When we think about restoring rainforests, our attention is often captivated by what we see above ground: towering trees laden with fruit supporting charismatic creatures like the Southern Cassowary or Tree Kangaroo. But there’s a whole world beneath the surface, working quietly and steadily to support the entire ecosystem. Soil regeneration, often overlooked, is fundamental to the health of rainforests and is a cornerstone of any effective restoration project.

The quality of soil directly influences the entire ecosystem’s resilience and success.”

Dr. Alex Cheesman.

A Look Below the Surface

Dr. Alex Cheesman, Rainforest Rescue’s Tropical Plant Ecophysiologist and Restoration Advisor, considers how 25 years can transform the soils in a former cane field or cattle paddock into the foundation for a self-sustaining rainforest.

“The quality of soil directly influences the entire ecosystem’s resilience and success,” Dr. Cheesman says.

But what exactly happens below ground over a quarter of a century? Soils that were compacted by heavy machinery or animal traffic can begin to regain their structure, nutrients and organic matter that were once removed by harvesting or swept downstream by soil erosion can be held in place, and microorganisms return, creating an environment where new trees can take root and thrive.

Factors Influencing Rainforest Restoration Success

The journey from degraded land to rainforest is guided by numerous factors, from initial soil structure, pH (soil acidity), and organic matter content to the presence of particular mycorrhizal fungi and invertebrates. Sites which have been used for agriculture for 50+ years may have no native seedbank, and lack the symbiotic fungi that trees need to thrive. Heavily tilled sites like sugarcane paddocks may lack the invertebrate and microbial community needed to effectively cycle organic matter back into the soils. Whilst sloped areas that have undergone heavy grazing may have seen nutrients lost downstream to erosion.

Rainforest Rescue’s methods, which include carefully selecting native plant species to suit specific soils, and introducing organic native soil during planting, aim to recreate conditions that foster soil health. For example, some species of rainforest tree like Cardwellia sublimis (Proteaceae) form ‘cluster roots’ fine root hairs specially adapted to extract phosphorus from low fertility heavily weathered soils, while the inclusion of native forest soils at planting provides saplings an inoculation of critical mycorrhizal fungi. As the plants grow site management may be needed to control weeds, but efforts are made to leave fallen branches and woody debris in place providing critical habitat and ecosystem complexity. This ‘bottom-up’ approach speeds up natural processes of restoration, helping soil regain the complex structure found in old-growth rainforests.

Is It Possible to Replicate Rainforest Soil?

While it’s hard to fully replicate the intricate make-up of ancient rainforest soil from day one, restoration efforts can set the stage. By encouraging the right conditions, soil can begin regenerating sooner. Over time, we observe a shift from hard, compacted earth towards richer soils, teeming with life—an encouraging signal that the land is moving closer to its natural state. Over a span of 25 years, certain milestones mark a restoring rainforest’s progress.

Early stages may see hard soils broken up by delving roots from early pioneers, and the return of deep leaf litter layer protecting it from erosion during high intensity rainstorms. As the soil matures, termites, fungi, and a diversity of microbes return, facilitating the cycling of this organic matter, returning nutrients in the topsoil that sustain forest growth.

However, this can take time, researchers working with Rainforest Rescue like Dr Baptiste Wijas (University of Miami) and our very own Alex Cheesman have shown that the return of termites (an important ecosystem engineer) to secondary forest in the Daintree can be a slow process with notable differences in the abundance and impact of termites between old growth and secondary forests even after two decades of recovery.

Rainforests, Rivers, and Reefs – A Connected Ecosystem

Rainforests play an essential role in protecting nearby rivers and the reef, acting as natural filters. A healthy rainforest soil absorbs and holds rainwater in the landscape not only protecting trees from seasonal drought but preventing sediment and nutrient runoff overwhelming rivers and reaching delicate reef ecosystems.

During severe climatic events, like cyclones or the floods seen across the Australian Wet Tropics in January 2024, the interconnectedness of these systems becomes even more vital. A resilient rainforest with robust soil can mitigate damage downstream, reducing runoff and helping forest resilience in the face of climate change.

Soil’s Role in a Sustainable Future

Looking forward, as Australia commits to Net Zero goals, soil has a powerful role to play. Healthy rainforest soils store significant amounts of carbon, keeping it out of the atmosphere while supporting biodiversity and ecosystem resilience.

To advance our understanding of how forest regeneration sequesters CO2 from the atmosphere, Rainforest Rescue is collaborating with researchers like Fred Holden and Prof. Paul Nelson at James Cook University, to study changes in carbon and other important nutrient levels within topsoils of former sugarcane paddocks transitioning to rainforests. This research will not only help quantify carbon sequestration into these soils but also help guide more effective restoration techniques.

Currently there is a gap in research to determine how quickly and successfully degraded rainforest soils can recover to a state akin to their ‘original’ form. So, these studies are supporting the enrichment of this area of knowledge, where history has shown it is easy to destroy but much harder to restore.

As Rainforest Rescue marks 25 years of habitat conservation and restoration, they continue to lay the groundwork for the next quarter century, partnering with researchers and community groups to help restore rainforests that support local biodiversity, combat climate change, and strengthen soil health.

Authored by Dr Alex Cheesman, Rainforest Rescue’s Tropical Plant Ecophysiologist and Restoration Advisor. Alex is a Senior Research Fellow working at James Cook University and the University of Exeter in the UK.