Higher temperatures and increased atmospheric carbon dioxide (CO2) levels resulting from climate change are thought to have stimulated tree growth and in turn greater carbon storage.
But trees that grow faster have shorter lifespans, according to a global analysis published in the journal Nature Communications, and that could have ramifications for carbon storage predictions.
While this growth-lifespan trade-off has been observed before in some tree species, the study is the first to show that it’s a “universal phenomenon” occurring across virtually all species and all climates, says lead author Roel Brienen from Leeds University, UK.
Forests have provided a vital carbon sink during recent decades as the globe has warmed, especially tropical forests such as the Amazon. However, Brienen and colleagues found previously that carbon accrual there had declined by a third since the 1990s.
This prompted them to explore the relationship between tree growth and longevity in as many species as possible, using tree rings.
“By measuring tree rings’ widths one can tell how fast trees grew, while counting rings provides information on tree ages and allows making inferences about trees’ maximum lifespan,” Brienen explains.
The team analysed more than 210,000 individual tree ring records for 110 diverse tree species from more than 70,000 sites around the world, ranging from the tropics to the Arctic. Results showed that, on average, tree lifespan dropped “exponentially” by 23% for a 50% increase in early growth.
The analysis linked the trade-off directly to shortened life span from accelerated tree growth, ruling out direct effects of environmental factors like temperature and soil type.
Dramatic increases in tree mortality could result from reaching maximum potential size, the team suggests. Rapid growth could also weaken tree defences against insects or disease or cause them to grow less densely, thus compromising their water transport systems.
There is likely to be a lag between increased carbon uptake as trees grow rapidly and carbon release as they die, eventually reversing any carbon storage gains and dampening hopes for the future capacity of the terrestrial carbon sink.
“Data-driven simulations show that trade-offs have the potential to reduce, or even reverse the global carbon sink of forests in the future,” Brienen and colleagues write in their paper.
“Extant Earth system model projections of global forest carbon sink persistence are likely too optimistic, increasing the need to curb greenhouse gas emissions.”
Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.
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