Looking to the future … how rising temperatures could reduce tree growth and carbon absorption
It has long been assumed our forests will play an increasingly important role in mitigating climate change, thanks to trees’ ability to absorb more carbon dioxide than they emit.
However, new research suggests the world’s trees may absorb less atmospheric carbon dioxide in the future, impeding their growth and reducing their capacity to reduce the impact of the changing climate.
The study focused on the Douglas fir, North America’s most prolific tree, which can be found in various states in the western half of the country.
The team of scientists collected data on more than 2.7 million Douglas fir tree rings, from across 2,700 sites. Tree rings are the annual layers of growth created from carbon absorbed by the tree; thinner rings indicate the absorption of less carbon during the year of their creation.
Margaret Evans, who led the research, is Assistant Research Professor of Forest Ecology in the Department of Ecology & Evolutionary Biology at the University of Arizona. She said tree rings offer a good indication of the environmental conditions surrounding a tree over time.
“Tree rings have proven themselves a powerful tool to investigate past climate,” Evans said. “Projection of future tree growth is an obvious extension of the same logic.”
Using their findings to predict the impact of future climate conditions on trees, the researchers found increasing global temperatures will lead to thinner tree rings in almost all Douglas firs, indicating impeded growth and a consequent reduction in the amount of carbon dioxide being removed from the atmosphere.
To date, there have been numerous opposing theories about the impact of climate change on our future forests. While some models have predicted a warmer planet will result in increased tree stress and death, other theories have suggested trees might actually absorb carbon dioxide more effectively under the new conditions and consequently experience enhanced growth.
The problem with existing models is their use of data acquired in controlled or small-scale environments, or their reliance on observations of trees currently growing in warmer climates to predict the future behaviour of the same species currently growing in cooler climates.
The latest research found the assumptions underlying the latter approach were overly simplistic and yielded poor predictions.
Evans explained trees adapt to their local climates, meaning examples of the same species found at different locations are unlikely to have the same genetic makeup, and therefore cannot be compared on a like-for-like basis.
The latest research also suggests a more accurate method of predicting future growth within a species might instead look at trees currently growing in warmer conditions and consider their ‘climate sensitivity’.
Average temperatures and precipitation impact a tree’s climate sensitivity, and researchers discovered the warmer or drier a local climate is on average, the more adversely impacted trees were.
"In the North-western states, every single tree ring looks the same because they're just not very sensitive to change," Evans explained.
"But in more desert terrains, year-to-year variability, or sensitivity, is much greater. In a wet year in the desert, the trees can grow great, fat rings.”
The late University of Arizona Professor Harold ‘Hal’ Fritts proposed the link between climate sensitivity and tree ring growth more than half a century ago, and this latest research suggests he may have been correct.
Going forward, improved predictive models for tree growth in response to different conditions brought about by climate change will require genetic studies, according to Evans’ team.
Source: https://phys.org/news/2020-06-trees-absorb-carbon-dioxide-world.html
