Amount of carbon stored in forests reduced as climate warms

Accelerated tree growth caused by a warming climate does not necessarily translate into enhanced carbon storage, an international study suggests.

Pines in winter. Photo: Annie Spratt/Unsplash

The team found that as temperatures increase, trees grow faster, but they also tend to die younger. When these fast-growing trees die, the carbon they store is returned to the carbon cycle.

The results have implications for global carbon cycle dynamics. As the Earth’s climate continues to warm, tree growth will continue to accelerate, but the length of time that trees store carbon, the so-called carbon residence time, will diminish.

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

Professor Ulf Büntgen from Cambridge’s Department of Geography, the study’s lead author, uses the information contained in tree rings to study past climate conditions. Tree rings are as distinctive as fingerprints: the width, density and anatomy of each annual ring contains information about what the climate was like during that particular year.

For the current study, Büntgen and his collaborators, sampled more than 1100 living and dead mountain pines from the Spanish Pyrenees and 660 Siberian larch samples from the Russian Altai: both high-elevation forest sites that have been undisturbed for thousands of years.

Using these samples, the researchers were able to reconstruct the total lifespan and juvenile growth rates of trees that were growing during both industrial and pre-industrial climate conditions.

The researchers found that harsh, cold conditions cause tree growth to slow, but they also make trees stronger, so that they can live to a great age. Conversely, trees growing faster during their first 25 years die much sooner than their slow-growing relatives.

This negative relationship remained statistically significant for samples from both living and dead trees in both regions.

The relationship between growth rate and lifespan is analogous to the relationship between heart rate and lifespan seen in the animal kingdom: animals with quicker heart rates tend to grow faster but have shorter lives on average. (University of Cambridge)

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