High in the treetops, canopy soils protect the forest below

In the rainforests of Costa Rica, a unique habitat forms high up in the trees. These “canopy soils” are an often overlooked aspect of forest ecology. 

A new study from Utah State University and Imperial College London not only exposes the wealth of biodiversity contained in canopy soils but also emphasizes their vulnerability in the face of climate change and deforestation.

Focus of the study

Canopy soils are essentially pockets of soil that accumulate along tree branches, creating a thriving ecosystem for various plant and animal species. But understanding how these soils form and the kind of biodiversity they nurture has been a challenge for ecologists.

“We found canopy soils most often form in cool, foggy areas in these tropical forests where there are large, old trees,” said study first author Jessica Murray. “Unfortunately, this describes some of the most at-risk forest types, which are threatened by changing climate and deforestation.”

The significance of these findings is further emphasized by Dr. Bonnie Waring, Murray’s PhD supervisor at Imperial.

“Canopy soils in tropical cloud forests are rich in life and nutrients and could be large carbon stores. The fact it’s taken until now to even start to recognise their importance is amazing – not least because we may be destroying them faster than we can study them,” said Dr. Waring.

A world above our heads

Murray climbed trees across six primary rainforest sites in Costa Rica, including some of the last continuous swaths of primary rainforest in South America. 

These varied landscapes, with altitudes spanning from 67 to 2,700 meters above sea level and temperatures ranging between 10-24 ◦C, offered diverse conditions ranging from varying rainfall patterns to dense fog.

Scaling trees as tall as 30 meters, Murray discovered that each soil mat – a combination of soil, moss, and plants – was a distinct microhabitat, akin to “its own little forest.”

Within these mats, a thriving community of plants and animals existed. Birds fed on abundant invertebrates. Epiphytes, which are plants that grow on other plants, flourished due to the nitrogen-rich environment created by mosses. 

Interestingly, trees themselves appeared to be aware of this nutrient abundance, as evidenced by roots growing into the canopy soils to extract their share of nourishment.

A fragile balance

The research indicates that the most abundant canopy soils are found in higher, cooler zones with a consistent fog presence and elder trees. The longevity of these soils remains uncertain, but estimates suggest that some might take over a century to fully mature, making their potential loss even more devastating.

With rising global temperatures, these cloud forests are experiencing shifting fog levels, potentially leading to the demise of many canopy soils. The towering trees that currently support them might soon be inadequate. 

Adding to the issue, significant portions of these forests face the impacts of deforestation. And in areas where recovery efforts are underway, the time might still be insufficient for the full restoration of these mature canopy soils.

“Determining the contribution of canopy soils to forest carbon budgets is important for assessing their role in mitigating climate change,” said Murray. 

“It could also be an important marker for recovery: if these soils are crucial for the forest to be an optimum carbon sink, then their formation should be a final marker of a truly recovered forest.”

Study implications 

Dr. Waring noted that the research demonstrates the value of intact forest ecosystems: restoration of logged forests is important, but some features can’t be put back. “We need some policies that focus on preserving landscapes that are ecologically diverse and carbon-rich.”

In an era where forest conservation is increasingly urgent, these findings shed light on an overlooked aspect of forest ecology and underscore the fragile nature of the relationships within these habitats.

The study is published in the journal Geoderma.

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