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Tropical trees’ growth and CO2 intake hit by more extreme dry seasons

Tree rings on a dead tree.

Tree rings on a dead tree. The study involved a team of 100 researchers who contributed tree-ring data from 30 tropical and subtropical countries. Image by danielkirsch via Pixabay.

  • A new study has found that dry seasons that are warmer and drier than usual can stunt the growth of tropical trees, causing them to take in less carbon dioxide.
  • While trees tend to grow more during the wet season, the researchers found that the dry season actually had a stronger impact on tree growth than the wet season.
  • As climate change continues to raise temperatures, tropical trees could face increased risk of mortality and the possibility of becoming a net source of carbon, rather than a carbon sink.

Slice through a tree trunk, and you’ll find a series of rings that tell a story of how that tree grew over the course of its life. These rings are easy to detect in trees in temperate regions since warmer months produce distinct light bands and colder months produce dark bands. In trees growing in the tropics, rings can be harder to notice — so much so that ecologists once believed that tropical trees didn’t produce rings at all.

Over the past few decades, this line of thinking has changed. Researchers now understand that trees in the tropics do have rings, although they tend to be lighter and subtler than those found in trees growing in temperate zones. In 2018, researchers even established the first tropical tree-ring network that collates research and hundreds of tree-ring chronologies from more than 30 countries in tropical regions.

A recent study published in Nature Geoscience draws on data from this network to look at how climate fluctuations influence tree growth in the tropics. Its key finding is that a dry season that’s warmer and drier than usual stunts the growth of tropical trees, and as a result, these trees are likely to take in less carbon during these periods. In the wet season, the trees did grow — but the dry season still had a more profound effect on fluctuations in growth, the study found.

“We know that tropical trees mainly grow during the wet season,” lead author Pieter Zuidema, a professor of tropical forest ecology at Wageningen University & Research in the Netherlands, told Mongabay. “So we would expect that variation from year to year in the climate and rainfall and the temperature during the wet season would be the main driver of fluctuations in growth. But it’s not. It’s a dry season that has a much stronger imprint signal on tropical tree growth than the wet season, and much stronger than we had anticipated.”

Amburana cearensis in Caatinga dry forest in northeastern Brazil. Image courtesy of Peter Groenendij.

Zuidema said this is the first global study that looks at how climate fluctuations impact trees in the pan-tropical region, although other studies have come to similar conclusions through local or regional analyses.

The study involved a team of 100 researchers who contributed tree-ring data from 30 tropical and subtropical countries.

As climate change raises global temperatures, producing drier, hotter weather throughout the tropics, this could have dire consequences for trees, Zuidema said.

“[As] climate change causes … an overall shift towards more water limitation and a stronger variability, then it may lead to an amplification of the year-to-year variation in tree growth, and therefore, also a higher probability of trees growing hardly anything in a year,” he said.

Zuidema said the lack of growth could undermine a tree’s ability to take in carbon dioxide, but it could also threaten the tree’s ability to survive. And when mortality occurs, this “​​increases the risk that tropical vegetation or tropical forests will flip from being a net sink of carbon to a source of carbon in dry years,” he said.

He added that further research is needed to quantify the reduction in growth and carbon sequestration in tropical trees during the dry season.

Another study published last year in Nature Climate Change suggested that while many tropical forests have remained carbon sinks, locking away 7.6 billion metric tons of carbon dioxide every year, many have, at times, turned into net sources of carbon dioxide, including the Brazilian Amazon.

Researchers taking tropical trees’ samples for the study. Images courtesy of Peter Groenendij.

Tropical forests shifting from carbon sinks into net sources of carbon would accelerate the climate crisis, experts say. According to the theory of planetary boundaries, there are nine Earth systems and processes support life on Earth, but each one has a limit to which it can stand environmental change. The theory suggests that humanity already breached the threshold for climate change when we surpassed 350 parts per million (ppm) of carbon dioxide in the atmosphere in the late 1980s. In 2021, the Mauna Loa Atmospheric Baseline Observatory in Hawai’i measured a peak of 419 ppm, the highest level since measurements began over 60 years ago.

Rutuja Chitra-Tarak, tropical tree expert and staff scientist at the Los Alamos National Laboratory, who was not involved in the paper published in Nature Geoscience, calls this new study “important and fascinating.”

“Zuidema and co-authors undertake a large meta-analysis of tropical tree rings over the past 50 years, adding a large dataset of tree ring chronologies of their own,” Chitra-Tarak told Mongabay in an emailed statement. “They shed new light on how and when changes in climate decline tropical tree woody growth, an important carbon sink.”

She added that while there are many uncertainties in understanding how climate change affects tropical trees, the increase of carbon dioxide into the atmosphere and other human-driven pressures are likely to have negative effects.

“Climate change is placing trees in environments that they haven’t witnessed in their historical past, crossing the range of climatic variability and tolerances that trees may be adapted to,” she said. “Increasing extremes in temperatures, water availability, increasing CO2 and other disturbances all are changing simultaneously … but how these factors will interact with each other, where do thresholds of negative response lie that would change or tip tropical forest function, needs to be worked out with data.”

Correction (04/20/2022): This article has been updated with the correct position of Rutuja Chitra-Tarak.

Banner image: Tree rings on a dead tree. The study involved a team of 100 researchers who contributed tree-ring data from 30 tropical and subtropical countries. Image by danielkirsch via Pixabay.

Citations:

Harris, N. L., Gibbs, D. A., Baccini, A., Birdsey, R. A., De Bruin, S., Farina, M., . . . Tyukavina, A. (2021). Global maps of twenty-first century forest carbon fluxes. Nature Climate Change, 11(3), 234-240. doi:10.1038/s41558-020-00976-6

Rockström, J., Steffen, W., Noone, K., Persson, Å., Chapin, F. S., Lambin, E. F., … Foley, J. A. (2009). A safe operating space for humanity. Nature, 461(7263), 472-475. doi:10.1038/461472a

Zuidema, P., Babst, F., Groenendijk, P., Trouet, V., Abiyu, A., Acuña-Soto, R., … Zhou, Z. K. (2022). Tropical tree growth driven by dry-season climate variability. Nature Geoscience, 15(4), 269-276. doi:10.1038/s41561-022-00911-8

Elizabeth Claire Alberts is a staff writer for Mongabay. Follow her on Twitter @ECAlberts.

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