How Mycorrhiza is the hidden engine behind reforestation in Bali and Borneo

Beneath the soil, invisible to the naked eye...

A collaboration unfolds that determines whether trees in Bali can survive.

At Restore the Legacy, we work daily on restoring forests, and we see firsthand how essential this collaboration is.

Mycorrhiza, the symbiosis between fungi and plant roots, has existed since plants first colonized the land around 400 million years ago. Yet this relationship is often underestimated in reforestation projects.

For us, mycorrhiza is one of the most powerful natural tools for soil restoration, plant growth, and ecosystem resilience.  

What is Mycorrhiza?

Mycorrhiza literally means “fungus-root”.

It refers to the mutualistic symbiosis in which fungi colonize plant roots.

The plant provides carbohydrates to the fungus, while the fungus supports the plant with the uptake of water, phosphate, nitrogen, and micronutrients. In addition, the fungus protects the plant against pathogens and improves soil structure.

Mycorrhizal fungi greatly increase the effective root surface area, allowing plants to absorb nutrients much more efficiently. This is a crucial factor in degraded tropical soils.

How Arbuscular Mycorrhiza transports nutrients

Arbuscular mycorrhiza (AM) is the most common type of mycorrhizal symbiosis and occurs in about 80% of all land plants.

In the root cells, these fungi form branched structures called arbuscules, which function as specialized exchange organs.

On the fungal side, phosphate transporters, such as PT4-like transporters, take up inorganic phosphate from the hyphal network. In return, the plant supplies sugars and lipids via SWEET-transporters. This process is regulated by the Common Symbiosis Signaling Pathway (CSSP), which is also involved in the rhizobium symbiosis, a topic we will explore further in a later blog post.

Thanks to this extensive exchange, the plant can absorb phosphate from soil fractions that would otherwise be inaccessible, such as Fe-bound phosphate in tropical soils. 

Besides arbuscular mycorrhiza, there is also ectomycorrhiza. This form occurs mainly in trees in temperate and boreal forests. Ectomycorrhiza forms a sheath around the root and does not penetrate the cells. The fruiting bodies of these fungi are visible as mushrooms emerging from the forest floor in autumn. 

Why Mycorrhiza is crucial in tropical soils

Despite their lush vegetation, tropical soils are often poor in available nutrients.

Due to high rainfall, minerals leach quickly from the soil.

Many nutrients are bound in organic material and are therefore not directly available to plants. In addition, tropical soils contain large amounts of iron and aluminum oxides. Phosphate binds strongly to these materials, making it nearly insoluble.

As a result, the concentration of free phosphate in the soil solution is extremely low, sometimes less than 1 µM. AM fungi can mobilize these bound phosphate fractions by secreting phosphates and organic acids. This makes them essential for plant growth in ecosystems such as Bali.

How Mycorrhiza strengthens reforestation 

1. Improved Growth and Survival of Young Trees 

   At Restore the Legacy we see that AM fungi enhance the uptake of essential nutrients. Their hyphae act as an extension of the root system and explore soil volumes that the plant itself cannot reach. This allows young trees to establish more successfully in degraded soils. AM fungi also improve water regulation by increasing the hydraulic conductivity of the root zone. This is a crucial factor during dry periods. 
   

2. Restoration of Soil Structure 

   Mycorrhizal fungi contribute to the formation of stable soil aggregates. This occurs partly through glomalin, a sticky glycoprotein that coats hyphae and binds soil particles together. Glomalin is resistant to degradation and can remain in the soil for decades. As a result, soil porosity, aeration, and water retention improve. These conditions are essential for successful reforestation in tropical regions such as Bali. 
   

3. Increased Biodiversity 

   AM fungi can reduce differences in plant fitness by distributing nutrients more evenly among species. This promotes coexistence and increases plant diversity. In tropical forests, where biodiversity plays a key role in ecosystem stability, this function is highly important. Research shows that areas with high AM fungi diversity often also have higher tree species diversity. We clearly observe this in our projects. 
   

4. Deep Carbon Storage 

   AM fungi acts as “deep carbon engineers”. They stimulate the formation of stable carbon compounds in deeper soil layers, partly through glomalin and by promoting root growth. This carbon is broken down much more slowly than organic material at the surface. As a result, mycorrhiza contributes not only to reforestation but also to climate mitigation through long-term carbon storage. This is an important goal within our projects. 
   

5. Use of Local Fungal Species 

   Research in Indonesia shows that local AM fungi significantly improve the growth of native tree species. These fungi are adapted to local soil chemistry, temperature, and humidity. That is why we work exclusively with local inoculum sources from similar ecosystems. Commercial mixes are often less effective because they are not adapted to regional conditions. By using local fungi, we increase the chance of successful colonization and accelerate soil life recovery. 

How reforestation projects can support Mycorrhiza

At Restore the Legacy, we take into account the sensitivity of arbuscular mycorrhiza to soil disturbance.

We avoid ploughing and heavy machinery, as these damage the underground fungal network. We also only use local mycorrhizal fungi sourced from the same ecosystem type as our planting sites.  

In degraded landscapes, the mycorrhizal community is often reduced in diversity and activity.

By adding organic material, such as locally produced compost, leaf litter, or wood chips, we feed soil life and help fungal networks recover.

As we plant more species, AM fungal diversity increases as well.

This leads to more resilient ecosystems that are better adapted to drought, nutrient scarcity, and disease.

Conclusion

For us at Restore the Legacy, mycorrhiza is not a detail but a fundamental building block of healthy tropical forests.

In Bali, where soils are vulnerable and ecosystems are under pressure, actively using mycorrhizal fungi can mean the difference between a plant that dies and a forest that returns.

By using local fungi, restoring soil life, and promoting diversity, we restore not just trees. We restore entire ecosystems.

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