The Danum River runs through a dense tropical forest.
Malaysia’s Danum Valley Conservation Area, above, was one of the regions studied by researchers. Credit: Gido, CC BY 2.0

Actively restored forests recover aboveground biomass faster than areas left to regenerate naturally after being logged. According to a new study on tropical forests in Sabah, Malaysia, areas that have undergone active restoration recovered 50% faster, from 2.9 to 4.4 metric tons of aboveground carbon per hectare per year. The findings suggest that the reduction in carbon associated with a single logging event would be recovered to the same level as unlogged forest after 40 years with active restoration, as opposed to about 60 years if the forest were left to regenerate naturally.

In addition to demonstrating the value in protecting previously logged forests, the study engages with the efficacy of carbon pricing, said coauthor Mark Cutler, professor of geography and environmental science at the University of Dundee in Scotland.

Cutler explained that “the costs associated with the most intensive forms of active restoration, if to be recovered through the voluntary carbon market, require a higher carbon price than has been seen in recent times.”

“However,” he added, “varying the type and intensity of restoration treatments according to the residual amount of carbon in the forest stand has the potential to reduce net costs and bridge the gap to financial sustainability, thus enabling much larger areas of forest to be restored.”

The “high potential for carbon storage during forest restoration is not being matched by an adequate price on the voluntary carbon market.”

“The paper raises an important issue regarding the effectiveness of carbon pricing if it is to serve as a financial incentive for restoring logged forests,” said Robin Chazdon, a professor emerita at the University of Connecticut and a research professor at the University of the Sunshine Coast in Queensland, Australia. Chazdon was not involved in the study. “Several other papers have also noted that high potential for carbon storage during forest restoration is not being matched by an adequate price on the voluntary carbon market.”

Researchers estimated the current cost to offset 1 metric ton of carbon dioxide equivalent (CO2e) in voluntary carbon offsetting schemes to be around US$2–$10. Carbon prices required to fulfill the 2016 Paris Agreement (US$40–$80 per metric ton CO2e) would provide an economic justification for tropical forest restoration, according to the paper.

Restoring the “Most Productive Forests on Earth”

Tropical forests contain 55% of global stores of aboveground forest carbon, but stocks are declining because of forest loss and degradation.

Pierre Taillardat, a postdoctoral researcher on coastal and terrestrial wetlands at the University of Québec, Canada, explained that “tropical forest management can be an effective way to mitigate climate change considering that they are the most productive forests on Earth.”

Study authors noted that protection of existing forests, even those that have been degraded or previously exploited, is paramount from the point of view of retaining aboveground carbon density, as well as maintaining biodiversity and other critical ecosystem services.

Researchers used a restoration approach called “assisted natural regeneration” at Malaysia’s Ulu Segama Forest Reserve and Danum Valley Conservation Area. In this case, assisted natural regeneration involved planting tree seedlings and cutting climbing plants that compete with young trees for access to light and nutrients. The open areas created for the tree seedlings were maintained free of competing plants by cutting them back for several years after the seedlings were planted. This process gave the seedlings a head start in the race to form a new forest canopy. These restoration treatments were applied annually to different places in the study areas.

Chazdon noted that this study is an excellent application of assisted natural regeneration measures to hasten the recovery of heavily logged forests, but it considers only one dimension of forest recovery: aboveground carbon density. This dimension can be assessed through making routine measurements of tree diameter and height on the ground as well as through remote sensing and airborne approaches.

“Further experiments are needed to guide cost-effective practices,” she explained. “Collecting seed and growing seedlings in nurseries and planting them out is much more expensive than merely cutting climbers.”

Moreover, Chazdon said, it is “important to note that these results cannot be generalized outside of the Sabah, Malaysia, context, as the scale and intensity of logging in forests there are much greater than in other tropical regions based on selective logging with less timber removed.”

—Mohammed El-Said (@MOHAMMED2SAID), Science Writer


El-Said, M. (2020), Restored tropical forests recover faster than those left alone, Eos, 101, Published on 03 September 2020.

Text © 2020. The authors. CC BY-NC-ND 3.0
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