‘Ents’ in Singapore? New research could uncover right soil conditions for giant trees

SINGAPORE – Nature is a many-splendoured thing, but it is big and beautiful things that most often draw praise. 

Yet, the most eye-catching of organisms sometimes rely on the unseen to thrive.

Take for example the dipterocarps, the “ents” of South-east Asia’s rainforests.

Bringing to mind the tree-like creatures in J.R.R. Tolkien’s fantasy world of Middle-earth, members of this family of trees are known to be among the tallest in South-east Asia’s rainforests.

As forest giants, these trees help to provide sanctuary and nesting places for birds and other species that dwell in the forest canopies.

Aided by ectomycorrhizal fungi in the soil that sheathe their roots and facilitate nutrient uptake, these formidable giants are known to reach heights of over 100m. 

A new research effort led by Singapore’s National Parks Board (NParks) wants to help the mighty by zooming in on the tiny. 

With the aim of improving dipterocarp growth in Singapore, NParks in February 2023 launched a grant call for research proposals that focus on understanding and enhancing the biological properties of soil and how dipterocarp growth can be enhanced in forested and urban areas.

On Sept 25, NParks said it is awarding the grant call to a research team led by A*Star.

As dipterocarps are also dominant trees of the native lowland forests of South-east Asia, this study will also contribute to forest restoration efforts in the region.

Dr Subhadip Ghosh, a senior scientist in plant science and health at NParks, said: “This will strengthen the resilience of these forests and ensure that the forests continue to provide many benefits for people and the planet.”

A healthy foundation

Of the over 500 dipterocarp species in South-east Asia, about 36 have been recorded in Singapore.

These trees are better known by the names of the wood that they produce, such as keruing (from trees of the genus Dipterocarpus) and meranti (from trees of the genus Shorea).

Dipterocarps form a symbiotic, or mutually beneficial, relationship with ectomycorrhizal fungi in soil.  

The fungi attach to the roots of the trees, and facilitate the uptake of nutrients by the trees. This promotes better growth and enhances the resilience of the trees against drought stress and disease. 

But while the association between the fungi and dipterocarps is well-known, there are limited studies on the diversity of ectomycorrhizal fungi in tropical soils, said Dr Ghosh, who is also a senior researcher at the NParks Centre for Urban Greenery and Ecology.

Previous studies have highlighted how these fungi form a diverse group of at least 6,000 species. 

“There is also limited knowledge on the different factors that affect the symbiotic relationship between the ectomycorrhizal fungi and dipterocarps,” said Dr Ghosh.

Botanists interviewed by The Straits Times pointed to a number of other unknowns in the relationship between the strapping trees and their earthy foundations.

Associate Professor Adrian Loo, a botanist and deputy director of the Centre for Nature-based Climate Solutions at the National University of Singapore (NUS), said he wanted to know which species of fungi form symbiotic relationships with different dipterocarps. Knowing this could be useful in priming or inoculating the soil with useful fungi, he added.

For his colleague, Dr Chua Siew Chin, a senior lecturer at NUS who is an expert on forest restoration, identifying the factors that can directly benefit plant growth is key.

“In the forests, there are many fungi and bacterial communities that are important to nutrient transformation in the soil and hence plant growth,” she noted. Human activities can also impact the physical and chemical properties of the soil, such as through pollution with building materials or plastics.

Dr Chua also pointed out that light is another important factor in dipterocarp growth, as many of these seedlings naturally prefer shade and are less drought-tolerant. 

The research findings from the NParks grant call could help to shed light on some of these unknowns. This could include, for example, learning about the variety of soil microbiome compositions associated with dipterocarps, said NParks’ Dr Ghosh.

Similar to how the human gut is home to a great diversity of microorganisms, such as bacteria, which help us break down food into nutrients that can be absorbed by the body, soils also host an assemblage of microorganisms. 

