• Scientists theorize that increased forest biodiversity also increases productivity (growth), and therefore carbon sequestration. But, a new large-scale study found no consistent relationship in tropical forests studied in the Amazon, Congo and Borneo.
• Research by 100+ scientists combines data from 360 1-hectare plots in Amazon, Congo, and Borneo forests, resulting in one of the largest datasets yet to examine the relationship between tropical tree diversity and carbon storage.
• Tropical forests differ markedly between continents, researchers found: Borneo forests were a triple hotspot for biodiversity, carbon and threat, making a compelling global case for prioritizing their conservation. African plots tended toward higher carbon stocks and lower diversity; South American plots had lower carbon stocks.
• The researchers urge conservationists not to generalize forest attributes when setting conservation strategies, but instead to measure the diversity, productivity, and carbon storage capabilities of each forest in order to make informed conservation decisions. This approach could enhance the success of REDD+ and other programs.

Tropical forests are home to more than 50 percent of terrestrial biodiversity, and they also play a globally important role in storing and sequestering carbon. Forest conservation can therefore help address two of the most urgent challenges facing the world today: stemming biodiversity loss and mitigating the effects of climate change.

More than 60 countries are developing projects to reduce carbon emissions from deforestation and forest degradation in exchange for economic benefits, under the United Nation’s REDD+ program. But a new study cautions that conservation strategies that focus on protecting only the most carbon-rich forests will “inevitably miss many high diversity ecosystems” that contribute less significantly to curbing greenhouse gases.

The study, published in the journal Scientific Reports, involved more than 100 scientists working within three international networks of forest plots; all using standardized methods so findings could be correlated across regions.

The networks are generating intensive, long-term studies of tree diversity and growth that enable the researchers to combine data from 360 1-hectare plots across the forests of the Amazon, Congo and Borneo. The result is the compilation of one of the largest datasets yet to examine the relationship between tree diversity and carbon storage in the tropics.

What is known is that, in general, there is a positive relationship between increased plant diversity and enhanced productivity (a measure of plant growth). And because trees sequester carbon as they grow, enhanced productivity should result in greater carbon sequestration.

However, scientists also know that isn’t always the case. Diversity can reach a saturation point, so that adding more species to the mix has no further effect on growth (and therefore carbon storage). Scientists also note that it’s unclear how the relationship between diversity and growth plays out in tropical forests where diversity levels are extremely high.

Determining that there is a positive link between forest diversity, growth and carbon sequestration in the tropics could, therefore, be a win-win for conservation and curbing climate change.

This question is what interested the research team, led by Martin Sullivan of the University of Leeds, UK. Surprisingly, when the scientists analysed their data, they found that there was no relationship between how diverse a forest plot was and how much carbon it stored, either within continents, or globally. Instead, diversity and carbon storage varied independently.

Some continent-wide patterns did emerge: African plots tended to have higher carbon stocks, and lower diversity; South American plots had lower carbon stocks, but both carbon and diversity varied a great deal throughout that continent; while Southeast Asian plots tended to have both high carbon stocks and high diversity.

The forests also differed in how diversity changed from plot to plot: differences between plots were smallest in Southeast Asia, possibly due to the dominance of wind-dispersed species, something that would help tree species spread across the landscape, explained Sullivan. The evolution and ecology of Southeast Asian tree species could explain why these forests emerged as being the most carbon-rich. Sullivan thinks that the dominance of Dipterocarp tree species — famously including the tallest tropical tree in the world — is partly responsible. “[T]hey underwent a huge radiation in Asia,” said Sullivan. “They have an unusual growth form, being very tall and straight, and are fast growing.”

The lack of a clear relationship between carbon and diversity contrasts with the findings of other recent studies, but the research team notes that this could be explained by differences in methodology and data: other studies have used smaller datasets that did not account for environmental variation as carefully as the new study, they argue. Sullivan’s team did find a positive relationship at smaller scales within plots, in line with other research, suggesting that any relationship between diversity and carbon storage might be scale-dependent.

Kyle Cavanaugh, of UCLA, led one of the earlier studies that reported a positive link between diversity and carbon, and he agrees that there are a number of possible explanations for the differing conclusions. “It is challenging to control for all of the climatic and environmental variables that influence both biodiversity and carbon storage and observational studies address this issue in different ways,” he explained. “Various studies have also spanned different gradients in climate, elevation, and other environmental factors.”

What do the results mean for large-scale conservation strategies? “Conserving forests for climate change mitigation is important, and the recognition of this in the Paris Agreement brings great opportunities for forest conservation,” said Sullivan. However, the link between carbon and biodiversity should not automatically be assumed to exist in any particular forest, with each one needing to be evaluated seperately.

“The protection of biodiversity needs to be considered alongside climate change mitigation, rather than [both] assumed to benefit automatically,” he said.

Biodiversity could also play other roles in maintaining carbon stocks in forests, roles that were not addressed by this study, Cavanaugh added. “[I]t is still unclear how biodiversity impacts the stability and resilience of carbon storage in tropical forests. Variability in the response of species to disturbances such as drought may enhance the ability of diverse forests to withstand and/or recover from such disturbances. This has important implications for the stability of tropical forest carbon storage.”

“[W]e really need long-term tropical forest monitoring data,” to explore these questions further, Cavanaugh urged. Sullivan explained that data availability is one of the biggest challenges facing conservationists attempting to determine the relationship between carbon and biodiversity, especially when trying to take into account the diversity of other groups, such as birds and mammals, alongside trees.

International networks, such as those that contributed to the latest study, are vital for research at this scale, Sullivan said. “Establishing and monitoring plots is very labor intensive, so it would not be possible for a single research group to conduct ambitious long-term continental and pan-tropical monitoring.”

The team’s results show that “[t]ropical forests differ markedly among continents, and I think it is important to conserve this diversity of forests across the world rather than concentrate on a single continent,” Sullivan said. He added that the patterns identified by the team could help guide conservation priorities.

“REDD+ schemes could be especially important in promoting forest conservation in central Africa, as there is a great need and opportunity to protect the extensive carbon-dense forests in the Congo Basin,” said Sullivan. The team also identified Bornean forests as “a triple hotspot for biodiversity, carbon and threat, [meaning that] there is a compelling global case for prioritizing their conservation.”

Citation:

Sullivan, M. J. P. et al. Diversity and carbon storage across the tropical forest biome. Sci. Rep. 7, 39102; doi: 10.1038/srep39102 (2017)