Riding his bike to work in the Netherlands’ Zeeland province, Tjeerd Bouma passed fields of pear and apple trees. His mind wandered. As a coastal ecologist at the Royal Netherlands Institute for Sea Research (NIOZ), Bouma had been searching for a material with which to create artificial reefs in the Wadden Sea, a system of intertidal sand and mud flats that’s been heavily modified by humans over thousands of years. He realized that pear trees could be used for this very purpose.
“They have these nice branching structures and complexity, and they’re biodegradable,” Bouma told Mongabay. Moreover, Bouma knew that fruit trees in orchards had a limited life span. Once pear trees reach 25 to 35 years of age, they no longer produce enough fruit to be profitable, so farmers cull them, turning them into firewood or wood pellets or simply sending the trees to landfills. But maybe, Bouma thought, they could be repurposed to create reefs.
Bouma shared his idea with colleagues, including Jon Dickson, a researcher at NIOZ whose current work focuses on artificial reefs and marine environmental restoration. Dickson believed the idea had merit, since pear trees have the structural complexity needed for an artificial reef and because planting “tree reefs” would mimic a natural process.
“If we look at history … wood came down rivers all the time and got spit out at sea,” Dickson said. “Lots of it washed up, but lots of it sank, too. We have fossil records dating back to the Jurassic [period] about marine wood deposits and the animals that live on it, so we know that wood has been going out to sea for hundreds of millions of years.”
In many parts of the world, this natural flow of wood into the oceans has largely ceased since humans began modifying coastlines. In the Wadden Sea, for instance, wood and other naturally hard substrates were covered in sand or purposefully removed. This process was also hampered by upstream logging, the damming of estuaries, and even bottom trawling and dredging.
In a new paper published in Frontiers in Marine Science, for which Dickson was the lead author, researchers investigated the usefulness of pear trees as artificial reefs and their ability to attract marine biodiversity. They constructed 32 pyramid-shaped “tree reefs” of six interconnected short-stemmed pear trees. Then they transported the tree reefs to the open waters between two Dutch barrier islands, Texel and Vlieland, and settled them on the sandy bottom — about 3-4 meters (10-13 feet) deep — with concrete feet. After that, they waited.
Four months later, the researchers raised the tree reefs onto a ship to see what sessile organisms (those that can’t move on their own) were living on them. They found 15 species, including barnacles, bryozoans, anemones, tunicates and algae.
The researchers lowered the tree reefs back into the water, along with fish traps. When they raised the fish traps 24 hours later, they found crabs, shrimps, prawns, and several fish species, including fivebeard rockling (Ciliata mustela), shorthorn sculpin (Myoxocephalus scorpius) and pout whiting (Trisopterus luscus).
“We were surprised at the speed that it happened,” Dickson told Mongabay. “The amount of life we saw living on the trees after four months, we were expecting that after something like five years. It was incredible how fast the trees were just a profusion of life.”
The researchers also documented the presence of species at control sites and found that fish were consistently more abundant at the tree reef sites. However, the authors say the data don’t indicate “whether the tree-reefs are simply aggregating them or if they [are] also increasing the overall abundance of these fish in the area.”
Since the study’s publication, the researchers have pulled the tree reefs out of the water again, 16 months after their original planting, and found an even greater assortment of marine life.
“This year, we have cuttlefish laying eggs on the reef, which is really exciting to see,” Dickson said. “We found close to 2,000 eggs on a reef this year, and that’s just one reef. With that many cuttlefish in an area, perhaps the predation of crabs goes up. If there are less crabs in an area, maybe mussels can grow, and more things are going to continue changing.”
Dickson said he believes these pear tree reefs could last 15 to 20 years in the Wadden Sea — or, based on a more optimistic outlook, 50-75 years.
“Trees are relatively cheap, and hopefully we can kick-start the formation of natural reefs where there used to be some,” Dickson said. “And that can make a change because once the reef is there, it can keep growing on itself.”
He also said the idea of using felled trees to make reefs could be replicated in other parts of the world, especially temperate regions. Tree reefs could also work in the tropics, but Dickson said marine boring worms (family Teredinidae) would likely chew through the trees faster in these areas.
“The woodworm is very hungry and very angry in the tropics,” he said.
Currently, felled trees aren’t widely used as artificial reefs. However, there have been some small-scale initiatives in places like El Salvador, where local fishers sank dead mangrove wood, bicycle parts and concrete to attract marine biodiversity. It’s also typical for concrete blocks or retired ships to be used as artificial reefs. In the U.S., anglers use old Christmas trees to attract fish and increase their catches in lakes and reservoirs.
Timothy Baxter, a scientist at the University of Oxford who studies marine biodiversity in harbors and breakwaters but was not involved in the new study, said he was surprised by how quickly the pear trees were colonized by marine wildlife.
“The study presents an innovative technique for increasing biodiversity in marine environments where hard substrates have been depleted due to human activity,” Baxter told Mongabay in an email. “As the felled pear trees are used as an alternative to naturally occurring woody debris that has been depleted due to damming upstream, they represent a viable way of enhancing marine wildlife. Given that wood is a natural material that will degrade over time, the felled pear trees have the potential to be significantly more eco-friendly than other ‘ecological enhancement’ techniques that use concrete and other engineering materials. The low cost is also a benefit.”
Baxter said he thinks the next research stage should be to attempt a “scaled-up version of the study, both in terms of time and space.”
“It would be interesting to see the effects of felled pear trees over longer time scales and a greater spatial area,” he said. “In particular, it would be interesting to see whether, when scaled up, felled pear trees have any negative impact on biodiversity. This includes, for example, facilitating the spread of non-native or invasive types of marine organisms as has been shown for some wooden shipwrecks and other artificial structures in marine environments.”
In another recent study led by Baxter, researchers found that historic masonry, concrete and rocky cliff habitats in harbors also supported diverse communities of marine organisms.
“[I]n soft sediment environments, both [harbors and tree reefs] are likely to increase biodiversity compared to the surrounding environment,” Baxter said. “A combination of methods will be required to ensure marine ecosystems remain resilient to threats posed by increasing coastal development and climate change.”