Amount of carbon stored in forests reduced to climate warming

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The team, led by Cambridge University, discovered that with increasing temperatures, trees grow faster, but also tend to die younger. When these fast-growing trees die, the carbon they store is returned to the carbon cycle.

The results, reported in the journal Nature Communications, have implications for the dynamics of the global carbon cycle. As the Earth's climate continues to warm, the growth of trees will continue to accelerate, but the period of time in which trees will store carbon, the so-called carbon residence time, will decrease.

During photosynthesis, trees and other plants absorb carbon dioxide from the atmosphere and use it to build new cells. Long-lived trees, such as high-altitude pines and other conifers found in the boreal forests of northern high latitude, can store carbon for many centuries.

"As the planet heats up, the plants grow faster, so the thought is that planting more trees will lead to more carbon being removed from the atmosphere," said Professor Ulf Büntgen of Cambridge's Department of Geography, lead author of the study . "But this is only half the story. The other half is one that was not considered: that these fast-growing trees retain carbon for shorter periods of time."

Büntgen uses the information contained in the tree rings to study the climatic conditions of the past. Tree rings are as distinctive as fingerprints: the width, density and anatomy of each annual ring contain information on how the weather was during that particular year. By taking basic samples from living trees and samples of discs of dead trees, researchers are able to reconstruct the behavior of the earth's climate system in the past and understand how ecosystems were and are responding to temperature changes.

For the current study, Büntgen and his collaborators from Germany, Spain, Switzerland and Russia sampled over 1100 live and dead mountain pines of the Spanish Pyrenees and 660 specimens of Siberian larch from the Russian Altai: both forest sites high altitude that have been undisturbed for thousands of years. Using these samples, the researchers were able to reconstruct the total life span and juvenile growth rate of the trees that grew during both industrial and pre-industrial climatic conditions.

Researchers have found that rigid and cold conditions slow down the growth of trees, but they also make trees stronger, so that they can live up to an advanced age. In contrast, trees that grow faster during their first 25 years die long before their slow-growing relatives. This negative relationship remained statistically significant for both living and dead tree samples in both regions.

The idea of ​​a carbon residence time was initially hypothesized by co-author Christian Körner, professor emeritus of the University of Basel, but this is the first time it has been confirmed by the data.

The relationship between growth rate and life span is analogous to the relationship between heart rate and life span observed in the animal kingdom: animals with a faster heart rate tend to grow faster but have a shorter average life.

"We wanted to test the hypothesis of live fast, die young, and we found that for trees in cold climates, it seems to be true," said Büntgen. "We are challenging some long-term assumptions in this area, which have implications for large-scale carbon cycle dynamics."

Reference:

Ulf Büntgen et al. & # 39; Limited capacity for tree growth to mitigate the global greenhouse effect under expected heating. & # 39; Nature Communications (2019). DOI: 10.1038 / s41467-019-10174-4

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