Long-Lived Tropical Trees Play Outsized Role as Carbon Sinks

Infertile “pioneer” giants play a big role in storing atmospheric carbon that causes climate change.

Around 300 tree species grow in 125 acres of old-growth forest at Barro Colorado Island, Panama. (Photo by Christian Ziegler)

(CN) — A new model for predicting how tropical forests develop over time could help guide the management and recovery of biodiversity hot spots that play an important role in climate change.

“By having a better understanding of the mechanisms driving forest carbon storage, we are much more likely to be able to accurately predict the future of the global carbon cycle, and thus the pace of climate change,” said Caroline Farrior, an ecologist and professor at the University of Texas, Austin, who co-authored a new study published in the journal Science on Thursday.

Using 34 years of real-world data collected from a tropical rainforest in Panama, the research team condensed 282 plant species into five key categories that can be fed into a computer model to create more accurate forecasts for forest development.

“With these five functional groups we can produce accurate predictions of forest recovery and biomass over time,” said Nadja Rüger, junior research group leader at the German Centre for Integrative Biodiversity Research and lead author of the study.

The team focused on a nearly 125-acre plot of land on Barro Colorado Island in Panama, which is home to “one of the best studied tropical forests in the world,” Rüger said.

By comparing computer simulations to real-world data from 1985 to 2010, the team found five plant types that help produce more accurate models.

The five plant types were identified as fast, slow, infertile giants, fertile dwarfs and an intermediate group. The “fast” plants grow and die quickly. The “slow” species grow sluggishly and live long. Infertile giants, or long-lived pioneers, grow quickly and live long but reproduce less. Fertile dwarfs, or short-lived breeders, grow and survive poorly but produce a lot of offspring. The intermediate group includes plants that grow at a mild pace and reproduce moderately.

“Here, we have shown we can capture all of that diversity with just five groups,” Farrior said.

By taking the results of computer-simulated forest growth using these five groups and comparing it to real-world data, the research team discovered an approach to more accurately predict how forests develop over time.

The findings could help forecast how tropical forests will respond to climate change over time and how much biomass will be available to help absorb carbon dioxide created by human activities, such as burning fossil fuels.

It could also help refine computer models used to predict global climate change over time by integrating data on how forests will develop and the resulting impact on their capacity to soak up carbon.

“If we can predict how young forests grow up and sequester carbon, we can support this direction to make better predictions about how the global climate will change,” Rüger said.

Additionally, the new framework could help in developing sustainable strategies for forest management and timber harvesting.

“We can apply this modeling framework to really see how long it takes for the timber stocks to recover and what sustainable cutting cycles would be, or sustainable management designs,” Rüger said.

Another major finding from the study was uncovering the important role that infertile giants play in old-growth forests. In the Barro Colorado Island forest, infertile giants can be hundreds of years old and grow up to 160 feet tall and 10 feet in diameter. It was assumed these types of trees were more valuable in middle-aged forests and that they might disappear in old-growth forests, but that assumption was proved wrong in this study.

“They account for about 40% of the biomass in this forest,” Rüger said. “They are the dominant strategy type among all forest ages and not only middle-aged forest as has been assumed so far.”

This “growth and survival” strategy is a special feature of the Barro Colorado Island rainforest. Comparing it to 10 other tropical forests around the world, the dominant role of infertile giants was found in only two forests, Rüger said.

Rüger hypothesizes that wind disturbance creates gap in the forest that provides more opportunities for these typically infertile giants to reproduce.

“When they die and fall, they create big gaps in the forest that allows this type of tree to recruit, but we’re not sure about that,” Rüger said.

In future studies, Rüger said she plans to broaden this approach to younger forests and forests with less complete data.

“We evaluated this model with forests that are 40 years old,” Rüger said. “I’d like to extend it to very young forests to support forest restoration projects.”

Rüger added that she also hopes this new framework will foster collaborations with scientists that specialize in forest trees to develop new tools for planning sustainable management practices.

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