Where birds used to jump, fish now swim: sunken treetops could be used to restore damaged marine ecosystems, according to a pilot project in the North Sea off the Netherlands. The artificial reefs made from the branches of felled pear trees quickly turned into hotspots for marine life. According to the scientists, the cost-effective concept may have far-reaching potential for projects to strengthen threatened coastal ecosystems.
Where there is stable ground, life thrives: Especially in coastal areas that are heavily characterized by soft sand and mud, solid structures made of rock or driftwood play an important role as a basis: Fixed creatures such as mussels and others can settle on them contribute to the development of reef structures with complex communities. However, human influence has reduced the availability of such hard substrates in many areas of the world's oceans: Damaged mussel beds are increasingly being covered by sand, and changes in land use in river areas mean that fewer large pieces of driftwood are getting into the sea.
"Tree planting" on the seabed
There are already projects in which artificial hard structures - such as concrete - are sunk in the sea to serve as a basis for life. The researchers led by Jon Dickson from the Royal Netherlands Institute for Marine Research in Den Hoorn, on the other hand, came up with the idea of using finely structured structures that do not have to be specially produced: disused fruit trees that are produced in agriculture. As they explain, they orientate themselves towards original processes: "Before humans domesticated the landscape through agriculture, logging and river controls, large numbers of trees fell into rivers and were washed into the sea. We know that some of them sank there, providing homes, shelter and food for sea creatures from time immemorial,” says Dickson.
This was also the case in the area the team chose to test the concept: it is shallow water in front of the Wadden Sea around the Dutch islands of Texel and Vlieland. For the project, 32 pyramid-like structures were assembled from the crowns of 192 felled pear trees that had exceeded their useful life. These structures were then taken by ship to four locations in the test area and sunk about three to four meters deep onto the soft seabed using concrete weights.
Oases of marine life
After six months, the tree reefs were briefly lifted out of the water for examination. The researchers found that a lush community had already established itself on the structures. "Within six months, they were covered with an abundance of sessile animals and algae, and harbored more fish than the surrounding areas," reports Dickson. The scientists found a total of 15 species of sessile organisms. These were different representatives of shellfish, algae, polyps or even starfish.
The evaluation of traps on the tree reefs also showed that the fish population there was around five times higher than at the control sites around 200 meters away. Only two species have been found there, while the tree reefs were inhabited by six different species. "Our results suggest that the reintroduction of tree reefs as biodegradable, structurally complex hard substrates can increase local marine biodiversity in soft sediment systems within short periods of time," the researchers conclude.
This means that there may be far-reaching potential for the use of the method. However, there are still a few questions that need to be clarified, Dickson concludes: “Since they are biodegradable, how long can trees function as reefs? Which species will live in, on and around them in the longer term? In addition, since we have only conducted our experiments in a specific area of the sea so far, we do not yet know how tree reefs off the coasts of other regions would develop. These are questions that we should now answer,” says the scientist.
Source: Frontiers, professional article: Frontiers in Marine Science, doi: 10.3389/fmars.2023.1213790