Carbon capture is substantial in secondary tropical forests, according to a new study. Regeneration helps reach global climate mitigation and forest restoration goals. So just protecting natural forest regrowth can help reduce climate change. Here we present reports from Clemson University and (below that) from the University of Connecticut describing this research which has appeared in Science Advances.
From Clemson University
One of the most effective methods for capturing carbon from the atmosphere in the tropics of Latin America requires doing very little. In fact, researchers say, just protecting natural forest regrowth can help reduce climate change.
A study published May 13 in Science Advances, shows that when land is left to regrow after forests have been cleared, these “secondary forests” could play a substantial role in removing carbon from the air even without costly tree plantings or promotion of land abandonment.
However, the practice of protecting regrowing forests has been overlooked by policy-makers and global organizations aimed at reducing climate change.
“The mantra has been, ‘we need to protect old-growth forests’,” said Saara DeWalt, an author of the study and an associate professor of biological sciences at Clemson University. “Protection of old-growth forests, which store substantial amounts of carbon, is absolutely needed, but we need to look to secondary-forest protection as well.”
Protection of secondary forests have not been as highly valued, so after land is cleared once, it might not be considered for protection.
“Policies to mitigate climate change should include land abandonment and natural regeneration as part of an overall plan, with protection of old-growth forests and reduction in the use of fossil fuels,” DeWalt said. “If we also start to promote protection of second-growth forests, or promote land abandonment, we will get a big payoff.”
DeWalt is part of a 60-member international team of scientists investigating the benefits of secondary forests for reducing global warming. In February, a study by the team published in Nature reported that forests in wetter areas of the Neotropics — Central and South America and the Caribbean — regrow more quickly than those in drier areas.
The Science Advances study first used a 2008 map of forest biomass to determine the extent of secondary forests of different ages across the Neotropics. The researchers estimated that of 8.7 million square kilometers studied more than 2.4 million square kilometers were made up of young secondary forests (aged 1 to 20 years) and middle-aged secondary forests (20-60 years). Then, they projected how much carbon these forests would store in above-ground biomass (trunks, limbs and leaves) between 2008 and 2048 based on rates of recovery they had reported in the Nature study. These rates take into account the effects of rainfall and seasonality on forest regrowth.
They found that if all of the young and middle-aged secondary forests were left alone between 2008 and 2013, they would have taken up more carbon than was put into the air by fossil fuel consumption and industrial processes in the Neotropics between 2010 and 2014.
To account for changes in the secondary forests over the projected 40-year period, the researchers also looked at how much carbon would be stored in the above-ground parts of the trees if only 20, 40, 60 and 80 percent were left alone to regenerate. If only 20 percent of the secondary forests were allowed to continue developing, more carbon would be released than would be captured because of carbon released when clearing forests. But a net carbon gain starts to appear when just 40 percent of secondary forests are allowed to persist.
The study assumed the secondary forests would be protected from fire, but did not account for possible changes in climate that might affect rates of growth. However, the researchers say their estimates are conservative for various reasons, including the fact that they did not include carbon stored below ground in soil and roots, which could make up 25 percent of the total carbon captured by forests.
The importance of secondary forests is beginning to take hold among scientists thanks in part to the two recent studies by this research team and activities by individual scientists, like the University of Connecticut’s Robin Chazdon, the lead author of the Science Advances paper. Chazdon brought attention to secondary forests when she presented some of the team’s research in Paris in 2015, DeWalt said. But there’s still much research to do.
“The next step is to go beyond carbon and biomass and look at other benefits of secondary forests,” DeWalt said. The research team will continue to work with the network of land plots to determine the patterns in tree species diversity and composition.
“We need to understand more details about benefits of secondary forests and which conditions promote resilience of secondary forests,” DeWalt said.
“There’s not going to be a one-answer solution to slowing climate change,” DeWalt said. “It’s going to take a lot of different parts. If we can promote secondary forests and their protection, then we can go part of the way. Let’s add that to the tool box.”
University of Connecticut news release
From the University of Connecticut
The importance of forest conservation and forest regrowth in climate mitigation and carbon sequestration – capturing carbon dioxide (CO2 ) from the atmosphere – has long been recognized by climate scientists. But, detailed information needed to make accurate estimates of this potential has been missing.
Now, an international team of 60 scientists – working together as the 2ndFOR Network — has completed studies on the effects of forest conservation and secondary forest regeneration across 43 regions in Latin America and has come up with real answers.
In “Carbon sequestration potential of second-growth forest regeneration in the Latin American tropics” published in the May 13 issue of Science Advances, University of Connecticut professor of Ecology and Evolutionary Biology Robin Chazdon and her colleagues report a series of major findings related to their research.
Chazdon reports the objectives of the study were to model the areas covered by regrowth forests across the lowlands of the Latin American Tropics in two age classes, to project above ground carbon storage in these young forests over four decades, and to illustrate alternative scenarios for carbon storage where 0-80% of these forests are allowed to regenerate and where 0-40% percent of pastures are allowed to regenerate into forests.