Different compositions of microorganisms could have different effects on how the ectomycorrhizal fungi influences the growth of dipterocarps. 

A better understanding of the microbial communities associated with dipterocarps could help NParks develop methods to introduce and optimise microbial communities in soil – not unlike how humans take probiotics to improve gut health – to help dipterocarps grow better in forests and urban areas, said Dr Ghosh. 

Return of the giants

Lowland dipterocarp forest once covered over 80 per cent of Singapore, but less than 1 per cent of it remains.

Today, these trees are mainly found in the nation’s remaining primary forest, patches of which can be found in the Central Catchment and the Bukit Timah nature reserves.

Given that dipterocarps are dominant trees in Singapore’s primary forests, the health of this group of trees is critical to the resilience of the Republic’s forests within the nature reserves, Dr Ghosh said.

The research outcomes could also pave the way for the reintroduction of dipterocarps in areas where they are currently not present or not growing optimally, such as in secondary forests and urban areas. 

There are currently about 100 dipterocarp species planted in Singapore’s urban areas, said Dr Ghosh.

Dr Shawn Lum, a senior lecturer at NTU’s Asian School of the Environment, told ST that dipterocarps can be seen along certain urban spaces in Singapore.

But these urban dipterocarps are usually of a species non-native to Singapore.

Hopea odorata, a dipterocarp with a cone-shaped crown typical of younger trees grown in the open, is native to seasonal areas in tropical Asia, from the far north of West Malaysia into Myanmar and Bangladesh, he said.

These trees can be found growing along the CTE and PIE, said Dr Lum.

In the forest, this species develops a lollipop shape typical of many other forest species, he added. 

“More recently, a few species of native dipterocarps have been introduced to our streetscapes, but these do not seem to have taken to our streets and urban landscapes as comfortably as Hopea odorata,” he added.

The reason for this could lie underground, since urban soil tends to differ from forest soil.

As Dr Lum puts it: “If I were a forest soil dweller, I would imagine urban soils could be a harsh place to live.”

NUS’ Dr Chua said urban soil tends to lack an organic layer as it no longer receives inputs of dead organic matter from vegetation, which means its nutrient content is likely low.

“Some of the urban soil might even have been excavated from our construction sites, which means it is likely highly compacted, clay-like soil that has poor porosity for healthy root growth,” she added.

NParks’ Dr Ghosh said primary forests, secondary vegetation and urban green spaces are of interest for this grant call topic. The aim, he added, is to introduce at least 15 species of dipterocarps currently not found in urban areas to these settings. 

On the research gaps that need to be plugged in order to improve understanding of how urban soil could be made more conducive for dipterocarp growth, NTU’s Dr Lum said a better understanding of the ectomycorrhizal community in the forest is first needed.

“We need to know the diversity, species composition, changes over time, interactions, and functions of these fungi, and see how, and which components, are resilient enough to adapt to the urban environment,” he said.

Only with this information would it be possible to see how the forest soil can be recreated for the urban setting, such as through the addition of organic material, and finding the optimal means of shading and temperature regulation, Dr Lum said.

Since these trees are large, their introduction into the urban landscape would require careful planning, said the three experts. But their presence would have many other benefits.

“As large trees, dipterocarps will do what other large trees do – they will provide shade and aid in the reduction of that unhealthy urban scourge, urban heat island effect,” said NTU’s Dr Lum.

“They will provide sanctuary and nesting places for birds and other canopy species.

“They are also indescribably regal and beautiful, and will bring nature’s magic stardust into urban settings.”

For NUS’ Dr Loo, their presence in urban areas could help people learn more about the region’s natural heritage, since dipterocarps are distinctive pillars of South-east Asian rainforests.

He pointed to how some of the dipterocarp trees in Singapore’s nature reserves can be a few hundred years old.

“Growing them in our urban spaces would be like bringing the Bukit Timah forest to our backyard. The urban landscape could be a living arboretum for these grand and statuesque trees,” he said.