Interviewed: Giacomo Sellan, plant scientist (email@example.com)
(Photo: Heath forest in Kabili-Sepilok Forest Reserve, Sabah | Pic by Giacomo Sellan)
The Ibans call heath forests “Kerangas”, which means “lands that will not grow rice”. And it is not just rice that could not make it there.
As one treks from the lush lowland forests into heath forests, the trees change. Tall, thick trees give way to stunted ones with gnarled branches and “gracious” leaves, looking similar to bonsai trees, says plant scientist Giacomo Sellan.
A unique ecosystem on poor soil
Giacomo has studied the heath forests in Kabili-Sepilok Forest Reserve, Sabah. But clusters of heath forests grow across the tropics, from Borneo to the Amazon. He explains what makes these forests less productive – and unique – than the rest.
In heath forests, white sands carpet the forest floor instead of the clay soil typical of tropical rainforests. This sandy soil is acidic; in the Kabili-Sepilok Forest Reserve, heath forests soil has a pH lower than 4, which makes it more than 1000-times more acidic than pure water. Sand also cannot hold nutrients, so the soil is nutrient-poor.
Therefore, plant species diversity is lower on heath forest soil. But those that grow there are specialists that have adapted to an acidic environment. Giacomo reveals that it is not just plants that are specialists, but also some frogs and birds that live only in heath forests.
What is it about acidic soil that makes life difficult for plants? Well, forests thrive on two primary nutrients: nitrogen and phosphorus. But heath forests’ acidic soil lacks these, which then severely limits the growth of trees.
Decomposers dish out nutrients
Luckily for the plants, they can get nutrients from crucial partners formed over millions of years of evolution: decomposers like bacteria and mushroom. As decomposers break apart organic materials on the forest floor, they release nutrients which plant roots eagerly absorb.
But decomposers, like the plants, do better in some soils than others. In acidic soils, mushrooms thrive while bacteria take a backseat, as Giacomo explains.
However, this delicate process of decomposition could be changed and pressured by human activities.
Take nitrogen fertilisers, for example. Although adding nitrogen, or urea, through fertilisers could spur plant growth, studies have found that up to 40% of the applied urea would be lost to the atmosphere as ammonia gas within weeks. That leaves less organic matter in the soil. In contrast, soil bacteria could keep more nutrients in the soil and around the roots.
That is not the only challenge for heath forests. Giacomo gives an example of sand extraction and how this threatens heath forests in the Amazonian region, removing an important carbon store.
This unique sandy and nutrient-poor soil heath forests make the forests resilient to drought and heat. But it also makes them very susceptible to logging.
“There was a study not so long ago stating that heath forests might struggle more to regrow after it has been cut when compared to lowland area in rainforest,” says Giacomo.
“When you cut a heath forest, it struggles to grow back because of the nature of its very sandy and unfertilised soil.”
[Edited by YH Law]
G. Sellan, et al. 2019. Soil characteristics influence species composition and forest structure differentially among tree size classes in a Bornean heath forest. Plant and Soil, 438: 173-185.
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