Old trees do the heavy carbon lifting

When it comes to storing carbon, an increasingly vital function of rainforests for the health and survival of the planet, all trees are not the same, according to an analysis published in the journal Science.

The study suggests those that grow fast, live to ripe old ages and reproduce slowly account for the bulk of the forest’s biomass, at any age, and far more carbon storage than previously thought.

“People have been arguing about whether these long-lived pioneers contribute much to carbon storage over the long term,” says senior author Caroline Farrior from the University of Texas at Austin, US. “We were surprised to find that they do.”

Tropical rainforests comprise dynamic ecosystems with hundreds to thousands of different tree species.

The complexity of this biodiversity needs to be accounted for when forecasting global climate in the coming decades, says first author Nadja Rüger, from the German Centre for Integrative Biodiversity Research.

Currently, models represent all the trees in a forest as essentially the same, but the new analysis highlights the flaws in this approach.

“We show that the variation in tropical forest species’ growth, survival and reproduction is important for predicting forest carbon storage,” Farrior says.

The international team used 40 years of data from 200,000 trees from 282 species collected from a tropical rainforest in Panama, one of the most well-researched tropical rainforests in the world.

Previously, they had identified that trees use different strategies during their development which could be classified according to two criteria.

“Pace of life” distinguishes “fast” species that grow and die quickly from “slow” species that grow slowly and reach an old age.

But there is a trade-off; stature and fertility can differ irrespective of the trees’ pace of life.

“Infertile giants” – known as long-lived pioneers – grow relatively fast but live a long life, reaching a large stature, and produce few offspring each year, while “fertile dwarfs” are small shrubs and treelets that grow slowly but produce abundant offspring.

The researchers plotted these different strategies in computer models that simulated how trees grow, die, produce offspring and compete for light as they would in a real forest.

They then compared this with the observed development of real, secondary forests regenerating after being cleared for timber or agriculture.

By simplifying the diversity with their groupings, they were able to reliably capture the variation in established forests and demonstrated the need to include both dimensions.

“We discovered that the nearly 300 unique tree species can be represented in our computer model by just five functional groups and still produce accurate forecasts of tree composition and forest biomass over time,” says Farrior.

The data-driven modelling approach presents a new method to predict the development of species-rich forests that can save time and resources, Rüger says.

“Basically, we were able to reduce the forest to its essence, and that was only possible because we know so much about the tree species in the forest in Panama,” she adds.

Next, the researchers plan to apply their approach to young, secondary forests and tropical dry forests with less complete data.

“If this succeeds,” says Rüger, “it will be much easier than today to scientifically support renaturation projects and sustainable timber use in tropical forests, and also, of course, to estimate how effectively re-growing forests contribute to carbon storage and, therefore, to climate mitigation.”