Planting trees has become a popular, easy, cheap, and accessible way to tackle climate change – or to pretend we’re doing so.
Indeed, trees are biochemical factories that soak up carbon dioxide from the air, metabolize it, and store the resulting carbon compounds in their tissues. That is why forests are a crucial part of a healthy carbon balance on Earth.
Also, the symbolic appeal of laying a young tree in the ground makes it a popular gesture for companies, politicians, and governments who wish to signal their green intentions. And it is not just a popular stunt – businesses use tree planting to offset their carbon emissions legitimately.
However, there may be a critical issue with the idea that trees can simply soak up much of our carbon pollution and store it indefinitely.
Terrestrial ecosystems are credited with removing about a third of anthropogenic carbon emissions in the past 50 years – which is quite impressive.
How Much Carbon on Land is Stored in Trees?
According to NASA, as much as 45 percent of the carbon stored on land is locked up in forests. The increased net uptake of carbon has been attributed to afforestation and the expansion of forests in cold regions where there were none before – due to the increases in temperature and the rising CO2 levels.
Additionally, all plants grow faster in warm conditions in an atmosphere rich with carbon dioxide. The faster growth of trees in warmer conditions soaks up more carbon dioxide, which has been seen as natural leverage on global warming. It is considered that forests have slowed down anthropogenic CO2 rates, and the relevant Earth System Models predict that they will continue to do so in the future.
That is where a hidden problem arises.
Faster Growth in Trees Leads to a Shorter Life Span
A new study by an international team led by the University of Leeds’ School of Geography scientists and published in the journal Nature Communications indicates that trees’ fast growth also promises their early deaths.
Besides stopping ecosystem services, the early end of a lifetime means the trees stop storing carbon sooner than predicted. Further down the line, the recent increases in carbon stocks may turn out to be transient due to the lagged increases in tree mortality.
The team dubbed the phenomenon “growth-lifespan trade-offs.”
In a warmer world, trees reach their maximum size faster. However, this seems to result in faster aging and premature death. Additionally, rapid growth possibly makes the trees more vulnerable to diseases and pests, shortening their lifespan even further.
When a dead tree trunk starts to decay, it slowly releases the stored carbon back into the atmosphere, mostly in the form of methane. While methane has a significantly shorter atmospheric lifetime than carbon dioxide, it is 84 times more potent as a greenhouse gas.
The sharp global increase of methane concentration could lead to more extreme heating within a short timeframe, enabling us to pass critical climate tipping points faster than predicted based on the CO2 projections.
The study’s findings have been consistent across all biomes, with growth-lifespan trade-offs occurring in almost all species and climates.
Of course, trees and forests are among the greatest climate stabilizers in our biosphere, and that is undeniable. The new findings are important for reconsidering our climate change response strategies and re-thinking the current climate models, specifically how they model forests as carbon sinks.
As David Lee, professor of atmospheric science at Manchester Metropolitan University (who was not involved with the study) pointed out:
“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.”
One of the long-standing concepts in forestry has been that planting new forests in place of mature (harvested) ones is beneficial for mitigating climate change.
The young trees’ faster growth rates make them theoretically more efficient in reducing the atmospheric CO2; however, the new findings on tree mortality cast a shadow on that.
Mature Forests are More Resilient to Climate Change
Additionally, other studies have pointed out that older mature forests with well-established and well-adapted trees are more resilient to climate change and make better carbon banks in the long run than young forests. Instead of continuously applying the “out with the old, in with the new” logic to forest management, maybe it’s time to look at old-growth forests with fresh eyes. In times of unprecedented exploitation of old-growth forest ecosystems, this lesson seems more important than ever.
Related: Structurally Complex Forests Better At Carbon Capture, Study Shows
All recent scientific findings seem to share a point – we need to be working on reducing emissions, turning away from fossil fuels, preserving pristine ecosystems, and finding technical solutions for carbon capture and storage. As much as we would like that, there are no quick, painless, and easy fixes to climate change – such as merely planting another tree.
Resources
The Study:
Brienen, J.R.W, et al. 2020. Forest carbon sink neutralized by pervasive growth-lifespan trade-offs. Nature Communications, vol 11, article no. 4241. https://doi.org/10.1038/s41467-020-17966-z
Articles
Shorter lifespan of faster-growing trees will add to climate crisis, study finds. The Guardian. 08 Sept 2020. https://www.theguardian.com/environment/2020/sep/08/shorter-lifespan-of-faster-growing-trees-will-add-to-climate-crisis-study-finds
Climate crisis making world’s forests shorter and younger, study finds. The Guardian. 28 May 2020. https://www.theguardian.com/environment/2020/may/28/climate-crisis-world-forests-shorter-younger-study
Seeing Forests for the Trees and the Carbon: Mapping the World’s Forests in Three Dimensions. NASA Earth Observatory. 9 Jan 2012. https://earthobservatory.nasa.gov/features/ForestCarbon
“Older forests resist change, climate change, that is.” Science Daily. 7 June 2019. https://www.sciencedaily.com/releases/2019/06/190607122408.htm
