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Forest restoration to boost biomass doesn’t have to sacrifice tree diversity

Image Credit: Light breaks through the canopy of Ulu Segama Forest Reserve in Sabah. Image courtesy of Nadine Keller.
  • Restoring degraded forests to boost biodiversity, store carbon and reconnect fragmented habitats is a burgeoning area of tropical forest conservation.
  • But uncertainty remains around the long-term impacts of various restoration approaches on forest biodiversity and functioning, with experts suggesting, for instance, that overly focusing on biomass accumulation for climate mitigation can come at the expense of species diversity.
  • A new study in Malaysian Borneo has found that actively restoring logged forest plots with a diversity of native timber species, coupled with management of competitive vegetation, actually boosted adult tree diversity after nearly two decades compared to plots left to regenerate naturally.
  • While the results add to a growing body of evidence that active restoration can lead to biodiversity gains, the authors caution that restoration approaches must be conducted in ecologically sensitive ways to avoid unintended outcomes.

Tropical forests the world over are in trouble. Vast swaths have been razed to the ground, lost forever, and studies indicate that at least 10% of those that remain standing are severely degraded. As humanity tries to backpedal on the destruction we’ve wrought, restoring degraded areas to their former glory through tree planting and forestry techniques has become a major endeavor. But scientists still know surprisingly little about the long-term effects of different restoration methods on forests.

Given that many restoration initiatives are in part motivated by climate change mitigation, methods that focus on the rapid accumulation of aboveground biomass, or tree growth, have drawn particular scrutiny. Some experts have suggested, for instance, that such human-assisted restoration can reduce overall forest diversity by favoring carbon storage at the expense of natural ecological processes. Instead, they advocate leaving forests to regenerate and recover of their own accord.

Now, a new study that investigates the long-term effects of forest restoration at sites in Malaysian Borneo indicates that planting trees for biomass accumulation can in some cases boost measures of biodiversity in the long run compared to natural regeneration.

Sungai Segama
Sungai Segama flows through Class I forest reserve, large swaths of which were logged during the 1980s and subsequently restored through various conservation initiatives. Image courtesy of Nadine Keller.

The researchers found enhanced adult tree diversity, including the recovery of rare species, in forest plots planted with timber species and subject to basic forestry maintenance, such as cutting climbers, compared to areas where forest regenerated naturally. The team from Malaysia, Switzerland and the U.K. published their findings in Forest Ecology and Management.

“The results were really surprising,” Nadine Keller, study lead author and a doctoral candidate at ETH Zürich at the time of the research, told Mongabay. “We expected to find lower biodiversity because fewer tree species were planted than would occur naturally, [but] we found the opposite.”

Keller and her colleagues studied sites in Ulu Segama Forest Reserve in Sabah state that had been selectively logged between 1981 and 1991 and subsequently partially restored with native timber species between 1993 and 2004. Although the initial aim of the restoration was to guarantee sustainable harvestable timber and maximize carbon storage, the area was protected as a Class I forest in 2012, so the site was never relogged.

A total of 76 native tree species were planted during the restoration period, including 53 dipterocarp species chosen for their timber quality, 15 fruit tree species, and three fast-growing “pioneer” species that can quickly colonize disturbed ground. Teams also cleared climbers and lianas from the site prior to planting seedlings and removed competing vegetation from around the young trees for three years after planting.

Restored forest
A forest plot studied by researchers in Ulu Segama Forest Reserve in Sabah, restoration began more than a decade ago. Image courtesy of Nadine Keller.

Keller said the high diversity of tree species planted could explain why this project successfully enhanced tree diversity nearly two decades after restoration. However, she added that the 76 planted species were distributed unevenly across a vast area of restoration plots that encompassed roughly 8,800 hectares (21,700 acres), so it’s unlikely that individual plots were planted with exceedingly high tree diversity.

“Seventy-six species sounds like a lot of species,” she said, “but in Danum Valley [a nearby forest reserve], there’s a 50-hectare [124-acre] forest monitoring plot, and they have 500 different tree species in just that 50 hectares!”