She says, “This research is vital because actively growing vegetation takes carbon dioxide out of the atmosphere and converts it to plant tissues such as wood and leaves. Old growth forests contain large stocks of carbon in their biomass. When these forests are cleared and burned, this carbon is released into the atmosphere contributing to global warming. This is one of the main reasons why it is important to halt deforestation.
“But, we have also learned that when forests regrow, their carbon stocks in above ground biomass increase over time, depending on climate, prior land use, and features of the surrounding landscape. This regrowth can happen without planting trees, through the spontaneous process of natural regeneration. This is a low-cost way of restoring forests and of reaching carbon mitigation goals that should receive greater attention from policy makers, non-governmental organizations, and international conventions.”
“This study uses the knowledge gained from long-term studies of tropical forests to address a pressing societal need,” says Saran Twombly, program director in the National Science Foundation’s (NSF)’s Division of Environmental Biology, which funded components of the research through NSF’s Long Term Studies in Environmental Biology Program and Dynamics of Coupled Natural and Human Systems Program. “It shows that natural processes alone can provide a solution to the excess carbon dioxide threatening the planet.”
Among the major findings of this study:
- Models of forest age in 2008 show that 17% of the forest area in lowland Latin America is young second-growth forest (1-20 years) and 11% is intermediate age (20-60 years).
- Assuming that 100% of the second growth persists and regenerates over 40 years, the carbon storage capacity doubles in young second growth and increases by 120% in intermediate age forests. In both forest age classes, a net gain of 8.48 Petagrams of carbon is stored over 40 years.
- This amount is equivalent to 31.09 Petagrams of CO2, which equals all of the carbon emissions from fossil fuel use and other industrial processes in all the countries of Latin America and the Caribbean from 1993 to 2014.
- Ten countries account for 95% of this carbon storage potential, led by Brazil (71%), Colombia, Venezuela, and Mexico. Chazdon says that it is remarkable that this huge amount of carbon storage doesn’t require any costly tree plantings or conversion of farmlands present in 2008. “It is all based on natural forest regrowth and only requires persistence and protection of the young forests and abandoned agricultural fields.”
In addition to forest-based solutions to mitigate climate change, there are many other benefits to this strategy, including hydrological regulation, habitats and corridors for conserving biodiversity, and provision of non-timber forest products to local people.
The importance of avoided deforestation and forest restoration for carbon storage and climate change mitigation are widely recognized and are included in Article 5 of the COP21 Paris agreement.
Prior emphasis has been on the avoidance of deforestation, however Chazdon says, “As a result of our analysis we can positively say that avoiding deforestation and supporting forest regeneration are complementary and mutually reinforcing activities.”
She acknowledges that natural regeneration is not always possible in cases where soils have been degraded or sites are isolated from seed sources, but says that multi-species tree planting can also be a solution. This option is costly and may be difficult to accomplish at large scales, but it is an alternative strategy that may be appropriate in some instances.
Hitting the Target
While forest regeneration and protection alone cannot fully compensate for greenhouse gas emissions on a global scale, the study affirms that this strategy can contribute significantly toward reaching national and international carbon mitigation targets.
Projections from the research also indicate the potential for significant contribution to forest restoration targets of the Bonn Challenge and the New York Declaration on Forests which call, respectively, for ending natural forest loss and restoring 150 million hectare of forest worldwide by 2020 and 350 million hectares of forest worldwide by the year 2030.
Regrowth of tropical secondary forests following complete or nearly complete removal of forest vegetation actively stores carbon in aboveground biomass, partially counterbalancing carbon emissions from deforestation, forest degradation, burning of fossil fuels, and other anthropogenic sources. We estimate the age and spatial extent of lowland second-growth forests in the Latin American tropics and model their potential aboveground carbon accumulation over four decades. Our model shows that, in 2008, second-growth forests (1 to 60 years old) covered 2.4 million km2 of land (28.1% of the total study area). Over 40 years, these lands can potentially accumulate a total aboveground carbon stock of 8.48 Pg C (petagrams of carbon) in aboveground biomass via low-cost natural regeneration or assisted regeneration, corresponding to a total CO2 sequestration of 31.09 PgCO2 . This total is equivalent to carbon emissions from fossil fuel use and industrial processes in all of Latin America and the Caribbean from 1993 to 2014. Ten countries account for 95% of this carbon storage potential, led by Brazil, Colombia, Mexico, and Venezuela. We model future land-use scenarios to guide national carbon mitigation policies. Permitting natural regeneration on 40% of lowland pastures potentially stores an additional 2.0 Pg C over 40 years. Our study provides information and maps to guide national-level forest-based carbon mitigation plans on the basis of estimated rates of natural regeneration and pasture abandonment. Coupled with avoided deforestation and sustainable forest management, natural regeneration of second-growth forests provides a low-cost mechanism that yields a high carbon sequestration potential with multiple benefits for biodiversity and ecosystem services.