A common way to restore a degraded forest is to plant seedlings and nurture these into full trees. Indeed, to most people restoration and tree-planting (with native species) are virtually synonymous. Planting and nurturing tree seedlings over a wide area, however, can be an expensive and labour-intensive affair. Not to mention the decades or even centuries it takes before seedlings grow into trees.

Another common path to restoration is to simply protect a degraded forest from further degradation, often by fencing it off. Over time, lost tree cover will return through natural regeneration. This option is a relatively inexpensive, but it can also take centuries to achieve full restoration and the environmental benefits it brings.

According to Rhett D. Harrison, tropical forest ecologist with the World Agroforestry Centre (ICRAF), there might be a middle road to the restoration of tropical forests: thinning a recovering forest.

Thinning is a forester’s term for removing undesirable stems to encourage the growth of more desirable stems.

When tropical forests are logged or otherwise disturbed, increased light levels lead to a big jump in the growth of pioneers —young trees, grasses and climbers—in the forest. Compared to the original forest, these pioneers have lower value for timber, carbon sequestration and biodiversity. Over time, however, some break through to become full trees.

In a collaborative experiment, Harrison, Tom Swinfield, Roki Afriandi and Ferry Antoni, researchers from ICRAF, Royal Society for Protection of Bird in UK, and PT Restorasi Ekosistem Indonesia respectively, were interested to see if selective cutting of some pioneers might increase the rate of forest recovery of a degraded tropical forest. And preliminary results, just published, appear to support the idea.

The research was conducted over a 280-hectare area in the Harapan Rainforest, an Ecosystem Restoration Concession in Sumatra, Indonesia. It sought to compare whether thinning pioneer species could shift the species composition towards the kind seen later in the forest’s regeneration.

“We hoped that by removing juvenile pioneer stems we could break the cycle of canopy refilling with pioneers,” says Harrison.

They also looked at the cost and ease of management of thinning, compared to enrichment planting (planting tree seedlings of specific species) for reforestation.

The thinning targeted several species of particularly abundant pioneers, including one invasive species (Bellucia pentamera). In addition to a control block left untouched, three thinning intensities were tested; the researchers removed pioneer vegetation based on its size, measured as the diameter at breast height (dbh):

Low-intensity: All undesired stems of less than10 cm dbh removed Medium intensity: All stems <15 cm dbh removed, and High intensity: All stems < 20 cm cm dbh removed.





Early results, published in the journal Forest Ecology and Management, indicate both a speed-of-recovery and cost benefit of thinning, opening up an exciting new possibility for the restoration of tropical forests.

In the low and medium-intensity treatments, thinning “only marginally increased understory light levels, but it substantially changed forest composition towards a later successional sere.” This correlates to a faster recovery of something approximating the original forest composition.

Harrison and colleagues also worked out that thinning cost less than 10% the cost of forest enrichment with seedlings. Furthermore, says Harrison, “thinning does not entail the subsequent costly management that is needed before planted seedlings can grow into full trees.”

“Moreover, the removed stems may have a market, for energy production for instance, which can help offset costs of restoration.”

Based on their initial results, the researchers recommend replication of thinning experiments in other tropical forests, as well as continued, long-term monitoring of their experimental site at Harapan Rainforest. This would help establish the applicability of thinning for restoration of tropical forests in Asia, Africa, and elsewhere—a global priority. It would also determine best practices—informed by the extent of degradation and socioeconomic factors such as resource and labour availability—for forest managers.

In addition, such studies would establish the impact of extreme weather events, such as drought, on forest recovery. This information is critical to the design of long-term restoration interventions. In the second year of monitoring the Harapan experiment, for instance, the severe 2015 El Nino drought dried up the foliage leading to greatly increased light in the understorey. And in highest intensity treatment in Harapan, the light reaching the forest floor encouraged the growth of pioneer species.

Harrison is quick to add, however, that natural regeneration of trees and deliberate enrichment planting will continue to be important interventions for forest and landscape restoration.

Indeed, enrichment planting, with desirable species such as high-value timber trees or those with high carbon potential, has worked extremely well in South Korea, Japan, and many other countries. And in highly degraded sites, it may be the only option.

With further research to establish best practices, thinning could become the middle road between expensive enrichment planting and cheap but slow natural regeneration, a path that will help us reach global commitments related to landscape restoration and climate change mitigation easier, cheaper and faster.

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Download full article: Accelerating tropical forest restoration through the selective removal of pioneer species. By Tom Swinfield, Roki Afriandi, Ferry Antoni, and Rhett D. Harrison. Forest Ecology and Management 381 (2016) 209–216. http://dx.doi.org/10.1016/j.foreco.2016.09.020

Read related blog by Rhett Harrison: Thinning – the restoration middle road

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This work was made possible by the funding agencies that have supported Harapan Rainforest, in particular the Federal Govern- ment of Germany through KfW [BMUB/IKI: 09_III_IDN_K_Harapan Rainforest] and the Government of Denmark through DANIDA Support to Harapan Rainforest [ESP3].

ICRAF The World Agroforestry Centre is one of the 15 members of the CGIAR. We would like to thank all donors who support research in development through their contributions to the CGIAR Fund.

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Further reading

What will it take to restore 100 million hectares of land in Africa?

Straight talk: Grants are needed to develop scalable community-based Forest Landscape Restoration (FLR) business models to achieve AFR100

Seeds of hope emerge across the world’s drylands

What makes a farmer grow a tree? It depends.

The Shinyanga revolution: Tanzanian success story creates momentum for land restoration

One small change of words – a giant leap in effectiveness!

Booklet: The quiet revolution: how Niger’s farmers are re-greening the croplands of the Sahel

https://www.youtube.com/watch?v=GKmlAjuZ6Hw