When orchards fall silent and fruiting trees are cut, their wood is often destined for firewood or decay. But imagine those discarded trunks becoming anchors of hope, submerged beneath shifting tides—places where fish breed, algae cling, and life pulses again.
That is exactly what is happening in the Wadden Sea of the Netherlands, where researchers are sinking culled pear trees to rebuild reefs.
The outcome? A revival of biodiversity so swift, so vivid, it challenges what many believed possible in degraded marine environments.
Experiment Born From Observation
It began with an insight from coastal ecologists who noticed the gradual disappearance of hard surfaces in soft, sandy seafloors. Over centuries, rivers once carried driftwood and branches to coastal seas.
These natural structures—wood, stones, shells—offered three-dimensional structure: refuge, attachment surfaces, feeding grounds. But human interventions—logging, damming, removal of wood, dredging—eroded that natural architecture. The seabed became flatter, poorer, less supportive of life.
Tjeerd Bouma with NIOZ (Royal Netherlands Institute for Sea Research) would pass orchards of pear and apple trees on his bike rides and think: what if these trees, past economic productivity, could find a second life underwater?
Building The Tree-Reefs
In April 2022, scientists harvested 192 pear trees deemed past their fruit-producing prime. They fashioned these into 32 pyramid-shaped reef structures, setting them at four different soft-bottomed sites in the Wadden Sea, at depths of roughly 3-4 metres. Concrete blocks anchored the bases.
Each pyramid used six interconnected, short-stemmed trees. These trees are “waste” in orchard economics, but structurally complex, biodegradable, and available in large numbers each year as orchards are pruned or replanted.
First Signs Of Life: Rapid Colonization
Within four months of submergence, life was already profusing. Sessile organisms—those that stay fixed in one place—were crawling across wood surfaces. Barnacles, bryozoans, hydroid polyps, sea grapes, and algae all found purchase along branches of submerged wood.
The vertical surfaces created by branching tree limbs allowed for a kind of zonation: near the sea floor lived algae and organisms less tolerant of strong currents; higher up, bryozoans and other filter-feeders thrived.
Mobile species soon followed. Fish traps set around the reefs returned not only more individuals but more species than traps in nearby sandy controls. Fish like five-bearded rockling, common goby, European eel, and crustaceans like prawns and shrimp appeared in greater numbers and larger sizes. In many cases, there were five times more fish around the tree reefs than in control sites.
Restoring What Was Lost: The Essential Fourth Point
This is perhaps the most important thread of the story: what has disappeared over centuries, and the possibility of bringing it back.
Historically, soft-bottom seas like the Wadden were far from lifeless. Driftwood, logs from upriver, shell debris, peat, stones—natural hard substrates—formed reefs: mussels, oysters, tubeworms, bryozoans, and many more.
These structures provided essential habitat complexity: hiding places for juvenile fish, attachment surfaces for algae and filter feeders, nursery grounds for crustaceans.
But over time, many of these substrates have been lost or buried:
- Rivers were dammed, reducing the wood flow downstream.
- Shellfish reefs have been over-fished or diseased.
- Seafloor disturbance by dredging, bottom-trawling, and sediment shifts buried hard surfaces.
The tree-reefs aim not only to provide temporary habitat, but to kick-start a regenerating ecosystem—to restore the natural processes and physical complexity that were once normal in these waters. By re-introducing these structures, researchers believe shellfish reefs (mussels, oysters), biogenic reefs built by living organisms, can reestablish, multiplying benefits: improving water quality, stabilizing sediments, offering more niches, supporting food webs.
What’s Surprising: Speed, Scale, And Behavior
Several findings have surprised the scientists involved:
- Speed: Much of what might take years elsewhere happened in months. Tree reefs were heavily colonised within four to six months.
- Abundance And Size: Fish caught around tree reefs were often larger than those in control sites. Rockling in particular reached sizes indicating mature individuals.
- Egg Laying And Reproduction: After 16 months, cuttlefish eggs were found—nearly 2,000 on one reef. This suggests these structures are not just aggregating organisms but providing conditions for reproduction and generational renewal.
Challenges, Unknowns And How To Adapt
Despite the optimism, the experiment raises critical questions:
- Longevity: How long will the tree-reefs function? Wood is biodegradable. Wood-boring organisms can accelerate decay, especially in warmer waters.
- Scalability And Location Dependence: Will results be similar in colder, warmer, clearer, or more turbid waters?
- Ecological Risks: Could artificial wood structures facilitate invasive species or change predator-prey balances? Continuous monitoring is essential.
Looking Forward: Scaling, Adaptation, And Impact
The promise of the pear-tree reefs is already leading to hopes of broader application. Restoration managers and conservationists see this as a tool—not a silver bullet—but one component of restoring coastal biodiversity.
Efforts are underway to monitor these tree reefs long term: observing seasonal shifts, successional changes in sessile communities, and whether larger reef-building shellfish eventually dominate. Researchers also consider how to adapt the model: experimenting with other wood species, adjusting reef shape, and combining with other restoration measures.
The ripple effect is profound: when fish and crustaceans return, birds and seals follow. When filter feeders thrive, water clarity improves. The sea becomes not just structurally richer, but functionally healthier.
Conclusion: What The Pear Trees Teach Us
What started as imagining a tree’s second life is showing how restoration rooted in humility and history can heal ecosystems. In the shifting sands of the Wadden Sea, these pear-tree pyramids remind us that lost habitats aren’t always gone—they can sometimes be coaxed back.
The pear tree reefs are more than structures underwater: they are symbols of hope. They show that restoration, when done thoughtfully, can yield results sooner and more beautifully than expected. If handled carefully, these tree-reefs might not only restore biodiversity but restore our belief that humans can partner with nature’s recovery, allowing seas to thrive once more.
Sources:
The Guardian
Mongabay
Frontiersin