Joannès Guillemot, a researcher at CIRAD, the French Agricultural Research Centre for International Development, who was not involved in the study, said it’s tricky to draw conclusions about what factors led to the boost in adult tree and rare species diversity. That’s because the results are influenced by a combination of complex natural forest processes and a variety of management techniques, he said. But it’s likely down to the control of grasses and competitive plants allowing seedlings to grow strongly and close over the canopy, which in turn would have enhanced natural regeneration in the restored plots.

Guillemot said studies conducted in other parts of the world, such as the Atlantic Forest in Brazil, have found similar patterns, where intensive management has been found to enhance the regeneration of both planted and spontaneously colonizing species, particularly those dispersed by animals. These patterns show that “establishment and maintenance of restoration tree plantations (or tree enrichment) can, in some cases, be a win-win strategy for climate mitigation and biodiversity conservation in human-modified tropical landscapes,” Guillemot said.

Red leaf monkey
A red leaf monkey (Presbytis rubicunda) in the forest in Sabah. Image courtesy of Nadine Keller.

Keller said it’s vital to realize that individual study findings can’t be generalized across the tropics, since myriad local conditions heavily influence project outcomes, making direct comparisons impossible. “The success of restoration depends on so many factors,” she said, “starting with the environmental and climate factors, but then also how degraded the site was. A site that’s been heavily logged, for example, but is still a forest is a huge difference to a site that has been clear-cut.”

Keller also cautioned that restoration initiatives, including those focused on biomass accumulation and carbon storage, must be conducted in “ecologically sensitive” ways to avoid unanticipated outcomes, such as carbon release and wasted funds. She cited initiatives that showcase celebrities planting trees in closed canopy forests where there’s not enough light for seedlings to grow, and projects plugging trees into grasslands that already store vast amounts of soil carbon as particularly concerning.

Keller said she next aims to investigate how restoration methods influence belowground processes, including the association between Southeast Asia’s dipterocarp trees and mycorrhizal fungi, which lock away carbon in their biomass in the soil. “We know that soils can store much more carbon below ground than trees can store above ground, so this is particularly relevant and could be another amazing outcome of active restoration.”

Orangutan
An orangutan in the forest in Sabah. Image courtesy of Nadine Keller.

Carolyn Cowan is a staff writer for Mongabay. Follow her on � @CarolynCowan11.

Citations:

Keller, N., Niklaus, P. A., Ghazoul, J., Marfil, T., Godoong, E., & Philipson, C. D. (2023). Biodiversity consequences of long-term active forest restoration in selectively-logged tropical rainforests. Forest Ecology and Management, 549, 121414. doi:10.1016/j.foreco.2023.121414

Vancutsem, C., Achard, F., Pekel, J., Vieilledent, G., Carboni, S., Simonetti, D., … Nasi, R. (2021). Long-term (1990–2019) monitoring of forest cover changes in the humid tropics. Science Advances, 7(10). doi:10.1126/sciadv.abe1603

Brancalion, P. H., Campoe, O., Mendes, J. C., Noel, C., Moreira, G. G., Van Melis, J., … Guillemot, J. (2019). Intensive silviculture enhances biomass accumulation and tree diversity recovery in tropical forest restoration. Ecological Applications, 29(2). doi:10.1002/eap.1847

Crouzeilles, R., Ferreira, M. S., Chazdon, R. L., Lindenmayer, D. B., Sansevero, J. B. B., Monteiro, L., … Strassburg, B. B. N. (2017). Ecological restoration success is higher for natural regeneration than for active restoration in tropical forests. Science Advances, 3(11). doi:10.1126/sciadv.1701345

Carolyn Cowan
Carolyn Cowan
  Carolyn Cowan is a staff writer for Mongabay, covering Southeast Asia. Follow her work on Twitter @CarolynCowan11 or at her website www.carolyncowanmedia.com.
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