Conservation and Biodiversity

Repeated logging depletes soil, pushing forests to ecological limits, finds new study

You cut a tree, you grow a tree. Sounds like basic arboreal math, right? Unfortunately, it might not be quite that simple.

A study published Tuesday in Global Change Biology found that repeated logging and re-growth of forests depletes soils of vital nutrients over time, pushing forest systems to ecological limits.

“Old-growth tropical forests that have been the same for millions of years are now changing irreversible due to repeated logging,” said first author Dr. Tom Swinfield in a press release from the University of Cambridge.

A tough leaf

Researchers used LIDAR-guided imaging spectroscopy to create high definition images of old- and new-growth tropical forest in northeastern Borneo, the third-largest island in the world. These images were then overlaid with nutrient measurements from 700 trees in the forest to map and compare old-growth with repeatedly logged forest.

The team found that logged and regrown areas had trees with tougher leaves and lower concentrations of phosphorus and nitrogen than old-growth trees. But tougher leaves must mean more resilient leaves, so that should be a good thing, right? Not exactly.

“Nutrient depletion and tougher leaves suggest reduced nutrient availability because plants respond to their environment by investing in defenses: Tougher leaves are harder to eat,” said Swinfield in an email. “Fewer nutrients also favors tree species well adapted to these conditions, leading to changes in species composition.”

In other words, for trees in the far-flung tropical forests of the Pacific Ocean, it’s survival of the fittest.

Phosphorus for the rest of us

So what is it about these soil nutrients — and phosphorus in particular — that makes trees and ecosystems so reliant on them? Phosphorus is a nutrient so essential to trees that Swinfield called it “the glue that holds DNA together… it literally holds life together.”

Phosphorus comes from rocks, but as Swinfield explained, most tropical soils are too deep for roots to reach the bedrock and tap into the phosphorus there. Also, phosphorus is tightly bound to inaccessible molecules in soil, which further limits its uptake by plants.

Old-growth forests, however, sidestep this issue through natural ecological processes — through decomposition and uptake by roots, nutrients are continually cycled. Essentially, when a tree falls in the forest, it rots and releases its nutrients back into the top levels of soil. These nutrients are eventually absorbed by other trees.

Unfortunately, logging not only prevents the natural cycles of tree fall, decomposition and nutrient release; logging also exports large quantities of phosphorus (trees) out of forests, reducing access to this critical nutrient, Swinfield explained. And as logged forests increase, the differences between old and new forests become more pronounced.

While the study focused on the primary forests of the Danum Valley and Maliau Basin as reference sites, Swinfield suspects this research to be relevant to lowland tropical forests on ancient soils more generally. And when Swinfield says ancient, he means ancient — the Borneo forests are the second-oldest continuously forested area in the world and are about 130 million years old.

These forests have witnessed the comings (and goings) of the dinosaurs. But they’re currently under threat from the cause of the sixth mass extinction: us.

Heart of the palm

So, who’s cutting down these forests in the first place?

The forests of Borneo have lost more than half of its forests, a third of which has happened in just the last three decades.The key drivers of this deforestation are palm oil plantations, pulp plantations, forest fires and illegal logging, which also helps illegal wildlife trade, since cleared forests make wildlife hunting and capture as easy as shooting fish in the proverbial barrel.

Recent research finds that palm oil is the main driver, responsible for at least 39% of forest loss in Borneo in the last 18 years. Indonesia and Malaysia produce about 85% of the world’s palm oil, which makes its way into everything from chocolate and instant ramen to shampoo and lipstick.

In recent years, many public-facing campaigns have emerged to inform consumers and call for boycotts on the product.

Growing causes for concern, but also hope

Reforestation efforts are underway to restore burnt and cleared areas of forest in Borneo, both for their carbon sequestration benefits and the wildlife habitat they provide to thousands of species native to the island.

Despite their relative fragility, the world’s trees are remarkably resilient, capable of maintaining growth even when phosphorus levels decline. Nonetheless, humans should take the study to heart and recognize that reforestation is not a remedy to our global logging problem.

“Our findings suggest [tropical trees] may be changed permanently by logging, which creates an additional reason to preserve them beyond their uniqueness for biodiversity and as a carbon asset,” said Swinfield. “I’m hopeful that carbon markets will continue to grow and promote a financial mechanism to pay for the conservation of this irreplaceable global asset.”

If a tree falls in the Borneo forest, and there’s no one there to hear it (or to log it), does it make a sound?

We leave that quandary to the philosophers, but we also need to leave that felled tree on the ground — as this week’s research shows what a powerful role natural cycles must play in the health of our soil, our trees, our forests and our planet.