Trees with faster growth rates die younger across multiple countries and species, which reduces overall carbon storage capacity, a new study claims.
Researchers analysed tree-ring data of more than 200,000 records of 82 species across Europe, Asia and the Americas.
They found tree faster growth is being offset by earlier mortality and release of the carbon back into the atmosphere.
Many scientists believe planting more trees will offset the amount of carbon dioxide (CO2) emissions generated from human activity.
But shorter lifespans of trees will actually make them grow fast and have a much shorter lifespan to absorb the CO2 than anticipated, the new study claims.
The new study further calls into question predictions that greater tree growth means greater carbon storage in forests in the long term.
Nothofagus pumilio (southern beeches) forests around Lago Argentino, in the Andes of southern Patagonia, Argentina, showing groups of dead trees in relation to the severe drought during the austral summer 2011-2012. Increment cores from dead trees provide ages ranging from 150 to 300 years for these individuals
‘While it has been known for a long time that fast-growing trees live shorter, so far this was only shown for a few species and at a few sites,’ said study author Dr Roel Brienen from the University of Leeds.
‘We started a global analysis and were surprised to find that these trade-offs are incredibly common.
‘It occurred in almost all species we looked at, including tropical trees.’
Currently, forests absorb large amounts of carbon dioxide (CO2) from the atmosphere, stimulating tree growth.
A relationship between faster tree growth rates and shorter tree lifespan has already been shown in some trees, particularly in cold-adapted conifers.
But whether this applies across species and climates has been disputed.
Dr Brienen and colleagues analysed a large dataset of tree-ring data representing tree species across all continents except Africa and Antarctica.
Faster growth, they report, is linked to reduced tree lifespan both across and within tree species.
Tree rings of Hymenaea courbaril (Leguminosae) from the Neotropics. These tree rings formed during the wet season are delimited by visually distinct bands known as marginal parenchyma bands (limits indicated by the white arrows)
The trade-off of fast growth rates and slow lifespans has the potential to slow down or even reverse the global forest carbon sink in the future.
Growth-lifespan trade-offs are also near universal, occurring across almost all tree species and climates.
This suggests that increases in forest carbon stocks – carbon sequestered from the atmosphere and stored within the forest ecosystem in living biomass, soil and dead wood – may be short lived.
A reduced future forest carbon sink further increases the urgency to curb greenhouse emissions.
‘Our modelling results suggest there is likely to be a time lag before we see the worst of the potential loss of carbon stocks from increases in tree mortality,’ said Dr Brienen.
‘They estimate that global increases in tree death don’t kick in until after sites show accelerated growth.’
‘This is consistent with observations of increased tree death trends across the globe.’
Previous research at the University of Leeds has found long-term increases in tree mortality rates are behind tree growth increases in the Amazon forest.
In other words, trees are dying before they’re big enough to store significant amounts of carbon.
And the chances of trees dying increase dramatically as they reach their maximum potential tree size.
The team admit other factors may still play a role in trees dying earlier than usual.
For example, trees that grow fast may invest less in defences against diseases or insect attacks.
As a result of this, they may develop wood of a lower density or with water transport systems that are more vulnerable to drought.
‘Our findings, very much like the story of the tortoise and the hare, indicate that there are traits within the fastest growing trees that make them vulnerable, whereas slower growing trees have traits that allow them to persist,’ said study author Dr Steve Voelker from the Department of Environmental and Forest Biology, New York.
‘Our society has benefited in recent decades from the ability of forests to increasingly store carbon and reduce the rate at which CO2 has accumulated in our atmosphere.
‘However, carbon uptake rates of forests are likely to be on the wane as slow-growing and persistent trees are supplanted by fast-growing but vulnerable trees.’
The study has been published in Nature Communications.
WHERE IS EARTH’S CARBON STORED?
Amazon rainforest: 200 billion tonnes
Siberian permafrost: 950 billion tonnes
Arctic: 1,600 billion tonnes
Oceans: As much as 38,000 gigatonnes, according to World Ocean Review
These figures are estimates, but true values may be higher. By contrast, humans produce an estimated 36 billion tonnes of carbon annually.