Trees naturally take up and store carbon dioxide, creating a vast and natural carbon sink that is crucial to putting the brakes on global warming. One assumed benefit of higher temperatures and increased CO2 levels is that they can stimulate the growth of trees, which means forests will take more CO2 as they grow. In other words, faster-growing trees remove more CO2 from the atmosphere. However, there may be a caveat.
Across almost all tree species, faster-growing trees have a shorter lifespan, according to a new international study published on 8 September in Nature Communications (1). Fast-growing trees reach their maximum size sooner, which increases their chance of dying – possibly up to 30 years earlier for every degree of warming (2). This could eventually reverse any supposed gains in forest carbon storage.
The trade-off between growth and lifespan
In the largest to date looking at the relationship between tree growth and tree lifespan, a team of researchers led by the University of leads analysed more than 200 thousand tree-ring records from 82 tree species around the world and confirmed a clear tradeoff between tree growth and lifespan, mainly due to faster growth at warmer temperatures rather than the higher temperatures themselves.
The authors suggest that certain traits of faster-growing trees might make them more vulnerable to environmental stressors such as drought, disease, and pests. For example, trees that grow faster may put fewer resources toward fighting off diseases and the wood is less dense, which could make faster-growing species are more susceptible to drought.
In a statement, lead author Dr Roel Brienen of the University of Leeds said he was “surprised to find that these trade-offs are incredibly common” and “occurred in almost all species we looked at, including tropical trees.”
“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. They estimate that global increases in tree death don’t kick in until after sites show accelerated growth”.
Brienen added: “This is consistent with observations of increased tree death trends across the globe. For example, previous Leeds research has shown long-term increases in tree mortality rates lagging behind tree growth increases in the Amazon forest”.
The new finding calls into question existing climate models. Standard climate models assume forest carbon sinks will continue to balance out the carbon dioxide produced by fossil fuel burning. But shorter lifespans will mean the benefits of these faster-growing forests might also be shortlived as forests die off sooner and end up storing less carbon. Moreover, when trees die, they slowly begin to release their carbon stores in the form of methane, a potent long-lived greenhouse gas.
Co-author Dr Steve Voelker, from Syracuse, New York, said: “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 paper echoes cautionary words of other climate scientists who argue that while planting trees is necessary, the approach is certainly not a magic solution to climate change. Trees are only a small piece of a broader strategy – one that should focus on not releasing greenhouse gases in the first place.
(1) Brienen, R.J.W. et al. Forest carbon sink neutralized by pervasive growth-lifespan trade-offs. Nature Communications (2020). DOI: 10.1038/s41467-020-17966-z
(2) Di Filippo, A. et al. The longevity of broadleaf deciduous trees in Northern Hemisphere temperate forests: insights from tree-ring series. Frontiers in Ecology and Evolution (2015). DOI: 10.3389/fevo.2015.00046