Secondary forests offer big opportunity to fight global warming

• Over half of the world’s tropical forests are not old-growth but naturally regenerating forests, and a large part of that is secondary forest.
• An international team analyzed the recovery of above-ground biomass in 1,500 forest plots at 45 sites across Latin America and found that carbon uptake is surprisingly fast in these young, regrowing forests.
• After 20 years, secondary forests recovered 122 tons of biomass per hectare (about 2.5 acres) on average.

Secondary forests are known to sequester carbon more rapidly than old growth forests, meaning they have an important role to play in our efforts to mitigate the impacts of climate change.

But new research shows that some regenerating forests might have an even higher rate of carbon uptake than previously thought, which can help identify the most important conservation priorities.

In an article published by the journal Nature this week, an international team of scientists led by researchers at Wageningen University in the Netherlands summarized the results of their study focused on secondary forests recovering from being almost completely destroyed for agriculture or cattle ranching.

The researchers say that their results contradict previous studies that suggested tropical forests may collapse or switch to an alternative stable state once a certain threshold of disturbance is reached.

“Our study shows that forests can be resilient, and that their biomass resilience strongly depends on water availability,” they write in Nature.

“By mapping potential for biomass recovery across the Neotropics, attention can be focused on particular areas that should be conserved or treated with extra caution because they are more difficult to restore (slow recovery).”

The team analyzed the recovery of above-ground biomass in 1,500 forest plots at 45 sites across Latin America and found that carbon uptake is surprisingly fast in these young forests.

Lourens Poorter, a professor at Wageningen and lead author of the article, said that after 20 years, secondary forests recovered 122 tons of biomass per hectare (about 2.5 acres) on average.

“This corresponds to an uptake of more than three tons carbon per ha each year. This is eleven times the uptake rate of old-growth forests,” Poorter said in a statement.

However, the team also found that secondary forests differ dramatically in their biomass resilience, which they say is mainly dependent on water availability, though several factors play a role.

“Climate change scenarios predict less and/or more variable rainfall for several regions in the tropics, which may potentially hamper biomass recovery and forest resilience in these regions,” Poorter and co-authors write.

Over half of the world’s tropical forests are not old-growth but naturally regenerating forests, and a large part of that is secondary forest. The authors suggest that relative recovery, that is, the ability of a regrowing forest to return to its original, pre-deforestation state, may be the better indicator of forest resilience than absolute recovery rates, or how quickly it recovers after being disturbed.

The researchers found that it takes a median time of 66 years for secondary growth forests to recover 90 percent of old-growth values.

Secondary forests have risen in prominence over the past decade as a conservation priority. A study published last year in Nature, for instance, suggested that just by reducing deforestation and forest degradation, allowing forests degraded by logging and agriculture to recover, and reforesting areas that have been cleared, we could sequester or avoid emitting the carbon equivalent of nearly half the current level of greenhouse gases emissions from the world’s use of fossil fuels.

In order to show how policy makers might use their findings about secondary forests’ carbon uptake rates, the authors of the present study produced a biomass resilience map of Latin America (seen above).

They write in the Nature article that the map can be used to identify areas that require special protections due to slow recovery rates, or to determine where spontaneous forest regrowth and reforestation have a high chance of successfully sequestering large amounts of carbon.

The latter might be suitable forests for programs administered under the UN Reducing Emissions from Deforestation and forest Degradation (REDD+) initiative, officially adopted as part of the Paris climate agreement signed in December by nearly 200 countries that committed to reducing their emissions in order to combat climate change.

“Such a spatially explicit, resource-based approach paves the road towards a more sustainable design and management of human-modified tropical landscapes,” the researchers write.

CITATION

• Poorter, L., Bongers, F., Aide, T. M., Zambrano, A. M. A., Balvanera, P., Becknell, J. M., ‚Ķ & Craven, D. (2016). Biomass resilience of Neotropical secondary forests. Nature online 3 February 2016. doi:10.1038/nature16512

Article published by Mike Gaworecki on February 12, 2016.