Rise in carbon capture as global warming speeds growth of forests would be negated by earlier deaths, say scientistshttps://www.theguardian.com/environment/2020/sep/08/shorter-lifespan-of-faster-growing-trees-will-add-to-climate-crisis-study-finds by Fiona Harvey Excerpts (with bolding by me):
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.
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.”
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.
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.
“[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.”
“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.”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.
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