areas some 200 meters away. The frequency was also higher in forest areas: for example, 5.1 times more individuals of the dominant species, the bearded vulture, were caught than in control areas.

rapid colonization

“The current results highlight that initial colonization of natural tree reefs is rapid and suggest that restoration of communities associated with woody substrates may be possible through active restoration,” the authors conclude.

“Since we only conducted our experiment in one sea, we don’t yet know how tree reefs on the coasts of other continents would behave. And how long will they function as reefs if they are biodegradable? Which species will live in and around them in the long term? Those are questions we need to answer,” says Dickson.


Who lives in a pear tree under the sea? A first look at tree reefs as a complex natural, biodegradable structure to enhance biodiversity in marine systems

Photo: CC by Jon Dickson

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