Shorter lifespan of faster-growing trees could add to climate crisis

This article was originally published by The Guardian and appears here as part of Canada's National Observer's collaboration with Climate Desk.

Live fast, die young is a truism often applied to rock stars but could just as easily describe trees, according to new research. Trees that grow rapidly have a shorter lifespan, which could spell bad news for tackling the climate crisis.

Trees grow faster in warmer conditions, and this should act as a natural brake on global heating, as they take up and store more carbon dioxide from the air as they grow. But the new study casts doubt on this beneficial cycle, finding that the faster trees grow, the sooner they die – and therefore stop storing carbon.

Some fast-growing trees, including conifer species in cold regions, have long been known to show shorter lifespans, but what was not known was the impact of warmer conditions that can spur growth as global heating accelerates. An international team of researchers, publishing their work on Tuesday in the peer-review journal Nature Communications, has found that the relationship between faster growth and shorter lifespan appears to hold good across tree species and latitudes.

Roel Brienen, associate professor of geography at the University of Leeds, the lead author of the paper, said: “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.”

Trees growing faster in warmer conditions reach their maximum size sooner, and that appears to increase their chance of dying. Trees that grow more quickly may also be more vulnerable to factors such as drought, disease and pests. When trees die, they give up their stored carbon gradually, in the form of methane, a greenhouse gas.

This means that many standard climate change models of how we can use forests as carbon sinks, to absorb the carbon dioxide we produce from fossil fuel burning, are likely to overestimate the benefits. This study suggests that although the forests of the future might grow faster as temperatures increase, they could also store less carbon as the trees die off sooner.

“Our findings 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 Steve Voelker of the department of environmental and forest biology at Syracuse University New York, a co-author of the study. “[The] 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.”

David Lee, professor of atmospheric science at Manchester Metropolitan University, who was not involved in the study, said: “Currently, Earth system climate models predict continuation or increases in the size of the carbon sink of mature forests and this study shows the opposite, that increased CO2 compromises forests as a carbon sink … The idea that fossil fuel-based emissions can be offset by planting trees or avoiding deforestation really does not stand up to scientific scrutiny.”

However, Keith Kirby, woodland ecologist at the University of Oxford, said the results did not negate the value of growing trees to stave off the climate crisis. “We cannot rely as much on increased growth per unit area to maintain and enhance the forest carbon sink potential, but this might be offset by slowing deforestation and increasing the expansion of the extent of forests, where this can be done in a sustainable way,” he said.

For the study, the international team of scientists analysed data from more than 200,000 tree ring samples representing 110 species of tree, across all continents except for Africa and Antarctica.

They found that faster growth was linked to shorter lifespans in trees of the same species, and across different species, and was not dependent on the climate or soil.

The researchers also conducted a computer simulation, using data on the black spruce (Picea mariana), to see what impact faster growth would have on carbon storage. The results showed that the greater propensity for the trees to die off after growing more quickly could reduce the capacity of global forests to absorb and store carbon dioxide, as temperatures rose.

Growing trees, and preserving existing forests, is one of the most important ways of staving off the worst impacts of the climate crisis. But several studies have cast doubt on the capacity of global forests to act as carbon sinks as the climate alters. A study published in March found that tropical forests were losing their ability to store carbon, and research published in May showed that the world’s forests were becoming shorter and younger.