Rare Fungi Drive Rainforest Restoration on Palmyra Atoll

A new study has found that native rainforest restoration on the remote Palmyra Atoll depends on symbiotic mycorrhizal fungi that were previously overlooked. Researchers discovered that the atoll's native pisonia trees form a complete dependency on a specific fungal genus called Tomentella, which captures nutrients from seabird guano that would otherwise wash into the ocean. The finding carries significant implications for an effort to restore Palmyra's forests by removing 1.5 million invasive coconut palms, since natural regeneration may fail in cleared areas where these essential fungi are absent from the soil.

 

The Hidden Fungal Partnership

 

Palmyra Atoll in the North Pacific ranks among the most remote island systems on Earth. A native rainforest tree there provides a critical ecological service by offering nesting sites for thousands of seabirds. The guano produced by these birds fuels the surrounding coral reefs, linking the forest to the wider marine ecosystem. The new study revealed that this entire cycle rests on an invisible partner in the form of symbiotic mycorrhizal fungi. Without these fungi, the connection between forest, birds and reef could not function as it does.

Researchers mapped fungal diversity across the atoll and uncovered a striking dependency in the native pisonia trees. Every tree sampled showed a complete association with the fungal genus Tomentella, meaning the trees rely entirely on these fungi to survive. The fungi capture intense pulses of nitrogen and phosphorus released by bird guano before those nutrients can escape into the sea. This relationship was present in every single tree the team examined. The consistency of the finding underlines how fundamental the partnership is to the ecosystem.

 

An Ancient Evolutionary Adaptation

 

The fungi involved display an unusual tolerance for nutrient-rich conditions. Most ectomycorrhizal fungi struggle in soils saturated with nutrients, yet the Tomentella associated with pisonia appear adapted to high phosphorus levels. These conditions are created directly by the accumulation of seabird guano across the atoll. This adaptation sets the fungi apart from many of their relatives found elsewhere. It allows them to thrive precisely where other fungi would falter.

Study researchers interpret this as evidence of a long shared evolutionary history. The adaptation suggests a partnership stretching back across time between the fungi, the trees and the seabird colonies. These massive colonies shape the atoll ecosystem through the nutrients they deposit. The three-way relationship points to co-evolution rather than a recent or incidental pairing. This deep history helps explain why the dependency is now so complete and consistent.

 

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Implications for Forest Restoration

 

The discovery directly affects the ongoing plan to restore Palmyra's native forests. That effort involves removing 1.5 million invasive coconut palms to allow native vegetation to return. However, the study found that Tomentella abundance drops sharply beyond 250 metres from an existing pisonia tree. This means large cleared areas may lack the fungi required for native trees to establish. Natural regeneration could therefore fail in exactly the places restoration aims to recover.

To address this risk, conservationists may need to actively introduce the fungi into cleared soils. Study researchers suggest that inoculating soils with Tomentella could support reforestation success. This may prove especially necessary in areas far from existing pisonia forests where the fungi are scarce. It could also apply to atoll systems where pisonia has been absent for long periods due to legacy coconut agriculture. Researchers caution that further field experiments are still needed to confirm whether inoculation improves seedling survival and growth.

 

A Reservoir of Rare Biodiversity

 

Beyond the restoration findings, the study revealed Palmyra as a centre of rare fungal diversity. The atoll hosts globally rare and potentially new fungal species never before recorded in worldwide databases. Some of these fungi were found colonising aerial roots suspended more than a metre above the ground. This unusual location suggests they may be dispersed by wind or birds rather than through soil alone. The presence of undocumented species highlights how little is known about these remote microbial communities.

Land crabs also emerged as key players in shaping this hidden fungal world. The atoll's hundreds of thousands of land crabs were identified as ecosystem engineers for the microbial community. By excavating and mixing the soil, these crabs significantly increase fungal richness within their burrows. Some of the crabs have a leg span exceeding three feet across, reflecting their scale as physical agents in the environment. Researchers stress that protecting this hidden biodiversity matters because such microbes support ecosystem health, resilience and forest regeneration.